Re: CoW behavior when writing same content

[Andrei Borzenkov <arvidjaar@gmail.com>]

09.10.2018 18:52, Chris Murphy пишет:
> On Tue, Oct 9, 2018 at 8:48 AM, Gervais, Francois
> <FGervais@distech-controls.com> wrote:
>> Hi,
>>
>> If I have a snapshot where I overwrite a big file but which only a
>> small portion of it is different, will the whole file be rewritten in
>> the snapshot? Or only the different part of the file?
> 

If you overwrite the whole file, the whole file will be overwritten.

> Depends on how the application modifies files. Many applications write
> out a whole new file with a pseudorandom filename, fsync, then rename.
> 
>>
>> Something like:
>>
>> $ dd if=/dev/urandom of=/big_file bs=1M count=1024
>> $ cp /big_file root/
>> $ btrfs sub snap root snapshot
>> $ cp /big_file snapshot/
>>

And which portion of these three files is different? They must be
identical. Not that it really matters, but that does not match your
question.

>> In this case is root/big_file and snapshot/big_file still share the same data?
> 
> You'll be left with three files. /big_file and root/big_file will
> share extents,

How comes they share extents? This requires --reflink, is it default now?

> and snapshot/big_file will have its own extents. You'd
> need to copy with --reflink for snapshot/big_file to have shared
> extents with /big_file - or deduplicate.
> 
This still overwrites the whole file in the sense original file content
of "snapshot/big_file" is lost. That new content happens to be identical
and that new content will probably be reflinked does not change the fact
that original file is gone.