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

[Jens Axboe <jaxboe@xxxxxxxxxxxx>]

>> 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.

-- 
Jens Axboe

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