[linux-lvm] Snapshots

[linux-lvm] Snapshots

[Jon Hardcastle]

Hi,

Do I need some free extents to create a READ ONLY snapshot? I dont have free extents but require a read-only snapshot before doing an upgrade.

Cheers.


-----------------------
N: Jon Hardcastle
E: Jon@eHardcastle.com
'Do not worry about tomorrow, for tomorrow will bring worries of its own.'
-----------------------


      

[linux-lvm] Re: Snapshots

[Mike Snitzer]

On Thu, Sep 17 2009 at 10:24am -0400,
Jon Hardcastle <jd_hardcastle@yahoo.com> wrote:

> Hi,
> 
> Do I need some free extents to create a READ ONLY snapshot? I dont
> have free extents but require a read-only snapshot before doing an
> upgrade.

Yes, even though you intend to only read from the snapshot the upgrade
will cause data to be copied out from the origin to the snapshot.  So
you definitely need some free extents.

Mike

Re: [linux-lvm] Snapshots

[Peter Keller]

Dear John,

On Thu, 17 Sep 2009, Jon Hardcastle wrote:

> Hi,
>
> Do I need some free extents to create a READ ONLY snapshot? I dont have 
> free extents but require a read-only snapshot before doing an upgrade.

Extents are written to the snapshot when changes take place on the volume 
that the snapshot is based on. If you have no free extents, trying to create 
a snapshot strikes me as pointless: you may as well mount the origin volume 
read-only and back up its contents directly (or unmount it and back it up at 
the level of the volume). Or do you have some special circumstances?

If you make the snapshot read-only, as soon as changes occur on the origin 
volume the snapshot will become useless: some of the data that are required 
to make the snapshot usable (i.e. unchanged copies of altered blocks on the 
origin volume)  will no longer exist.

Regards,
Peter.


-- 
Peter Keller                                     Tel.: +44 (0)1223 353033
Global Phasing Ltd.,                             Fax.: +44 (0)1223 366889
Sheraton House,
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Cambridge CB3 0AX
United Kingdom

[linux-lvm] Snapshots

[jonr]

Hello List,

I don't get it!

I have a xen system that has a DomU of FC11 with a 20GB LV for the  
disk. I created a snapshot using this command:

lvcreate -L +10G -s -n s_cgate-be1 /dev/xenvg/cgate-be1

This is the output of an 'lvdisplay' of the snapshot LV:

lvdisplay /dev/xenvg/s_cgate-be1
   --- Logical volume ---
   LV Name                /dev/xenvg/s_cgate-be1
   VG Name                xenvg
   LV UUID                7c1kEs-xYoO-Qxem-2E2M-hlee-pc4Q-zm6TZ5
   LV Write Access        read/write
   LV snapshot status     active destination for /dev/xenvg/cgate-be1
   LV Status              available
   # open                 2
   LV Size                20.00 GB
   Current LE             5120
   COW-table size         10.00 GB
   COW-table LE           2560
   Allocated to snapshot  0.11%
   Snapshot chunk size    4.00 KB
   Segments               1
   Allocation             inherit
   Read ahead sectors     auto
   - currently set to     256
   Block device           253:15

I then shutdown the DomU and pointed the 'Disk...' line in the config  
file for the DomU to the snapshot. It boots.

OK, I must be missing the simple answer because I cannot seem to grasp  
the concept of snapshots.


1. How can I have a 20GB LV as a disk and the snapshot be 10GB and  
boot the entire OS?

2. Can I create a DomU and then snapshot the LV and use the snapshot  
to create other DomU's?

3. If 2 is yes, would I want to continue using the snapshot as the  
disk or is there something else that should be done, i.e. dd the drive  
to a new LV?

In the 'lvdisplay' of the snapshot I have these extra lines:
   LV snapshot status     active destination for /dev/xenvg/cgate-be1
   # open                 2
   COW-table size         10.00 GB
   COW-table LE           2560
   Allocated to snapshot  0.11%
   Snapshot chunk size    4.00 KB

4. Does the 0.11% mean that the 10GB snapshot file is only using .11%  
of the 10GB LV?


If there is a good doc out there explaining all of this could someone  
just post a link. I don't mind reading but am not getting the answers  
to the above. I have found lots of docs explaining how to do it but  
nothing going much further than that.

Thanks for any help,

Jon

[linux-lvm] Re: Snapshots

[Mike Snitzer]

On Thu, Sep 10 2009 at  7:25pm -0400,
jonr@destar.net <jonr@destar.net> wrote:

> Hello List,
>
> I don't get it!

The snapshot diagrams from Alasdair's old FOSDEM slides should help you
visualize the kernel layers involved beneath LVM (e.g. dm-snapshot):

http://people.redhat.com/agk/talks/FOSDEM_2005/

Starting with the "Snapshot" slide through "Two Snapshots".  Without
having text associated with each slide it may be too terse for you
but I recommend having a look...

> 1. How can I have a 20GB LV as a disk and the snapshot be 10GB and boot 
> the entire OS?

The snapshot LV is backed by the "COW" in the diagrams above.  If you
wanted to accommodate _every_ block changing in the origin LV (copied out
to the snapshot LV) you'd have to size the snapshot LV to be a bit
larger than the origin LV (snapshot metadata has some small overhead).

Sizing your snapshot LV is all about how you intend to use the
snapshot.  E.g. what is the expected rate of change to the origin LV
while you intend to have the snapshot active?  If you'll be changing the
origin extensively then you want to use a larger snapshot LV.

> 2. Can I create a DomU and then snapshot the LV and use the snapshot to 
> create other DomU's?

Sure, but I'd imagine you'd have to change some unique identifiers in
the DomU so that it can co-exist with the original.  These changes would
get written through the "snapshot" layer in the kernel and directly out
to the COW (would never touch the origin LV; not to be confused with
"origin" layer :).

> 3. If 2 is yes, would I want to continue using the snapshot as the disk 
> or is there something else that should be done, i.e. dd the drive to a 
> new LV?

Depends how permanent you need the new DomU to be.  And how much
simultaneous change you expect to the original DomU.

The emerging "multi-snapshot" implementation that Mikulas Patocka is
actively working on is really geared toward this Xen DomU snapshot
use-case.  Mikulas has posted his patches to dm-devel but it'll be a bit
longer before they are all merged into an upstream Linux kernel.

Mike