Re: One random read streaming is fast (~1200MB/s), but two or more are slower (~750MB/s)?

[Josef Bacik <jbacik@fusionio.com>]

On Wed, Apr 17, 2013 at 07:50:09PM -0600, Matt Pursley wrote:
> Hey All,
> 
> Here are the results of making and reading back a 13GB file on
> "mdraid6 + ext4",  "mdraid6 + btrfs", and "btrfsraid6 + btrfs".
> 
> Seems to show that:
> 1) "mdraid6 + ext4" can do ~1100 MB/s for these sequential reads with
> either one or two files at once.
> 2) "btrfsraid6 + btrfs" can do ~1100 MB/s for sequential reads with
> one file at a time, but only ~750 MB/s with two (or more).
> 3) "mdraid6 + btrfs" can only do ~750 MB/s for these sequential reads
> with either one or two files at once.
> 
> 
> So, seems like the speed drop is related more to the btrfs files
> system, then the experimental raid.
> Although it is interesting that btrfs can only do the full ~1100 MB/s
> with a single file on the btrfsraid6, but not mdraid6.
> 
> 
> Anyway, just some more info and reproducible results.
> 
> I have also opened a ticket in bugzilla.kernel.org for this issue here...
> https://bugzilla.kernel.org/show_bug.cgi?id=56771
>

This is strange, and I can't see any reason why this would happen.  I'll try and
reproduce next week when I'm back from LSF.  Thanks,

Josef