Re: IOPS higher than expected on randwrite, direct=1 tests

[Jeff Moyer <jmoyer@xxxxxxxxxx>]

Jens Axboe <jaxboe@xxxxxxxxxxxx> writes:

>>> I wonder how many times I saw someone - including myself - fire up
>>> bonnie++ or iozone with a rather small test file compared to the full
>>> disk size ... Thanks very much everyone! This was an (overdue) eye
>>> opener.
>> In addition, testing with a file is dependent on the state of the file
>> system. For example, if you're using a 100GB file and there's not enough
>> contiguous free space to lay it out in one chunk you could have regions on
>> different parts of the disk. Further, even there is enough space, that space
>> could be near the inner or outer diameter which would also affect
>> performance.
>> 
>> There is a way to get a file address to lba map (which would give you some
>> insight into how a file is laid out). I think there's a syscall. Jens should
>> know.
>
> Yep, you can use FIBMAP/FIEMAP to walk the extents of the file and get a
> full map of how it's laid out on disk. Not sure if there are tools
> that'll do this for you, if not is pretty trivial to write.

man 8 filefrag; it's part of e2fsprogs.  I'm sure xfs has its own
utility to do this, too.

Cheers,
Jeff
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