On 10/31/2012 08:44 AM, David Sterba wrote:
> On Wed, Oct 31, 2012 at 08:34:38AM +0800, Liu Bo wrote:
>> Besides 'btrfs fi defrag', mounting with autodefrag may also do the same thing.
>
> Ok, autodefrag, good point. Then I suggest to make the snapshot-aware a
> mode of autodefrag, not a separate option (because it would make no
> sense other than an alias for "autodefrag=snapshotaware")
>
Hmm, you might be right.
But I have to say 'snapshot-aware defrag' is kind of trade-off.
1. The good case:
Say a file is full of fragments and make a snapshot based on file's root
fs root, snapshot
/ | \
/ | \
| - - | ... | - - - | ... | - - - |
p1 p2 p3
then we do a snapshot-aware defrag, it will be
fs root snapshot
\ /
\ /
| - - - - - - - - |
a whole new extent
We achieve the goal!
2. The bad case:
Say we have a file with an whole extent and a snapshot on it at the very first:
fs root snapshot
\ /
\ /
| - - - - - - - - |
then, we write into part of the file, with COW it will be:
| - - || - - - || - - - | ... | - - - |
p1 p2 p3 p2_new
(file in snapshot -> p1 + p2 + p3)
(file in fs root -> p1 + p2_new + p3)
then, we do a snapshot-aware defrag, it will be
| - - || - - - || - - - | ... | - - - |
p1 p2 p3 p2_new
||
VV
| - - | | - - - | ... ... | - - | - - - | - - - |
p1 p3 new extent
(file in snapshot -> p1 + p3 + middle of new extent)
(file in fs root -> new extent)
So we're making file in snapshot worse than before, although we get a good one for file in fs root.
thanks,
liubo
>
> david
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