Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!sobmips!roe From: roe@sobeco.com (r.peterson) Newsgroups: comp.sys.mips Subject: Re: Experience with DAT drives? Message-ID: <1990Mar28.053504.14178@sobeco.com> Date: 28 Mar 90 05:35:04 GMT References: <766@cluster.cs.su.oz> Distribution: comp Organization: Sobeco Group - Montreal, Canada Lines: 107 From article <766@cluster.cs.su.oz>, by yar@cs.su.oz (Ray Loyzaga): > In article <1990Mar11.160020.27837@sobeco.com> roe@sobeco.com (r.peterson) writes: >> >> However, both the transfer rate and storage capacity are about half the >> exabyte. > I have used a JVC DAT called a Gigatape, it plugs straight into the > SCSI port and works well on the M/120, there seems to be a firmware > mismatch once it is on the system that makes the M120 fail the cold > boot diagnostics, this doesn't stop the device from being usable, but > should be fixed by mips and the tape manufacturer ... (This is interesting to start with, since Gigatape is already a tradename for OEM exabyte drives...) Hmmm... I've had no problems whatever with the WANGDAT drive - passes all boot tests, and can be used to both boot and install the operating system (both miniroot and the full distribution). > The nice thing about this drive is that it has a display that gives > constant info about what is going on, seeking, writing etc, with > a number that increments as the tape is used, so you can get perfectly > accurate numbers about how much of the tape has been written. Ahah! A blinkenlights man! After my own heart, but: Why is this information useful? The same thing is available on several OEM versions of the exabyte drive, but I find it meaningless. Either a write works or not. What do you plan to do? Hire someone to watch the front panel of the tape drive and hammer the control-c if the drive is nearly full? Furthermore, (at least on the exabyte), all such statistics are available with SCSI inquiry commands. Much better to grab error stats and log them than depend on users noticing the front panel display. [ What follows is an unabashed plug for exabyte drives. ] [ I do not work for exabyte, but [standard disclaimer ]]. > The speed of writing was slower than the Exabyte, but certainly not > by half, maybe 80% as fast. True, given that you are using the standard mips scsi device driver. The exabyte drive transfer speed is a real big win in buffered mode; simply modify the (mips-supplied) device driver to set the buffered mode bit, and you will see *real* drive transfer rates. We get between 242KBS and 244KBS (the drive itself is rated at 246). DAT just simply, physically can't read/write faster than about 180KBS. (Specs say 176 KBS for the WANGDAT). > the real win is that the tape can do end to end > seeks in 10-20 seconds, whereas the Exabyte is absolutely painful > with each command. Why is this a real win? How many backup/restore utilities take advantage of seek time? Who cares how fast a drive rewinds? Also, I should point out that the DAT drives can *not* perform block seeks from end to end in 20 seconds - they can search file marks this fast. We've set up our system to use exabytes in 100MB savesets; worst- case restore time (the last file on the last saveset) is about 18 minutes (forward space 19 files, read the balance, and grab the last file). > The Gigatype, being fully digital, will be more > reliable and it does not lose sync due to a tape error. This is incorrect. The error correction on an exabyte drive provides extensive ECC, with read-after-write correction (with no loss whatever in terms of write speed). Simply being "fully digital" has nothing whatever to do with reliability. Also, any tape drive (digital or not) can "lose sync" if the tape is garbled enough (magnetic field, physical damage, etc). The exabyte simply does not "lose sync" without major tape damage. > If there is a media error early in the tape, the drive will still > be able to find any later files without trouble. This is great for > multi-machine backups and restores. Well, I've a problem with this statement too. Having used exabytes extensively for the last 2+ years, without *one* read error, your point simply isn't. Also, an exabyte drive is probably your best bet for recovery of bad tape spots. Check out the extensive error detection/recovery that is automatically encoded by the drive. Furthermore, in worst case, most dump/restore systems will simply choke and die when a bad block is encountered. > In my opinion, the DAT style systems are much better to use for > multi-file tape use, most of this comes from the speed of positioning. Again, my question remains - what dump/restore utility uses seek? What tape device driver (ATT 3B2 aside) supports it? Also, if an exabyte can forward space the whole tape in less than 15 minutes, who cares? -- "Norf gribble mitz der bork bork bork?" Roe Peterson "Lick spittle dwing dtz fritz slppp!" roe@sobeco.com - Uncle Duke uunet!sobeco!roe