On Mon, Jan 28, 2019 at 01:50:44PM +0100, Jan Kara wrote: > 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. That makes me want to run screaming for the hills. > > 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? Comparitively speaking: compared to copying a large file, reflink is cheap on any filesystem that implements it. Sure, reflinking on XFS is CPU limited, IIRC, to ~10-20,000 extents per second per reflink op per AG, but it's still faster than copying 10-20,000 extents per second per copy op on all but the very fastest, unloaded nvme SSDs... > > (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. I suspect for "very large read-mostly file" it's still an expensive operation on btrfs. Really, though, for this use case it's make more sense to have "per file freeze" semantics. i.e. if you want a consistent backup image on snapshot capable storage, the process is usually "freeze filesystem, snapshot fs, unfreeze fs, do backup from snapshot, remove snapshot". We can already transparently block incoming writes/modifications on files via the freeze mechanism, so why not just extend that to per-file granularity so writes to the "very large read-mostly file" block while it's being backed up.... Indeed, this would probably only require a simple extension to FIFREEZE/FITHAW - the parameter is currently ignored, but as defined by XFS it was a "freeze level". Set this to 0xffffffff and then it freezes just the fd passed in, not the whole filesystem. Alternatively, FI_FREEZE_FILE/FI_THAW_FILE is simple to define... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
