XFS write speed drop

[Jan Engelhardt]

Hello,


I have noticed that after an upgrade from 2.6.16-rcX -> 2.6.17-rc4, writes 
to one (hdc) xfs filesystem have become significantly slower (factor 6 to 
8), like if -o sync (or funky journal options on ext3) was on. Also, reads 
would stall until writes have completed. I would only expect such behavior 
when /proc/sys/vm/dirty_* are set to high values (like 95%, like I do on a 
notebook). hda remained fast.

It eventually turned out that it are the log barriers; hda does not support 
barriers and XFS gave a nice kernel message hint that pointed me to try out 
a mount flag. -o nobarrier makes it fast again.




Jan Engelhardt
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