[Discuss] LVM Re: A really interesting chain of functionality

[bogstad at pobox.com (Bill Bogstad)]

On Mon, Sep 26, 2011 at 9:45 PM, Mark Woodward <markw at mohawksoft.com> wrote:
> On 09/26/2011 07:17 PM, Edward Ned Harvey wrote:
>>
>> So, this all serves to rather emphasize my point, which is to say...
>> (LVM) Create snapshot, mount it, monitor it with nagios or whatever,
>> lvextend it, lvextend the filesystem, resize2fs, unmount and release
>> snapshot...
>> versus
>> (ZFS, Netapp, Volume Shadow Services, etc.) ?Do nothing, and don't worry
>> about it. ?It's all automatic and dynamic and just works.
>
> I don't think this is right. Running nagios on a snapshot would do nothing.
> A snapshot is protected from change.

This is neither true in the logical nor physical sense with LVM.   It
was never true in a physical sense, in that the storage for the
snapshot is slowly used up due to copy-on-write as applications write
to the original copy of the filesystem.   It's not true in the logical
sense because LVM snapshots have actually been read/write for quite a
while.  A common usage pattern for this appears to be when you want
multiple copies of essentially the same virtual machine image.
You start with a single gold copy and then create writable snapshots
for each virtual machine.

Bill Bogstad