Open Stack

Hi clayg, thanks your suggestion, it is very helpful for me.
We also can store the refcount metadata in mysql or nosql,  but I think it
is suitable for island, because island is just a plugin...

I run island in devstack, it look like work fine :)

2012/10/25 Clay Gerrard <clay.gerrard at gmail.com>

> On Wed, Oct 24, 2012 at 1:17 PM, Rongze Zhu <zrzhit at gmail.com> wrote:
> > Hi stackers,
> > Island is a plugin for cinder, it will allow to create volume in local
> > storage and back up incremental snapshots of your data to Swift for
> durable
> > recovery. Island design idea is local storage + qcow2 image + dependent
> > snapshot + swift.
> >
>
> So cool!
>
> So I wanted to confirm something I wasn't clear on during the
> discussion at the summit regarding the refcount metadata, so nice to
> be able to just look at the code :D
>
> In volume.island.snapshot._unlink_snapshot_block you guys use
> "get_object_meta" to pull in the "X-refcount" attribute, decrement it,
> and then set the new value with "objectpool.set_object_meta"
>
> This probably works fine with the ext IMPL, but when the swift IMPL
> gets out there it'll be sorta dangerous.  The object metadata (as with
> all objects in swift) is "eventually" consistent - if you update
> existing objects subsequent reads can't be guaranteed to have the
> latest X-refcount meta (the X-newest header helps, and may even be
> good enough short term, but that's really only the newest "available",
> in a storage node failure condition you may still be reading old
> data).  The story get's a lot simpler if you  can design something
> that only PUT's to *create new* objects (say for each snapshot) then
> if a GETsucceeds you know you have the "newest" copy cause theirs only
> one.
>
> Just something to think about.  I'm definitely going to play around
> with this.  Looks like it should work fine on devstack.
>
> This is awesome, great work!
>
> -clayg
>
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-- 

Rongze Zhu - 朱荣泽
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