Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!munnari.oz.au!metro!cluster!yar From: yar@cs.su.oz (Ray Loyzaga) Newsgroups: comp.sys.mips Subject: Re: Experience with DAT drives? Message-ID: <798@cluster.cs.su.oz> Date: 30 Mar 90 01:59:17 GMT References: <766@cluster.cs.su.oz> <1990Mar28.053504.14178@sobeco.com> Sender: news@cluster.cs.su.oz Reply-To: yar@cluster.cs.su.oz (Ray Loyzaga) Distribution: comp Organization: Basser Dept of Computer Science, University of Sydney, Australia Lines: 85 In article <1990Mar28.053504.14178@sobeco.com> roe@sobeco.com (r.peterson) writes: > 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 drive I tested made the SCS master test fail. > 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. We have type 8mm video tape backup devices, neither supports anything other than a red and a green light, the green light turns on when the tape is correctly inserted, the red light tells you there is power connected. I find the Gigatape display more useful because I like knowing what the tape is doing, like is someone writing it? is it rewinding, has it stopped, specially since the drive locks up sometimes requiring a system reboot. > 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? Very funny, I was trying to show that this info is very useful because it allows you to get the numbers right for systems dumps, you know exactly how much can be written, we got these numbers from the exabyte by using lots of dd's with different block sizes and waiting till the tape runs out, then backing off 10%. I find it useful to have information available that can tell you what is happening at a glance, the "Exabyte" style drive we have does not give enough information and the Gigatape gives more than enough, I know which situation I prefer. > 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. > I prefer instant feedback, we differ, I also run my terminal with echo turned on. > > 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. I care, I happen to search through tapes made up of lots of files and I much prefer a system that allows me to stick the tape in step many files to the end and take a look at the data contained in a matter of seconds rather than the 8mm video system we have that takes an inordinate amount of time to load, retension, seek. I tried using it for an archival system that would search through the tape and it took so long that I gave up, it was unusable, because the file mark search time was so slow. > 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). Actually you are saying it takes 18 minutes to get to the last tape file so that you can search it to see if it is on that tape, if it isn't you need to load another tape which will take several minutes to rewind and unload, then you have to load a new tape. We cannot afford to keep a table of contents of the dumps on line, so we have to make some guesses based on the last change time of the file to be restored. > 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. I have lost big with these tapes before, many times, all info on later tape files was also lost. The DAT tape works for seeks to later files I know, I scratched the early part of a tape to test it.