[Novalug] LVM Snapshots

James Ewing Cottrell 3rd JECottrell3@Comcast.NET
Sun Dec 19 20:01:42 EST 2010


  On 12/19/2010 10:07 AM, American Dave Kline wrote:
> On Sun, Dec 19, 2010 at 03:52:39AM -0500, James Ewing Cottrell 3rd wrote:
>>    Peter Larsen wrote:
>>
>> "Snapshots are fine; just don't mount your root on a snapshot."
>>
>> This is precisely the question I asked!
>>
>> Dave says "Yes, this is OK". Now who is right?
> Peter mentioned things that were important, that
> snapshots consume I/O and you'll have to worry about running out of
> space in a snapshot -- very important things to worry about with a root
> filesystem.  Hence my suggestion of --merge, which means if you like
> your snapshot you can merge it to its origin.
Once again, Efficiency is only important *after* Safety has been 
achieved. So once again....I ask my question....

IS IT SAFE TO BOOT OFF A SNAPSHOT? Peter says NO, Dave says YES. WHO IS 
RIGHT?
> Peter also gave a great LVM talk.
I am sure he has given many.
>> And BTW, there is No Such Thing as "lvconvert --merge". At least not in
>> CentOS 5.5.
> Correct.  RHEL 6, Fedora 14, etc, contain --merge.  CentOS is based off
> of RHEL 5 at present.  Red Hat is offering more affordable ways to get
> RHEL 6, you might want to have a look.
We should be getting RHEL 6 at work soon. And I probably should have 
known that newer versions exist out there.
> Consider purchasing RHEL as an investment in CentOS's upstream.
In any case, this conversation could have just as easily occurred this 
summer.
> That said, I suppose you could try booting a live distro and merging
> the snapshot(s) using the live LVM.  I've never done this, but would
> investigate if it was important.
As I mentioned, I want to avoid reboots. And the rsync method I 
mentioned would probably merge the data.

> Regards,
> -A. Dave
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