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@xxxxxxxx> SUSE Labs, CR
