Date: Thu, 29 Jun 2006 20:10:49 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Ivan Voras <ivoras@fer.hr> Cc: freebsd-fs@freebsd.org Subject: Re: Is the fsync() fake on FreeBSD6.1? Message-ID: <20060629195739.L77878@delplex.bde.org> In-Reply-To: <44A2E0FD.6060302@fer.hr> References: <a0cd7c070606270032h3a42de6ahf21cd11abedb6400@mail.gmail.com> <44A1B958.4030204@fer.hr> <20060628230439.M75051@delplex.bde.org> <44A2E0FD.6060302@fer.hr>
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On Wed, 28 Jun 2006, Ivan Voras wrote: > Bruce Evans wrote: > >>> But I see strange results with iostat. It shows 16KB transactions, ~2900 >>> tps and 46 MB/s. On the other hand, the program runs for ~36 seconds, >>> which gives ~1390 tps (this is a single desktop drive). Since 36 seconds >>> of 46MB/s would result in a file 1.6 GB in size, while it's clearly >>> 50000*512=25MB, iostat is lying. >> >> This is because you fsync() every 512 bytes. The file system then writes >> a 16K inode block and a 16K data block, giving 64 times as much i/o as >> necessary. > > Ok, so you're saying that it actually does 46MB/s, rewriting 16K FS blocks > over and over? Yes. It's actually surprising that the speed is only 46MB/s if the drive caches the write. > In that case, wouldn't all writes to the FS, especially with soft-updates be > minimally 16K+16K? It doesn't look like it when I monitor a live server - > there are 8KB and 4KB writes... maybe UFS fragments complicate the > (ac)counting. Yes, the minimum i/o size is the fragment size, and the average file size is still probably smaller than 16K. However, for sequential writes to large files, most writes should be 64K+0K (DFLTPHYS+0K) or 128K+0K (MAXPHYS+0K) depending on how broken MAXPHYS vs MINPHYS is. Clustering combines 16K-blocks into either 64K or 128K-blocks, and at least without soft updates and without sync mounting or fsync(), inode updates are normally delayed longer than data updates. Bruce
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