Date: Mon, 12 Nov 2001 13:23:52 -0500 From: Bill Moran <wmoran@potentialtech.com> To: Daniel Lang <dl@leo.org> Cc: freebsd-stable@FreeBSD.ORG Subject: Re: dirpref benefit on virtual disks Message-ID: <3BF013B8.8070009@potentialtech.com> References: <20011112152726.A10505@atrbg11.informatik.tu-muenchen.de>
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[since I've yet to see any other replies] Daniel Lang wrote: > Hi, > > as I understood, the new dirpref algorithm can improve > performance a lot, but only applies to new created directories. > To be able to use it, old existing directories would have to > be created new. That's consistent with my understanding of it. > 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. That being said, I don't _know_. It would be interesting to test it. Since you're already considering recreating directories, why not do a test to see if it's worth it? Just take a directory tree that's pretty complicated and that was created before you updated the dirpref code. Run a find operation of some sort that traverses the tree and time it. Then, back that tree up and recreate it, re-run the find and time it again and let everyone know if it's worth it. You only need do this with one section of a tree on the drive to determine how much difference it's going to make. > 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. -- Bill Moran Potential Technology http://www.potentialtech.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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