DLT FAQ

This is the DLT FAQ maintained by Larry Kaplan from Quantum. My copy is dated April 1995. I will try to keep a copy here until Larry resurfaces.

More info (including DLT autochangers) in the comp.arch.storage FAQ.

my home page at Caltech



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From: lkaplan@tdh.qntm.com (Larry Kaplan)
Newsgroups: comp.arch.storage
Subject: DLT FAQ as request - needs updating, maybe I'll get to it someday - dlt.faq [1/1]
Date: 13 Apr 1995 18:27:41 GMT
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+-----------------------------------------------------------------+
| Larry Kaplan                       |                            |
| Quantum Corporation                |                            |
| Shrewsbury, MA 01545 USA           |                            |
| Voice: (508) 770-6872 (office)     |                            |
|        (508) 770-6721 (lab)        |                            |
| FAX:   (508) 770-2869              |                            |
| lkaplan@tdh.qntm.com         (work/professional)                |
| kaplan@ultranet.com          (home/personal)                    |
+-----------------------------------------------------------------+
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Content-Type: Application/octet-stream; name=dlt.faq


    +------------------------------ Start Of Post ---------------------------+

    +------------------------------------------------------------------------+
    DLT.FAQ
    +------------------------------------------------------------------------+
    Frequently Asked Questions about DLT Tape Drives
    Written and Maintained by:
    Larry Kaplan
    Manager, Firmware Development, DLT Products
    Quantum Corporation 
    Shrewsbury, MA, USA 01545
    E-MAIL: lkaplan@tdh.qntm.com
    Voice: (508) 770-6872
    Technical questions only, please
    +------------------------------------------------------------------------+
    Last Updated: 
             3-Oct-1994 - New to this posting:
              New contact address
              Many new Q/A.  Appended to the end of the previous version.
            30-Sep-1994 - New contact information
            29-Aug-1994 - New to this posting:
              DLT Product Software Connectivity Chart
              Pointer to application notes for certain attachments
    +------------------------------------------------------------------------+

    Q: I'm thinking of buying a DLT.  What is available and what are the
        specifications ?

    A: Presently three generation of product are available.

        Product Name    Comp?  Capacity (uncomp)   User Data Rate (Uncomp)
        +-------------+-------+-------------------+----------------------+
         TZ85/THZ01      N              2.6 GB        800 KB/sec
         TZ86/THZ02      N              6.0 GB        800 KB/sec
         TZ87/DLT2000    Y             10.0 GB       1.25 MB/sec


    Q: Do these product support previous formats ?

    A: Yes, refer to table below:

        Product Name    Format         R/W
        +--------------+------------+--------------+
         Tx85/THZ01      TK50          R only
                         TK70          R only
                         TZ85          R/W

         Tx86/THZ02      TK50          R only
                         TK70          R only
                         TZ85          R/W
                         TZ86          R/W

         TZ87/DLT2000    TK50          R only (not supported on TZ87N or
                                               DLT2000)
                         TK70          R only (not supported on TZ87N or
                                               DLT2000)
                         TZ85          R/W
                         TZ86          R/W
                         TZ87          R/W


    Q: Is fast/wide SCSI available ?

    A: Presently, all three products support 8-bit, 5 MB/sec, SCSI, both
        single ended and differential.


    Q: What products are under development ?

    A: There's lots of bandwidth left in the technology.  Currently,
        prototypes are under test for the DLT4000, which increases 
        capacity to 20 GB (uncompressed), increases the User Transfer
        Rate (uncompressed) to 1.5 MB/sec, and provides support for
        10 MB/sec (fast) SCSI, single-ended and differential.
        Other advancements will be announced in due time.


    Q: I've recently connect a DLT to my system.  It's unbelievably slow.
        What gives ?

    A: It's common for a host to disable the drive's internal cache
        if it doesn't recognize the product name string.  If this is
        true, the DLT's performance can be reduced to the sub 100KB/sec
        range.

       A second possibility is the record size being used by the host.
        The DLT 2000 is especially sensitive to record size.  In general,
        the bigger, the better.  We recommend 32K, or even 64KB records
        if possible (up to 16MB records are supported).  In general,
        any record size greater than 8KB should result in a
        bottleneck-free tape drive.  Any record size less than 8KB will
        give performance problems.  Many software packages default to
        512 byte records.  This will result in throughput in the
        100-200 KB/sec range.


    Q: The performance isn't bad, but I'm not getting the full rating
        of the tape drive.

    A: Many other factors contribute to the actual performance as seen by 
        a user.  Host speed, host adapter, bus configuration, host
        software, disk characteristics, are all bottleneck considerations.
           

    Q: What Platforms and Software Products (Non DEC) Support DLT

    A: See Charts below:

        Novell/NLMs
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada Backup Exec                  Yes      Yes      Yes      Q3
          Avail - HSM                         Yes      Yes      Yes      Q4
          Cheyenne ARCserve NLM 4.05          Yes      Yes      Yes      Q3
          Cheyenne ARCserve NLM 5.01          Yes      Yes      Yes      Q3
          Conner - HSM                        Yes      Yes      Yes      Q4
          Legato - Networker                  Yes      Yes      Yes      Q3
          NovaStor NovaNet                    Yes      Yes      Yes      Q4
          Palindrome Backup Director          Yes      Yes      Yes      Q4
          Palindrome Network Archivist - HSM  Yes      Yes      Yes      Q4
          Systems Enhancement Total Recall    Yes      Yes      Yes      Q4

        UNIXware
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCserve/Open 1.1           Q4       Q4       Q4      Q4
          Legato UNIX                         Yes      Yes      Yes      Q3
          NovaStor - NovaVault                Yes      Yes      Yes      Q195

        SCO
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCserve/Open 1.1           Q4       Q4       Q4      Q4
          NovaStor - NovaMarch                Yes      Yes      Yes      Q195

        DOS          
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada - Backup EXEC for DOS        Yes       No       No      Q4
          Cheyenne ARCsolo for DOS            Yes       No       No      Q4
          NovaStor - NovaBack for DOS         Yes      Yes      Yes      Q4

        Windows
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada - Backup EXEC for Windows    Yes       No       No      Q4
          Cheyenne ARCsolo for Windows        Yes       No       No      Q4
          NovaStor - NovaBack for Windows     Yes      Yes      Yes      Q4
          
        OS/2
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCsolo for OS/2            Q4       Q4       Q4      Q4
          NovaStor - NovaBack for OS/2        Yes      Yes      Yes      Q4


    Q: Can I attach a DLT2000 to a HP9000-700 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.


    Q: Can I attach a DLT2000 to an IBM RS/6000 AIX 3.2 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.

    
    Q: Can I attach a DLT2000 to a SUN SPARC (Solaris 2.3) ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.


    Q: Can I attach a DLT2000 to a SUN SPARCSTATION SunOS 4.1.3 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.

    
    Q: I understand that there is a directory of data objects stored at the 
       start of the tape. If I issue a SCSI LOCATE command, does the drive 
       go back to BOT to access this directory and then position? What is 
       the worst case locate time?

    A: The directory is read into the drive's memory during the Load
       process and accessed from there (and updated as needed; the updated
       directory is written back on the unload.)  Typical worst case Locate
       time is about 100 seconds; there may be some cases that will be longer
       due to retries, but that is not normal.


    Q: Which SCSI commands will utilize the data object Directory and 
       provide the best file search performance?

    A: The SCSI LOCATE command will give the best possible performance,
       especially relative to a Space by filemarks/Space-blocks combination.
       If the application simply wants to get to a particular filemark
       (versus a data block), then the Locate command will give equivalent
       performance to a Space-filemarks command.  The ideal situation is to
       be able to specify the ultimate destination that the application is 
       trying to get to on the media with a single command, and the Locate 
       command is the most general and effective way to do that.


    Q: Will any SCSI Commands lose performance without any filemarks 
       on the tape?

    A: The answer is no.  In fact, without any filemarks on tape, a 
       Space-blocks command becomes almost equivalent to a Locate cmd 
       (except the one uses a relative count, and the other an absolute 
       address)


    Q: The IBM IDRC compression algorithm checks to make sure that it is not 
       expanding data.  What algorithm is used in the DLT and is it prone to 
       failure with precompressed data?  What does the compression algorithm do 
       to the error rate?
        
    A: An LZ type algorithm is used.  It does not "fail" per-se, but highly
       compressed data will expand somewhat. (IDRC compressed data usually
       gets compressed slightly more by DLT2000).  I have seen 5% expansion
       when "compressing" random data on the DLT2000.  The error rate is
       media based, so compression on/off does not affect it.  


    Q: I understand that the drive will stream with data blocks that are 
       at least 8KB in size. Does compression affect this? Does compression 
       affect data packing and efficient use of tape?

    A: Yes. But tape streaming is not the real issue; whether the tape drive 
       can keep up with the rate that the host can pump data to it or not is.
       With high compression ratios (above 3:1) the drive's data compressor
       (or the SCSI bus for that matter) may not be able to keep up with
       the data rate into the drive cache buffer to keep the drive streaming.
       In this scenario, as the cache gets full enough, the drive will be 
       started, but will be able to empty the cache faster than compressed
       data can be put into it.  Under these conditions, an 8K block size 
       may not be enough to ensure streaming performance at all compression 
       ratios.

       The drive will always do data packing, whenever appropriate, so
       compression does not affect that.  Compression naturally makes for
       more efficient use of the tape media.


    Q: Is logical space truely sequential? Does compression affect the logical
       space?

    A: The device handles "defects" in a way that is completely
       logically transparent.  The Locate command deals with logical
       blocks (whether BT flag is set or clear) so it skips over
       blocks and filemarks in a logical fashion.  The only time a
       defect is not transparent is when a hard read or write error
       occurs.  A hard read error does not normally prevent accessing
       data (if any) on the media beyond the defect.  

       Compression has no affect on the logical space and the host can still 
       logically position to any block on the media.


    Q: Is it reasonable to always use compression?  Are there Performance 
       issues?  How is data compression selected?

    A: In general, an applications will benefit from enabling data compression.
       The exception is when data has already been compressed
       with a Lempel-Ziv (or an even more powerful algorithm) compression
       method.  Such compressed data, or very random data (including
       encrypted data), will likely expand slightly (~5%).  If in doubt,
       and if there is a significant amount of such data, run some tests
       to see what sort of compression rates you actually get (use Log
       Sense Compression Status page to get compression ratios).

       If the data is compressing, throughput will increase roughly in
       direct proportion to the compression ratio from the native data
       rate, until the drive's maximum is reached.

       If a section of tape has compressed data, the front panel will indicate
       when the drive is reading compressed data.  The DLT drive will 
       automatically match to the way the data was written and decompress 
       whenever compressed data is requested. 

       Compression can be turned on/off at any time when writing.  There
       are basically 4 ways to do this: Mode Select Device Configuration
       Page, Mode Select Compression Control Page, Mode Select VU density
       code values, and the front panel (which overrides any SCSI selection).


    Q: In compressed mode, if 5 16kbyte blocks are written will logical block 
       address increment by 5? etc.  What about accessing a non-zero offset of
       the data stream?

       Whether the blocks are written in variable or fixed block
       mode, compressed or uncompressed, each block is a distinct logical
       block on the media.  The Read Position command will reflect this
       and the effect of compression is transparent.

       As for "seeking into a nonzero offset of the data stream" that
       is a filesystem function and at the SCSI level would have to be
       decomposed to some kind of Space/Locate command (possibly) and
       then issuing Read command(s).  The host would then ignore data 
       until the desired byte offset is reached, and start returning
       data from that point to the application.  SCSI sequential devices
       do not have the capability to return data starting with a byte offset.


    Q: How much space does a Write Filemarks command use?

    A: It could use 0 to 8KBytes.  A Write Filemarks (WrFMs) command of 
       0 will force any data in the cache onto tape.  If the last physical 
       block is only partially full it must still be "closed out" and 
       flushed.  This normally results in an insignificant capacity loss,
       unless WrFMs zero (alias a "flush") commands are issued very frequently.
       If this is done, it would also impact throughput performance
       because of the flush operation.  A WrFMs zero will *not* result in
       any logical block of itself (only what it flushes from previous
       write commands).  Again, it is equivalent to a "cache flush".

       A WrFms of 1 will cause the current physical block to be closed and
       flagged as containing 1 filemark, so this filemark takes up from
       zero to 8K bytes (however much unused space was left in the 8K
       physical block).  After writing 1 filemark, consecutive filemarks
       written right after that (i.e. sequential filemarks) will each
       use 1 physical block (8KB).


    Q: What happens after a powerfail or SCSI RESET during writing?  Can we 
       append to the tape?

    A: On a SCSI Bus RESET, all data that is in cache is flushed to the
       media.  This might result in a partial block on media, especially
       with very big blocks (ex: 10 MB).  A power failure will naturally
       clear the device (and cache) since there is no provision for 
       battery back-up within the DLT drive.

       Once a Write Filemarks with count=0 completes, the media is guaranteed
       to be synchronized with the cache (i.e. cache is flushed, all data
       and filemarks up to the flush are now on tape).  A bus Reset before
       the WrFMs zero completes will not kill the flush itself.  A power
       failure will.  The application can Space to EOD and check position
       to see if all data got flushed, but since the WrFMs zero did not
       complete (due to the powerfail) some data might be missing, and
       there might be a partial block.  However, the application can always
       go back to the position of the last *successful* flush.


    Q: For fixed block mode, what is the suggested block size?

    A: 16 KB or larger for good performance and compression rates.  Between
       8 KB and 16 KB performance should be okay.  But, below 8 KB device 
       throughput starts to fall off, so 8 KB or higher should be used
       if possible.  This applies to fixed and variable block modes.


    Q: What are the number of blocks that can fit on tape?

    A: The number of blocks you can fit on a particular cartridge will vary.
       Generally you can take the rated capacity and simply divide by the
       block size you plan to use.  If compression is used, the number of
       blocks can only be roughly estimated, unless the compression ratio
       with that particular data is uniform and well characterized.


    Q: What happens when you try to select the drive during powerup, or during 
       tape load/unload?

    A: After powerup, the drive will not respond for about 1 second to
       any Selection attempts, while the drive's Power-On Self Tests (POST)
       checkout the SCSI interface hardware.  After that, Selections will
       be responded to but any CDB will receive a BUSY Status, for the next
       10 or so seconds.  Then commands will be responded to normally,
       although the media might not be ready so media access commands will
       generally fail with a NotReady Sense Key.

       For load/unload, after an Unload is initiated, the media is not ready.
       After a Load command (Imm=0) completes the media will be at BOT and
       ready for media access commands.


    Q: How is media shelf life defined and specified?

    A: Shelf life is 10 years min. @ 20 degC and 40% RH.  After 500k passes 
       the tape should continue to function normally, unless it is being used
       in an evironemnt with heat, humidity or contamination levels beyond
       those recommended.


    Q: What is a "tape pass"?  When do you recommend retiring tapes?

    A: A pass is the media passing over the head once.  A full pass would
       mean going from one end of the tape to the other.

       At this point we cannot suggest a number of passes after which a 
       cartridge should be retired, because the media does not seem to
       wear out and actually improves with use.  We have tested the media
       up to 500,000 passes without degradation and use this number as
       our specification, even though a much larger number of passes could
       be attained.


    Q: How is the error rate calculated? How many retries are assumed?  Does 
       the error rate apply to virgin tape or to tape which has seen 500,000 
       passes?  Does it apply with compression on?
        
    A: Error rate is calculated based on rewrite, over-write, ECC's and
       re-reads.  The error rate for virgin vs. used tape is expected to 
       be about the same.  The media tends to improve with usage and the 
       media error rate applies to raw data on the media, so compression 
       is not a factor.

    +------------------------------- End Of Post ----------------------------+
--*-*-*- Next Section -*-*-*--



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Date: Wed, 10 May 1995 12:29:16 -0400
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To: rdv@alumni.caltech.edu
Subject: Re: REALISTIC DLT4000 perform...

*** EOOH ***
From: 
Date: Wed, 10 May 1995 12:29:16 -0400
To: rdv@alumni.caltech.edu
Subject: Re: REALISTIC DLT4000 perform...

sorry it took me so long to reply.

we are currently getting over 2mb's sec sustained on the sgi platform with
some modifications to the firmware and inquiry strings.

drop me some mail in about a week and I will let you know how the tests
turned out.

jim
inline corporation
800 465 4637


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Date: Mon, 22 May 1995 10:48:42 -0700
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To: rdv@alumni.caltech.edu

*** EOOH ***
From: rdv@alumni.caltech.edu (Rodney D. Van Meter)
Date: Mon, 22 May 1995 10:48:42 -0700
To: rdv@alumni.caltech.edu

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From: Ralf-Peter Rohbeck 
Newsgroups: comp.arch.storage
Subject: Re: DLT2000 on HP887
Date: 19 May 1995 19:02:57 GMT
Organization: Quantum GmbH
Lines: 429
Message-ID: <3piq11$2ji@worf.qntm.com>
References: <1995May15.161808.22423@dsi.bc.ca>
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wtw@dsi.bc.ca (William Wong) wrote:
>* I read form the FAQ that there is a DLT FAQ, could somebody tell me
>where can I find it?
Here we go. It resides on a machine inside qntm.com, and it hasn't been 
maintained for a while.


    +------------------------------------------------------------------------+
    DLT.FAQ
    +------------------------------------------------------------------------+
    Frequently Asked Questions about DLT Tape Drives
    Written and Maintained by:
    Larry Kaplan
    Manager, Firmware Development, DLT Products
    Quantum Corporation 
    Shrewsbury, MA, USA 01545
    E-MAIL: lkaplan@tdh.qntm.com
    Voice: (508) 770-6872
    Technical questions only, please
    +------------------------------------------------------------------------+
    Last Updated: 
             3-Oct-1994 - New to this posting:
              New contact address
              Many new Q/A.  Appended to the end of the previous version.
            30-Sep-1994 - New contact information
            29-Aug-1994 - New to this posting:
              DLT Product Software Connectivity Chart
              Pointer to application notes for certain attachments
    +------------------------------------------------------------------------+

    Q: I'm thinking of buying a DLT.  What is available and what are the
        specifications ?

    A: Presently three generation of product are available.

        Product Name    Comp?  Capacity (uncomp)   User Data Rate (Uncomp)
        +-------------+-------+-------------------+----------------------+
         TZ85/THZ01      N              2.6 GB        800 KB/sec
         TZ86/THZ02      N              6.0 GB        800 KB/sec
         TZ87/DLT2000    Y             10.0 GB       1.25 MB/sec


    Q: Do these product support previous formats ?

    A: Yes, refer to table below:

        Product Name    Format         R/W
        +--------------+------------+--------------+
         Tx85/THZ01      TK50          R only
                         TK70          R only
                         TZ85          R/W

         Tx86/THZ02      TK50          R only
                         TK70          R only
                         TZ85          R/W
                         TZ86          R/W

         TZ87/DLT2000    TK50          R only (not supported on TZ87N or
                                               DLT2000)
                         TK70          R only (not supported on TZ87N or
                                               DLT2000)
                         TZ85          R/W
                         TZ86          R/W
                         TZ87          R/W


    Q: Is fast/wide SCSI available ?

    A: Presently, all three products support 8-bit, 5 MB/sec, SCSI, both
        single ended and differential.


    Q: What products are under development ?

    A: There's lots of bandwidth left in the technology.  Currently,
        prototypes are under test for the DLT4000, which increases 
        capacity to 20 GB (uncompressed), increases the User Transfer
        Rate (uncompressed) to 1.5 MB/sec, and provides support for
        10 MB/sec (fast) SCSI, single-ended and differential.
        Other advancements will be announced in due time.


    Q: I've recently connect a DLT to my system.  It's unbelievably slow.
        What gives ?

    A: It's common for a host to disable the drive's internal cache
        if it doesn't recognize the product name string.  If this is
        true, the DLT's performance can be reduced to the sub 100KB/sec
        range.

       A second possibility is the record size being used by the host.
        The DLT 2000 is especially sensitive to record size.  In general,
        the bigger, the better.  We recommend 32K, or even 64KB records
        if possible (up to 16MB records are supported).  In general,
        any record size greater than 8KB should result in a
        bottleneck-free tape drive.  Any record size less than 8KB will
        give performance problems.  Many software packages default to
        512 byte records.  This will result in throughput in the
        100-200 KB/sec range.


    Q: The performance isn't bad, but I'm not getting the full rating
        of the tape drive.

    A: Many other factors contribute to the actual performance as seen by 
        a user.  Host speed, host adapter, bus configuration, host
        software, disk characteristics, are all bottleneck considerations.
           

    Q: What Platforms and Software Products (Non DEC) Support DLT

    A: See Charts below:

        Novell/NLMs
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada Backup Exec                  Yes      Yes      Yes      Q3
          Avail - HSM                         Yes      Yes      Yes      Q4
          Cheyenne ARCserve NLM 4.05          Yes      Yes      Yes      Q3
          Cheyenne ARCserve NLM 5.01          Yes      Yes      Yes      Q3
          Conner - HSM                        Yes      Yes      Yes      Q4
          Legato - Networker                  Yes      Yes      Yes      Q3
          NovaStor NovaNet                    Yes      Yes      Yes      Q4
          Palindrome Backup Director          Yes      Yes      Yes      Q4
          Palindrome Network Archivist - HSM  Yes      Yes      Yes      Q4
          Systems Enhancement Total Recall    Yes      Yes      Yes      Q4

        UNIXware
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCserve/Open 1.1           Q4       Q4       Q4      Q4
          Legato UNIX                         Yes      Yes      Yes      Q3
          NovaStor - NovaVault                Yes      Yes      Yes      Q195

        SCO
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCserve/Open 1.1           Q4       Q4       Q4      Q4
          NovaStor - NovaMarch                Yes      Yes      Yes      Q195

        DOS          
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada - Backup EXEC for DOS        Yes       No       No      Q4
          Cheyenne ARCsolo for DOS            Yes       No       No      Q4
          NovaStor - NovaBack for DOS         Yes      Yes      Yes      Q4

        Windows
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Arcada - Backup EXEC for Windows    Yes       No       No      Q4
          Cheyenne ARCsolo for Windows        Yes       No       No      Q4
          NovaStor - NovaBack for Windows     Yes      Yes      Yes      Q4
          
        OS/2
        +====================================================================+
          Product                           DLT2000  DLT2500  DLT2700  DLT4000
          +--------------------------------+--------+--------+--------+------+
          Cheyenne ARCsolo for OS/2            Q4       Q4       Q4      Q4
          NovaStor - NovaBack for OS/2        Yes      Yes      Yes      Q4


    Q: Can I attach a DLT2000 to a HP9000-700 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.


    Q: Can I attach a DLT2000 to an IBM RS/6000 AIX 3.2 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.

    
    Q: Can I attach a DLT2000 to a SUN SPARC (Solaris 2.3) ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.


    Q: Can I attach a DLT2000 to a SUN SPARCSTATION SunOS 4.1.3 ?

    A: Yes - Mail to the FAQ maintainer and request the appropriate 
        application notes.

    
    Q: I understand that there is a directory of data objects stored at the 
       start of the tape. If I issue a SCSI LOCATE command, does the drive 
       go back to BOT to access this directory and then position? What is 
       the worst case locate time?

    A: The directory is read into the drive's memory during the Load
       process and accessed from there (and updated as needed; the updated
       directory is written back on the unload.)  Typical worst case Locate
       time is about 100 seconds; there may be some cases that will be longer
       due to retries, but that is not normal.


    Q: Which SCSI commands will utilize the data object Directory and 
       provide the best file search performance?

    A: The SCSI LOCATE command will give the best possible performance,
       especially relative to a Space by filemarks/Space-blocks combination.
       If the application simply wants to get to a particular filemark
       (versus a data block), then the Locate command will give equivalent
       performance to a Space-filemarks command.  The ideal situation is to
       be able to specify the ultimate destination that the application is 
       trying to get to on the media with a single command, and the Locate 
       command is the most general and effective way to do that.


    Q: Will any SCSI Commands lose performance without any filemarks 
       on the tape?

    A: The answer is no.  In fact, without any filemarks on tape, a 
       Space-blocks command becomes almost equivalent to a Locate cmd 
       (except the one uses a relative count, and the other an absolute 
       address)


    Q: The IBM IDRC compression algorithm checks to make sure that it is not 
       expanding data.  What algorithm is used in the DLT and is it prone to 
       failure with precompressed data?  What does the compression algorithm do 
       to the error rate?
        
    A: An LZ type algorithm is used.  It does not "fail" per-se, but highly
       compressed data will expand somewhat. (IDRC compressed data usually
       gets compressed slightly more by DLT2000).  I have seen 5% expansion
       when "compressing" random data on the DLT2000.  The error rate is
       media based, so compression on/off does not affect it.  


    Q: I understand that the drive will stream with data blocks that are 
       at least 8KB in size. Does compression affect this? Does compression 
       affect data packing and efficient use of tape?

    A: Yes. But tape streaming is not the real issue; whether the tape drive 
       can keep up with the rate that the host can pump data to it or not is.
       With high compression ratios (above 3:1) the drive's data compressor
       (or the SCSI bus for that matter) may not be able to keep up with
       the data rate into the drive cache buffer to keep the drive streaming.
       In this scenario, as the cache gets full enough, the drive will be 
       started, but will be able to empty the cache faster than compressed
       data can be put into it.  Under these conditions, an 8K block size 
       may not be enough to ensure streaming performance at all compression 
       ratios.

       The drive will always do data packing, whenever appropriate, so
       compression does not affect that.  Compression naturally makes for
       more efficient use of the tape media.


    Q: Is logical space truely sequential? Does compression affect the logical
       space?

    A: The device handles "defects" in a way that is completely
       logically transparent.  The Locate command deals with logical
       blocks (whether BT flag is set or clear) so it skips over
       blocks and filemarks in a logical fashion.  The only time a
       defect is not transparent is when a hard read or write error
       occurs.  A hard read error does not normally prevent accessing
       data (if any) on the media beyond the defect.  

       Compression has no affect on the logical space and the host can still 
       logically position to any block on the media.


    Q: Is it reasonable to always use compression?  Are there Performance 
       issues?  How is data compression selected?

    A: In general, an applications will benefit from enabling data compression.
       The exception is when data has already been compressed
       with a Lempel-Ziv (or an even more powerful algorithm) compression
       method.  Such compressed data, or very random data (including
       encrypted data), will likely expand slightly (~5%).  If in doubt,
       and if there is a significant amount of such data, run some tests
       to see what sort of compression rates you actually get (use Log
       Sense Compression Status page to get compression ratios).

       If the data is compressing, throughput will increase roughly in
       direct proportion to the compression ratio from the native data
       rate, until the drive's maximum is reached.

       If a section of tape has compressed data, the front panel will indicate
       when the drive is reading compressed data.  The DLT drive will 
       automatically match to the way the data was written and decompress 
       whenever compressed data is requested. 

       Compression can be turned on/off at any time when writing.  There
       are basically 4 ways to do this: Mode Select Device Configuration
       Page, Mode Select Compression Control Page, Mode Select VU density
       code values, and the front panel (which overrides any SCSI selection).


    Q: In compressed mode, if 5 16kbyte blocks are written will logical block 
       address increment by 5? etc.  What about accessing a non-zero offset of
       the data stream?

       Whether the blocks are written in variable or fixed block
       mode, compressed or uncompressed, each block is a distinct logical
       block on the media.  The Read Position command will reflect this
       and the effect of compression is transparent.

       As for "seeking into a nonzero offset of the data stream" that
       is a filesystem function and at the SCSI level would have to be
       decomposed to some kind of Space/Locate command (possibly) and
       then issuing Read command(s).  The host would then ignore data 
       until the desired byte offset is reached, and start returning
       data from that point to the application.  SCSI sequential devices
       do not have the capability to return data starting with a byte offset.


    Q: How much space does a Write Filemarks command use?

    A: It could use 0 to 8KBytes.  A Write Filemarks (WrFMs) command of 
       0 will force any data in the cache onto tape.  If the last physical 
       block is only partially full it must still be "closed out" and 
       flushed.  This normally results in an insignificant capacity loss,
       unless WrFMs zero (alias a "flush") commands are issued very frequently.
       If this is done, it would also impact throughput performance
       because of the flush operation.  A WrFMs zero will *not* result in
       any logical block of itself (only what it flushes from previous
       write commands).  Again, it is equivalent to a "cache flush".

       A WrFms of 1 will cause the current physical block to be closed and
       flagged as containing 1 filemark, so this filemark takes up from
       zero to 8K bytes (however much unused space was left in the 8K
       physical block).  After writing 1 filemark, consecutive filemarks
       written right after that (i.e. sequential filemarks) will each
       use 1 physical block (8KB).


    Q: What happens after a powerfail or SCSI RESET during writing?  Can we 
       append to the tape?

    A: On a SCSI Bus RESET, all data that is in cache is flushed to the
       media.  This might result in a partial block on media, especially
       with very big blocks (ex: 10 MB).  A power failure will naturally
       clear the device (and cache) since there is no provision for 
       battery back-up within the DLT drive.

       Once a Write Filemarks with count=0 completes, the media is guaranteed
       to be synchronized with the cache (i.e. cache is flushed, all data
       and filemarks up to the flush are now on tape).  A bus Reset before
       the WrFMs zero completes will not kill the flush itself.  A power
       failure will.  The application can Space to EOD and check position
       to see if all data got flushed, but since the WrFMs zero did not
       complete (due to the powerfail) some data might be missing, and
       there might be a partial block.  However, the application can always
       go back to the position of the last *successful* flush.


    Q: For fixed block mode, what is the suggested block size?

    A: 16 KB or larger for good performance and compression rates.  Between
       8 KB and 16 KB performance should be okay.  But, below 8 KB device 
       throughput starts to fall off, so 8 KB or higher should be used
       if possible.  This applies to fixed and variable block modes.


    Q: What are the number of blocks that can fit on tape?

    A: The number of blocks you can fit on a particular cartridge will vary.
       Generally you can take the rated capacity and simply divide by the
       block size you plan to use.  If compression is used, the number of
       blocks can only be roughly estimated, unless the compression ratio
       with that particular data is uniform and well characterized.


    Q: What happens when you try to select the drive during powerup, or during 
       tape load/unload?

    A: After powerup, the drive will not respond for about 1 second to
       any Selection attempts, while the drive's Power-On Self Tests (POST)
       checkout the SCSI interface hardware.  After that, Selections will
       be responded to but any CDB will receive a BUSY Status, for the next
       10 or so seconds.  Then commands will be responded to normally,
       although the media might not be ready so media access commands will
       generally fail with a NotReady Sense Key.

       For load/unload, after an Unload is initiated, the media is not ready.
       After a Load command (Imm=0) completes the media will be at BOT and
       ready for media access commands.


    Q: How is media shelf life defined and specified?

    A: Shelf life is 10 years min. @ 20 degC and 40% RH.  After 500k passes 
       the tape should continue to function normally, unless it is being used
       in an evironemnt with heat, humidity or contamination levels beyond
       those recommended.


    Q: What is a "tape pass"?  When do you recommend retiring tapes?

    A: A pass is the media passing over the head once.  A full pass would
       mean going from one end of the tape to the other.

       At this point we cannot suggest a number of passes after which a 
       cartridge should be retired, because the media does not seem to
       wear out and actually improves with use.  We have tested the media
       up to 500,000 passes without degradation and use this number as
       our specification, even though a much larger number of passes could
       be attained.


    Q: How is the error rate calculated? How many retries are assumed?  Does 
       the error rate apply to virgin tape or to tape which has seen 500,000 
       passes?  Does it apply with compression on?
        
    A: Error rate is calculated based on rewrite, over-write, ECC's and
       re-reads.  The error rate for virgin vs. used tape is expected to 
       be about the same.  The media tends to improve with usage and the 
       media error rate applies to raw data on the media, so compression 
       is not a factor.



=========================================================================
Ralf-Peter Rohbeck                                      rrohbeck@qntm.com
Quantum GmbH-Application Engineering-Central Europe    (+49) 69-950767-18
Berner Str. 28, 60437 Frankfurt, Germany           fax (+49) 69-950767-91




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From: Ralf-Peter Rohbeck 
Newsgroups: comp.arch.storage
Subject: Re: DLT2000 on HP887
Date: 19 May 1995 19:06:54 GMT
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P.S. Have a look at
http://www.quantum.com/support/bulletins/tape.html

=========================================================================
Ralf-Peter Rohbeck                                      rrohbeck@qntm.com
Quantum GmbH-Application Engineering-Central Europe    (+49) 69-950767-18
Berner Str. 28, 60437 Frankfurt, Germany           fax (+49) 69-950767-91