Date: Tue, 20 Nov 2001 11:52:56 +1030 From: Greg Lehey <grog@FreeBSD.org> To: Christopher Farley <chris@northernbrewer.com> Cc: questions@freebsd.org Subject: Re: Slow restores on a DLT4000 Message-ID: <20011120115256.H76318@monorchid.lemis.com> In-Reply-To: <20011119150711.A7781@northernbrewer.com> References: <20011119150711.A7781@northernbrewer.com>
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On Monday, 19 November 2001 at 15:07:13 -0600, Christopher Farley wrote: > I am a new owner of a pair of DLT4000 drives. I've been testing them > using dump and restore. > > I get fairly good throughput on dumps; about 1.6 megs/second, which > seems similar to 'average' expectations that have been published. > During a dump, the drive does not stream, but runs, stops, rewinds, > runs, stops, rewinds... My thinking is that the tape drive is able to > write to a tape faster than my platters can provide the bits. Hmm. I have one of these drives too, and I see data rates of up to 3 MB/s. Are you using compression? > Restoring data off a level 0 dump takes about twice as long as the dump > itself, and it also fails to stream. This is almost certainly due to the metadata updates. Soft updates will help, but basically the disk is becoming the bottleneck. > Also, it takes just as long to retrieve a few files off of a dump as > it does to restore an entire dump. Well, I suspect it seems that way. In fact, it should be able to stream up to where it finds the files, so it should be a little faster. > It seems like the drive simply reads the tape sequentially, and if > it sees a file I've marked, it writes it to the disk. Consequently, > if the file is at the end of the tape, it will take 4 hours to find > the file on a 20 Gig dump. Yes, that's correct. The obvious choice here is to make several smaller dumps and then search to the beginning of the dump, which is much faster (a minute or two maximum). > Is this (slow) behavior due to the way dump/restore works, or due to > the DLT4000? Is there any kind of drive/backup program that allows > for some kind of random access on the tape drive? By definition, tapes are serial access devices. The DLT4000 is not much difference from other ones. > (I used to have a 4mm DAT, and I seem to recall that restoring a > single file or two was very fast and efficient. It has been a while > since I've done it, though.) They're pretty much the same. There are two basic speeds: file skip, which is relatively fast, but can only skip to the beginning of a file, and transfer. If the drive doesn't stream, it can be *much* slower. The DLT4000 should stream for a few seconds at a time, then back up and start again. The real killers are where this streaming is only a fraction of a second. Greg -- When replying to this message, please copy the original recipients. If you don't, I may ignore the reply. For more information, see http://www.lemis.com/questions.html See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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