Performance impact of mkfs.xfs vs mkfs.xfs -f

[Eric Sandeen]

On 8/25/15 3:32 PM, Shrinand Javadekar wrote:
> Hi,
> 
> I have 23 disks formatted with XFS on a single server. The workload is
> Openstack Swift. See this email from a few months ago about the
> details:
> 
> http://oss.sgi.com/archives/xfs/2015-06/msg00108.html
> 
> I am observing some strange behavior and would like to get some
> feedback about why this is happening.
> 
> I formatted the disks with xfs (mkfs.xfs) and deployed Openstack Swift
> on it. Writing 100GB of data into Swift in batches of 20GB each gave
> us the following throughput:
> 
>  20 GB: 93MB/s
>  40 GB: 65MB/s
>  60 GB: 52MB/s
>  80 GB: 50MB/s
> 100 GB: 48MB/s
> 
> I then re-formatted the disks with mkfs.xfs -f and ran the experiment
> again. This time I got the following throughput:
> 
>  20 GB: 118MB/s
>  40 GB: 95MB/s
>  60 GB: 74MB/s
>  80 GB: 68MB/s
> 100 GB: 63MB/s
> 
> I've seen similar results twice.

How did you do the above twice, out of curiosity?  If it's the same set of disks,
the 3rd mkfs would require "-f" to overwrite the old format.

> Any ideas why this might be happening?

With the paucity of information you've provided, nope!

What version of xfsprogs are you using?
What was the output of mkfs.xfs each time; did the geometry differ?

-f sets force_overwrite, which only does 3 things:

1) overwrite existing filesystem signatures
3) zeros out old xfs structures on disk
2) allow mkfs to proceed on a misaligned device
 
I don't see why any of those behaviors would change runtime behavior.

Maybe you have other variables in your performance testing, and two
tests isn't enough to sort out noise?

-Eric

> Thanks in advance.
> -Shri
> 
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