Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics

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On Tue, Jan 24, 2012 at 01:05:50PM -0500, Jeff Moyer wrote:
> - buffered writes and buffered O_SYNC writes, all 1MB block size show 4k
>   I/Os passed down to the I/O scheduler
> - buffered 1MB reads are a little better, typically in the 128k-256k
>   range when they hit the I/O scheduler.
> ext4:
> - buffered writes: 512K I/Os show up at the elevator
> - buffered O_SYNC writes: data is again 512KB, journal writes are 4K
> - buffered 1MB reads get down to the scheduler in 128KB chunks
> xfs:
> - buffered writes: 1MB I/Os show up at the elevator
> - buffered O_SYNC writes: 1MB I/Os
> - buffered 1MB reads: 128KB chunks show up at the I/O scheduler
> So, ext4 is doing better than ext3, but still not perfect.  xfs is
> kicking ass for writes, but reads are still split up.

All three filesystems use the generic mpages code for reads, so they
all get the same (bad) I/O patterns.  Looks like we need to fix this up

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