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

Michael Monnerie
2011-06-06T07:29:13Z

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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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