Is xfs defrag *reliably* aligned to stripes?

Is xfs defrag *reliably* aligned to stripes?

[Spelic]

Hello xfs people,

does the xfs defrag (xfs_fsr, I think) reliably aligns files to stripes?

There is one scenario which requires this: if you make raw disk images 
for virtual machines, you want such files to be reliably aligned to 
stripes.

Even if a complete defrag for one file is not possible because there is 
no contiguous free space (you remain e.g. with 2 trunks for one of such 
files you still need to ensure that every trunk begins exactly at the 
beginning of a stripe and ends exactly at the end of a stripe.

This is the only way to be able to align the guest filesystems to the 
RAID, inside the virtual machine.
(obviously this also requires some work by the system administrator 
because he needs to ensure that partitions inside the guest also are 
offsetted by multiples of the stripe size, ok, but it's still anyway 
needed that xfs_fsr guarantees total alignment to the stripes like I 
described)

Does it do that? Will it do that in the future?

Thank you

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Re: Is xfs defrag *reliably* aligned to stripes?

[Dave Chinner]

On Mon, Nov 22, 2010 at 08:18:27PM +0100, Spelic wrote:
> Hello xfs people,
> 
> does the xfs defrag (xfs_fsr, I think) reliably aligns files to stripes?

No, the allocator does best effort attempts only.

> Does it do that? Will it do that in the future?

If the filesystem has large enough contiguous free space extents, it
will align the initial allocation to stripe unit, but once you age
the filesystem there is no guarantee this will be possible. use
"xfs_bmap -vvp" to look at the alignment of files.

Of course, this all only works if you mkfs your filesystem with
sunit/swidth or su/sw parameters....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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