discussion of the patch
The next3 filesystem patch, which added snapshots to the ext3 filesystem,
appeared just over one year ago; LWN's
at the time concluded
that it needed to move forward to ext4 before it could possibly be merged.
That change has been made, and recent ext4
patches are starting to look close to being ready for merging
into the mainline. That has inspired the airing of new concerns which may
slow the process somewhat.
One complaint came from Josef Bacik:
I probably should have brought this up before, but why put all this
effort into shoehorning in such a big an invasive feature to ext4
when btrfs does this all already? Why not put your efforts into
helping btrfs become stable and ready and then use that, instead of
having to come up with a bunch of hacks to get around the myriad of
weird feature combinations you can get with ext4?
Snapshot developer Amir Goldstein's response is that his employer (CTERA Networks)
wanted the feature in ext4. The feature is shipping in products now, and
btrfs is still not seen as stable enough to use in that environment.
There are general concerns about merging another big feature into a
filesystem which is supposed to be stable and ready for production use.
Nobody wants to see the addition of serious bugs to ext4 at this time.
Beyond that, the snapshot feature does not currently work with all variants
of the ext4 on-disk format. There are a number of ext4 features which do
not currently play well together, leading Eric Sandeen to worry about where the filesystem is going:
If ext4 matches the lifespan of ext3, in 10 years I fear that it
will look more like a collection of various individuals' pet
projects, rather than any kind of well-designed, cohesive project.
How long can we really keep adding features which are semi- or
wholly- incompatible with other features?
Consider this a cry in the wilderness for less rushed feature
introduction, and a more holistic approach to ext4 design...
Ext4 maintainer Ted Ts'o has responded with
a rare (for the kernel community) admission that technical concerns are not
the sole driver of feature-merging decisions:
It's something I do worry about; and I do share your concern. At
the same time, the reality is that we are a little like the Old
Dutch Masters, who had take into account the preference of their
patrons (i.e., in our case, those who pay our paychecks :-).
In this case, he thinks that there are a lot of people who are interested
in the snapshot feature. He worried that
companies like CTERA could move away from ext4 if it can't be made to meet
their needs. So his plan is to merge snapshots once (1) the patches are good
enough and (2) it looks like there is a plan to address the remaining issues.
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