Re: Are nocow files snapshot-aware

[Kai Krakow <hurikhan77+btrfs@gmail.com>]

David Sterba <dsterba@suse.cz> schrieb:

> On Tue, Feb 04, 2014 at 08:22:05PM -0500, Josef Bacik wrote:
>> On 02/04/2014 03:52 PM, Kai Krakow wrote:
>> >Hi!
>> >
>> >I'm curious... The whole snapshot thing on btrfs is based on its COW
>> >design. But you can make individual files and directory contents nocow
>> >by applying the C attribute on it using chattr. This is usually
>> >recommended for database files and VM images. So far, so good...
>> >
>> >But what happens to such files when they are part of a snapshot? Do they
>> >become duplicated during the snapshot? Do they become unshared (as a
>> >whole) when written to? Or when the the parent snapshot becomes deleted?
>> >Or maybe the nocow attribute is just ignored after a snapshot was taken?
>> >
>> >After all they are nocow and thus would be handled in another way when
>> >snapshotted.
>> >
>> When snapshotted nocow files fallback to normal cow behaviour.
> 
> This may seem unclear to people not familiar with the actual
> implementation, and I had to think for a second about that sentence. The
> file will keep the NOCOW status, but any modified blocks will be newly
> allocated on the first write (in a COW manner), then the block location
> will not change anymore (unlike ordinary COW).

Ah okay, that makes it clear. So, actually, in the snapshot the file is 
still nocow - just for the exception that blocks being written to become 
unshared and relocated. This may introduce a lot of fragmentation but it 
won't become worse when rewriting the same blocks over and over again.

> HTH

Yes, it does. ;-)

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