Re: largeio mount and performance impact

[Eric Sandeen <sandeen@sandeen.net>]

Sebastian Brings wrote:
> In a different thread I read about the largeio mount option for XFS. Now
> I wonder if the problems I recently ran in have been caused by this.
> 
> After a system uprgade from sles9 sp2 to sp3 one app started
> misbehaving. Before the upgrade it used 15% CPU, after the upgrade it
> was 90+% and the performance dropped by about 50%. The app is writing a
> wave audio file, and for every 3840 bytes of audio samples it appends,
> it updates the RIFF header of the file. All of this was done using the
> buffered fopen/fwrite/... C library functions. 
> An strace showed that seeking to the beginning of the file also
> triggered a 12MiB read(2) call, and seeking to the end, for example to
> 13MiB, translated to a seek(2) to offset 1mib and a read(2) of 12 MiB.
> 
> Initiall I assumed something very strange had happened to the C lib
> defaults. Otoh 12MiB is the swidth of the filesystem, so I assume that
> the C lib got the 12 MiB optimal IO size from XFS and therefore behaved
> as described above. Could that be?

Unless you specify the largeio mount option, I don't -think- any of this is exposed.

What does stat -c %o <file> say?

Did this strace behavior change from sp2 to sp3?

-Eric