Subject: DLT FAQ - dlt.faq [1/1] From: lkaplan@tdh.qntm.com (Larry Kaplan) +------------------------------------------------------------------------+ 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.  ----------------------------------------------------------------------------- Kevin Martinez kevinm@beavis.qntm.com #include Quantum Corp. Fry's Electronics: where Incompetence is a Standard -----------------------------------------------------------------------------