[dm-devel] Re: New kernel shared snapshots

[Jonathan Brassow]

Thanks for the recent repost (18-Aug-2009) of your patches at:
http://people.redhat.com/mpatocka/patches/kernel/new-snapshot/devel

The interface seems to be in-line with what we want.  I noticed that  
you can't have dm-snapshot and dm-multisnapshot loaded in the kernel  
at the same time (kmem_cache_create: duplicate cache  
dm_snap_tracked_chunk -- in the module init function).  You will also  
want to run your patches through 'linux/scripts/checkpatch.pl'.  It  
mostly gets angry about the 80 characters / line breakage, but there  
is some other things it catches.

  brassow

On Jul 27, 2009, at 6:24 AM, Mikulas Patocka wrote:

> Hi
>
> The new release of shared snapshots can be found here:
> http://people.redhat.com/mpatocka/patches/kernel/new-snapshots/devel/
>
> It has the interface we talked about on Linux Tag.
> So look at it as a last chance to review the interface. And also  
> test it.
>
>
> To create a snapshot, you have to:
> - send a message
> - ask for status, get the snapshot id
> - suspend (this creates the snapshot, when the filesystem is  
> quiescent)
> - resume
>
> Then, you can attach the snapshots by loading "multisnap-snap" target.
>
> If the table line contains argument "sync-snapshots", it  
> synchronizes the
> snapshot list against what is given on the table --- i.e. creates
> snapshots that are on the table but are not in the store and deletes  
> those
> that are not in the store but are on the table.
>
> Other changes:
> - make more structures private (move them to dm-multisnap- 
> private.h), so
>  that ABI doesn't change if these are chaged.
> - snapshot IDs are treated as strings, not numbers.
> - metadata cache size can be specified as an argument to the "mikulas"
>  exception store.
> - an argument "preserve-on-error" that says that on error or overflow,
>  writes to the origin should be disabled.
> 	- if you don't specify this, the origin continues operation, but
> 	snapshots are irreversibly damaged by this (use for non-important
> 	snapshots such as backups)
> 	- if you specify this, the data are always preserved, the whole
> 	thing just halts on error (user when snapshots contain some
> 	important data.
> - fixed two bad bugs in dm-bufio.
>
>
> Known bugs:
> - that read-vs-realloc race (that I already fixed in the old  
> snapshots a
> year ago) is present there. I'll fix it, but it isn't so trivial as in
> unshared snapshots.
>
>
> Mikulas