Strange write starvation on 2.6.17 (and other) kernels

From: Andrzej Szymanski
Date: Mon Aug 14 2006 - 12:33:53 EST


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:

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)


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at