Re: [LSF/MM TOPIC] Lazy file reflink

Jan Kara
2019-01-28T12:50:45Z

Hi,

On Fri 25-01-19 16:27:52, Amir Goldstein wrote:
> I would like to discuss the concept of lazy file reflink.
> The use case is backup of a very large read-mostly file.
> Backup application would like to read consistent content from the
> file, "atomic read" sort of speak.
>
> With filesystem that supports reflink, that can be done by:
> - Create O_TMPFILE
> - Reflink origin to temp file
> - Backup from temp file
>
> However, since the origin file is very likely not to be modified,
> the reflink step, that may incur lots of metadata updates, is a waste.
> Instead, if filesystem could be notified that atomic content was
> requested (O_ATOMIC|O_RDONLY or O_CLONE|O_RDONLY),
> filesystem could defer reflink to an O_TMPFILE until origin file is
> open for write or actually modified.
>
> What I just described above is actually already implemented with
> Overlayfs snapshots [1], but for many applications overlayfs snapshots
> it is not a practical solution.
>
> I have based my assumption that reflink of a large file may incur
> lots of metadata updates on my limited knowledge of xfs reflink
> implementation, but perhaps it is not the case for other filesystems?
> (btrfs?) and perhaps the current metadata overhead on reflink of a large
> file is an implementation detail that could be optimized in the future?
>
> The point of the matter is that there is no API to make an explicit
> request for a "volatile reflink" that does not need to survive power
> failure and that limits the ability of filesytems to optimize this case.

Well, to me this seems like a relatively rare usecase (and performance
gain) for the complexity. Also the speed of reflink is fs dependent - e.g.
for btrfs it is rather cheap AFAIK.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR