[linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

[linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

[Tomasz Chmielewski]

Vijai Babu Madhavan, Thu, 11 Jan 2007 11:18:13 -0700, wrote:

> The problem of DM snapshots with multiple snapshots have been discussed 
> in the lists quiet a bit (Most recently @ 
> https://www.redhat.com/archives/dm-devel/2006-October/msg00034.html).
> 
> We are currently in the process of building a DM snapshot target that scales 
> well with many snapshots (so that the changed blocks don't get copied to each 
> snapshot). In this process, I would also like to validate an assumption.
> 
> Today, when a single snapshot gets created, a new cow device of a given size 
> is also created. IMO, there are two problems with this approach:
> 
> a) It is difficult to predict the size of the cow device, which requires a prediction 
> of the number of writes would go into the origin volume during the snapshot 
> life cycle. It is difficult to get this prediction right, as very high value reduces 
> utilization and low value increases the chances of snapshot becoming full.
> 
> b) A new cow device needs to be created every time.

Hi,

Any news on that?

Still, with multiple snapshots write performance degrades linearly - is 
any work done to change that anytime soon?



-- 
Tomasz Chmielewski
http://wpkg.org

Re: [linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

[Dan Kegel]

On Jan 23, 2008 8:16 AM, Tomasz Chmielewski <mangoo@wpkg.org> wrote:
> Vijai Babu Madhavan, Thu, 11 Jan 2007 11:18:13 -0700, wrote:
>
> > The problem of DM snapshots with multiple snapshots have been discussed
> > in the lists quiet a bit (Most recently @
> > https://www.redhat.com/archives/dm-devel/2006-October/msg00034.html).
> >
> > We are currently in the process of building a DM snapshot target that scales
> > well with many snapshots (so that the changed blocks don't get copied to each
> > snapshot). In this process, I would also like to validate an assumption.
>
> Any news on that?
>
> Still, with multiple snapshots write performance degrades linearly - is
> any work done to change that anytime soon?

Yes.  Dan Phillips has implemented a shared snapshot exception store.
You can try it out now if you like; it's at http://zumastor.org.
It feels a bit different from the user's point of view than LVM, though.
It plays well with the device mapper and can be used with any
block device (LVM or non-LVM).
- Dan

Re: [linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

[Tomasz Chmielewski]

Dan Kegel schrieb:
> On Jan 23, 2008 8:16 AM, Tomasz Chmielewski <mangoo@wpkg.org> wrote:
>> Vijai Babu Madhavan, Thu, 11 Jan 2007 11:18:13 -0700, wrote:
>>
>>> The problem of DM snapshots with multiple snapshots have been discussed
>>> in the lists quiet a bit (Most recently @
>>> https://www.redhat.com/archives/dm-devel/2006-October/msg00034.html).
>>>
>>> We are currently in the process of building a DM snapshot target that scales
>>> well with many snapshots (so that the changed blocks don't get copied to each
>>> snapshot). In this process, I would also like to validate an assumption.
>> Any news on that?
>>
>> Still, with multiple snapshots write performance degrades linearly - is
>> any work done to change that anytime soon?
> 
> Yes.  Dan Phillips has implemented a shared snapshot exception store.
> You can try it out now if you like; it's at http://zumastor.org.
> It feels a bit different from the user's point of view than LVM, though.
> It plays well with the device mapper and can be used with any
> block device (LVM or non-LVM).

Does it use device mapper (or it just "plays well with the device 
mapper")? Or is it a totally different technology?

I didn't find it explained very clearly in Zumastor HOWTO on 
http://zumastor.googlecode.com/svn/trunk/doc/zumastor-howto.html (other 
than pointers to /dev/mapper/zumatest, which probably means it uses 
device mapper).



-- 
Tomasz Chmielewski
http://wpkg.org

Re: [linux-lvm] Re: [RFC] Multiple Snapshots - Manageability problem

[Dan Kegel]

Tomasz Chmielewski <mangoo@wpkg.org> wrote:
> Dan Kegel schrieb:
> > Yes.  Dan Phillips has implemented a shared snapshot exception store.
> > You can try it out now if you like; it's at http://zumastor.org.
> > It feels a bit different from the user's point of view than LVM, though.
> > It plays well with the device mapper and can be used with any
> > block device (LVM or non-LVM).
>
> Does it use device mapper (or it just "plays well with the device
> mapper")? Or is it a totally different technology?

It is a device mapper target, so yeah, it uses device mapper.

> I didn't find it explained very clearly in Zumastor HOWTO on
> http://zumastor.googlecode.com/svn/trunk/doc/zumastor-howto.html (other
> than pointers to /dev/mapper/zumatest, which probably means it uses
> device mapper).

A little further down the page, http://zumastor.org links to
http://zumastor.googlecode.com/svn/trunk/doc/desimplnotes.html
which does mention it explicitly.
- Dan