Date: Tue, 6 Oct 2009 18:03:16 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Dieter <freebsd@sopwith.solgatos.com> Cc: freebsd-performance@FreeBSD.org Subject: Re: tuning FFS for large files Re: A specific example of a disk i/o problem Message-ID: <20091006174121.V25604@delplex.bde.org> In-Reply-To: <200910051755.RAA11047@sopwith.solgatos.com> References: <200910051755.RAA11047@sopwith.solgatos.com>
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On Mon, 5 Oct 2009, Dieter wrote: > I found a clue! The problem occurs with my big data partitions, > which are newfs-ed with options intended to improve things. > > Reading a large file from the normal ad4s5b partition only delays other > commands slightly, as expected. Reading a large file from the tuned > ad4s11 partition yields the delay of minutes for other i/o. > ... > Here is the newfs command used for creating large data partitions: > newfs -e 57984 -b 65536 -f 8192 -g 67108864 -h 16 -i 67108864 -U -o time $partition Any block size above the default (16K) tends to thrash and fragment buffer cache virtual memory. This is obviously a good pessimization with lots of small files, and apparently, as you have found, it is a good pessimization with a few large files too. I think severe fragmentation can easily take several seconds to recover from. The worst case for causing fragmentaion is probably a mixture of small and large files. Some users fear fs consistency bugs with block sizes >= 16K, but I've never seen them cause any bugs ecept performance ones. > Even this isn't tuned the way I wanted to. > -g * -h must be less than 4 G due to 32 bit problem (system panics). The panic is now avoided in some versions of FreeBSD (-8 and -current at least). > Note the 32 bit problem is in the filesystem code, I'm running amd64. > IIRC there is a PR about this. (I'm assuming the bug hasn't been fixed yet) > Result is that I must specify -g and -h smaller than they should be. I bet you can't see any difference (except the panic) from enlarging -g and -h. > And they have way more inodes than needed. (IIRC it doesn't actually > use -i 67108864) It has to have at least 1 inode per cg, and may as well have a full block of them, which gives a fairly large number of inodes especially if the block size is too large (64K), so the -i ratio is limited. Bruce
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