[linux-lvm] Snapshots

[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

Re: [linux-lvm] Snapshots

[Peter Keller]

Hi Jon,

You seem to be right that there isn't much written down about how snapshots 
work under the bonnet in the LVM context, but the same ideas have been used 
for other snapshot implementations for years. You could try searching for 
filesystem snapshot technology in general, rather than anything 
LVM-specific. For example, this article explains some of the basic concepts:

   http://www.ibm.com/developerworks/tivoli/library/t-snaptsm1/index.html

LVM uses the copy-on-write method (this is what "COW" stands for).

On Thu, 10 Sep 2009, jonr@destar.net wrote:

> 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?

Because the snapshot only stores copies of the disk blocks that have been 
modified on the original volume since the snapshot was taken (the blocks 
that would otherwise be overwritten if you hadn't created the snapshot). 
When you read from the snapshot, LVM uses never-modified blocks from the 
original volume and pre-modification copies of blocks from the snapshot as 
needed.

If the original volume became corrupt, the snapshot would also be unusable.

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

Yes - there are articles around about how to do this. If you intend to run 
the DomU's on the same network, you should boot the copies into single-user 
mode initially to modify their network identities so that they don't 
conflict with each other. (Maybe you can find some other way of doing this 
without booting the DomU's: I know that you can do this with VMWare, at 
least for Linux guest OS'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

You can continue use the snapshot in principle, although you might like to 
make the snapshot the same size as the original volume in that case to make 
sure that it never fills up. In your example, when the accumulated changes 
on the original volume since the snapshot was taken reach 10Gb, the snapshot 
will be full and go invalid and you won't be able to use it.

If your aim is to have a single starting point to create more than one DomU, 
you should copy the data out and keep it safe somewhere. You can then use 
that copy to create further copies as needed. Personally, I would only boot 
up the first copy (or the snapshot) to keep the OS up to date (which will 
save you having to update every copy that you make from it).

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

Yes, that's right.

> 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,

Have fun,
Peter.

Re: [linux-lvm] Snapshots

[André Gillibert]

jonr@destar.net wrote:
> 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?
> 
That's because the snapshot uses these 10GB as a "COW-table". i.e. A "Copy On Write table".
When the snapshot is first created, no copy is performed. All the blocks of the snapshot are identical to the blocks of the original LV. Accessing the snapshot or original LV access the same block.
Then, if you modify some data in either the original LV or the snapshot LV, an "exception list" listing data chunks that are different in the snapshot LV than in the original LV, is created.
When reading a snapshot LV block, LVM first looks at the "exception list". If the chunk accessed is the in the "exception list", the chunk data in the exception list is returned. If the chunk isn't in the exception list, the chunk data is extracted from the original volume.
When writing a snapshot LV block, it's written in a chunk of the exception list, either existing, or created at this time.
When writing a chunk in the original volume, it's first copied in the "exception list", and then, modified in the original volume.

The chunk size is specified with the --chunksize parameter (4k to 512k) to lvcreate(8), when creating a snapshot.

If ever you've so many exceptions that the snapshot volume overflows, it's dropped.
Consequently, you must take care of allocating enough space in snapshots. This space is the size of the data that may be modified in the snapshot or original volume.
Snapshots are fine for short lived snapshots (e.g. when backing up) but aren't designed to cope with heavy modifications of the entire file system.

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

I'm not a Xen user. Sorry, I don't know.
Here, Google may help:
<http://www.google.com/search?rls=en&hl=en&source=hp&q=xen+device+mapper>

> 
> 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?
> 
It means that the internal LVM data structure + the exception chunks are using 0.11% of the 10GB "COW-table partition".

Create snapshot, dd(1), delete snapshot is a way to create a "forked" copy of the original volume.
That may, or may not be what you want. If you intend to share much data with the original volume, a simple snapshot may be the way to go. If a small or near-zero amount of data is to be shared, in that case, snapshotting won't save data space, but may incur performance penalties (see <http://www.nikhef.nl/~dennisvd/lvmcrap.html>), and so, my advice is to create snapshot, dd(1), and delete snapshot.

I hope it helps.
-- 
André Gillibert

[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

[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