Re: I/O hang, possibly XFS, possibly general

[Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>]

On Samstag, 4. Juni 2011 Peter Grandi wrote:
>   vm/dirty_ratio=2
>   vm/dirty_bytes=400000000
> 
>   vm/dirty_background_ratio=60
>   vm/dirty_background_bytes=0
> 
>   vm/dirty_expire_centisecs=200
>   vm/dirty_writeback_centisecs=400

Why dirty_background_ratio=60? This would mean you start to write dirty 
pages only after it reaches 60% of total system memory... Setting it to 
=1 would be the thing you want I guess.
Also, setting both dirty_background_(ratio|bytes) is not supported. The 
latter wins, according to sysctl/vm.txt

Similarly, dirty_ratio and dirty_bytes belong together and exclude each 
other. Maybe you specified both to fit older and newer kernels in one 
example?

dirty_expire_centisecs to 200 means a sync every 2s, which might be good 
in this specific setup mentioned here, but not for a generic server. 
That would defeat XFS's in-memory grouping of blocks before writeout, 
and in case of many parallel (slow|ftp) uploads could lead to much more 
data fragmentation, or no?


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Michael Monnerie, Ing. BSc

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