[Lustre-discuss] disk fragmented I/Os

[Brian J. Murrell]

On Fri, 2010-04-02 at 17:45 +0800, Lu Wang wrote: 
> Hi, 
> 	We set up a test file system with same patrition and same hardware. When the system is empty, the disk I/O is less fragemented.

So, I think you now have confirmation as to what's causing your disk I/O
fragmentation problem on your production system, yes?

> However, the disk I/O in flight is still low (mostly at "1"). Is there any way to increase this value through configuration? 

I think you are chasing a red herring.  The number of disk I/Os in
flight is only an indicator as to what could be wrong when other things
are not working correctly.

But as you can see from your brw_stats, the only items that are not
absolutely *perfect* are that 2% of your disk I/Os were fragmented:

> read      |     write
> disk fragmented I/Os   ios   % cum % |  ios   % cum %
> 1:                       0   0   0   | 2289  97  97
> 2:                       0   0   0   |   69   2 100

and 4% of your disk I/Os were not a full 1M.

> read      |     write
> disk I/O size          ios   % cum % |  ios   % cum %
> 8K:                      0   0   0   |    1   0   0
> 16K:                     0   0   0   |    1   0   0
> 32K:                     0   0   0   |    0   0   0
> 64K:                     0   0   0   |    1   0   0
> 128K:                    0   0   0   |    4   0   0
> 256K:                    0   0   0   |   12   0   0
> 512K:                    0   0   0   |   58   2   3
> 1M:                      0   0   0   | 2350  96 100

I would think those two small deviations from perfect would be within
the realm of acceptable, yes?  If you agree then you need to stop
chasing the disk I/Os in flight.  One likely explanation is simply that
the disk(s) is(are) able to drain the I/Os in flight as fast as the
OST(s) is(are) able to push them -- which is good!

Cheers,
b.

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