09.10.2018 18:52, Chris Murphy пишет: > On Tue, Oct 9, 2018 at 8:48 AM, Gervais, Francois > <fgerv...@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.
