Date: Mon, 12 Nov 2001 19:52:55 -0000 From: Matthew Whelan <muttley@gotadsl.co.uk> To: Bill Moran <wmoran@potentialtech.com>, Daniel Lang <dl@leo.org> Cc: freebsd-stable@FreeBSD.ORG Subject: Re: dirpref benefit on virtual disks Message-ID: <20011112195257.53C2137B416@hub.freebsd.org>
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12/11/2001 18:23:52, Bill Moran <wmoran@potentialtech.com> wrote: >> Now I have some huge filesystems on RAID partitions. To recreate >> all their directories involves some hassle, but I would think >> about doing it. But since these are no real but virtual disks, >> spread over a set of disks in a hardware raidbox, I'm not sure, >> if I would even benefit from the better algorithm. It sounded >> a bit like designed for filesystems on a (single?) disk? > >My thought would be that, yes, it will improve performance. If >you've got a typical 3 disk, RAID-5, a read of the directory still >causes a seek on all three disks, and if there's continual seeking >across the three disks, you'll see performance degredation. If >dirpref can layout the directory info so that it's close together, >seek time is reduces, whether on 3 disks or one. There were some benchmarks posted to this list, which confirmed this. A speedup, and a very healthy one, but maybe not -quite- as much as on a single disk, ISTR maybe a 6x improvement on rm -rf /usr/ports where you'd get 10x on a single disk. Check the archives if you want better figures than my dim memory can provide. >> Also I would like to know, if there is a certain limit of free >> space, on the disk, so that the algorithm can actually use >> the better layout? The disks have some space left, in an >> absolute way, but not that much from a relative point of view >> (like 12GB left which is just 6% minfree not taken into account). > >Have a read of the original paper on FFS (which is in /usr/share/doc >if you installed docs with FreeBSD) there is an explanation of why >8% is reserved on the drive - man tunefs comments about this as well. >I would assume that the new dirpref code needs plenty of free space >to use the optimal layout policy, but I don't know if the minimal >free space is still 8% or not, and I don't know if anyone has even >tested the new dirpref code to see if that number has changed. I'd imagine a lot would depend on how fragmented your free space is, in particular, avg. fragment size vs. avg. directory size (as configured in sysctl). People have tried tar -c/rm -rf/tar -x cycles, with wildly varying improvements - from nearly no difference to nearly as good as newfs. Matthew To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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