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On Mon, 14 Aug 2006 18:36:44 +0200 Andrzej Szymanski <szymans@agh.edu.pl> wrote: > I've encountered a strange problem - if an application is sequentially > writing a large file on a busy machine, a single write() of 64KB may > take even 30 seconds. But if I do fsync() after each write() the maximum > time of write()+fsync() is about 0.5 second (the overall performance is, > of course, degraded). > > The point is, that some applications (samba+smbclient) time out after > 20s waiting for write(). > > Does anybody have an idea how to tune the kernel to avoid this strange > delay in write()? > > I've tried to experiment with cfq and deadline IO scheduler - without > success. Decreasing /proc/vm/dirty_ratio to 5% helps a little. > > If somebody want to test it, the tool I've written for measuring maximum > write() time is here: http://galaxy.agh.edu.pl/~szymans/writetimer > > 1. Compile writetimer.c > 2. Put a large background read from the disk > 3. Simultaneously write 10 files 200MB each (write() without fsync()) > for i in 1 2 3 4 5 6 7 8 9 0 ; do ./writetimer 200 > testfile$i & done > 4. and with fsync() after each write() > for i in 1 2 3 4 5 6 7 8 9 0 ; do ./writetimer -200 > testfile$i & done > (negative file size turns on fsync()) > > Tested on > - 2.6.15-23 (512MB RAM, Pentium-M 1.7, Ubuntu 6.06, ATA disk) > - 2.6.17-1.2145_FC5 (512MB RAM, Pentium-M 1.7, Fedora Core 5, ATA disk) > - 2.6.12-2.3.legacy_FC3smp (2GB RAM, Fedora Core 3, software RAID 5 on 4 > ATA disks) Which filesystem? If ext3 in ordered-data mode: any fsync() will sync the whole filesystem (it has to). Mounting with `-o writeback' should help. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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