bcachefs - snapshots

bcachefs - snapshots

[Kent Overstreet]

Snapshots have been merged! 9 months of work and 3k lines of new code, finally
released. Some highlights:

 - btrfs style subvolumes & snapshots interface
 - snapshots are writeable
 - highly scalable: number of snapshots is limited only by your disk space
 - highly space efficient: no internal fragmentation issues

Design doc here: https://bcachefs.org/Snapshots/

The core functionality is complete - snapshot creation and deletion works, fsck
changes are done (most of the complexity was in making fsck work without
O(number of snapshots) performance - tricky). Everything else is a todo item:

 - still need to export different st_dev for files in different subvolumes
   (we'll never allocate a new inode with an inode number that collides with an
   inode inother subvolume - but snapshots will naturally result in colliding
   inode numbers)

 - need to hide dirents that point to snapshots when inside snapshots...

 - snapshot creation is not atomic w.r.t. page cache, we do sync_fs() but don't
   block buffered writes

 - other niggling page cache stuff - need to walk page cache and mark blocks as
   no longer reserved on snapshot creation

 - we no longer have quota support, since old style quotas interact badly with
   snapshots

 - we need per subvolume disk space accounting before i can implement btrfs
   style subvolume quotas

 - all the things I neglected to think of yet, and all the bugs I haven't found
   yet

Go wild, please try and break it.

Re: bcachefs - snapshots

[Bernd Schubert]

On 9/27/21 3:49 AM, Kent Overstreet wrote:
> Snapshots have been merged! 9 months of work and 3k lines of new code, finally
> released. Some highlights:
> 
>  - btrfs style subvolumes & snapshots interface
>  - snapshots are writeable
>  - highly scalable: number of snapshots is limited only by your disk space
>  - highly space efficient: no internal fragmentation issues
> 
> Design doc here: https://bcachefs.org/Snapshots/
> 
> The core functionality is complete - snapshot creation and deletion works, fsck
> changes are done (most of the complexity was in making fsck work without
> O(number of snapshots) performance - tricky). Everything else is a todo item:
> 
>  - still need to export different st_dev for files in different subvolumes
>    (we'll never allocate a new inode with an inode number that collides with an
>    inode inother subvolume - but snapshots will naturally result in colliding
>    inode numbers)

With my limited high level view on it - shouldn't you discuss with Neil
about a solution and to avoid going the btrfs route for colliding inode
numbers?

RE: bcachefs - snapshots

[Frank Filz]

> On 9/27/21 3:49 AM, Kent Overstreet wrote:
> > Snapshots have been merged! 9 months of work and 3k lines of new code,
> > finally released. Some highlights:
> >
> >  - btrfs style subvolumes & snapshots interface
> >  - snapshots are writeable
> >  - highly scalable: number of snapshots is limited only by your disk
> > space
> >  - highly space efficient: no internal fragmentation issues
> >
> > Design doc here: https://bcachefs.org/Snapshots/
> >
> > The core functionality is complete - snapshot creation and deletion
> > works, fsck changes are done (most of the complexity was in making
> > fsck work without O(number of snapshots) performance - tricky). Everything
> else is a todo item:
> >
> >  - still need to export different st_dev for files in different subvolumes
> >    (we'll never allocate a new inode with an inode number that collides with an
> >    inode inother subvolume - but snapshots will naturally result in colliding
> >    inode numbers)
> 
> With my limited high level view on it - shouldn't you discuss with Neil about a
> solution and to avoid going the btrfs route for colliding inode numbers?

I was going to ask that also having been watching the btrfs subvolume saga. As maintainer of the Ganesha user space NFS server I have an interest in this also though we haven't had anyone talk about bcachefs yet.

Frank