Re: LVM

[Tom Golson <tgolson@tamu.edu>]

Because we went live very quickly, and didn't really appreciate the idea
of creating new mail volumes, we're running ReiserFS on LVM over iSCSI.
   Normally, I'm LVM's biggest fan -- the technology is great.  However,
unless the LVM folks have recently changed something, I really don't
recommend it for anything that involves multipath capable storage.

LVM fundamentally seems to manage physical slices by their /dev/[sh]d
device name, and not the disk id's.  Disk id's are persistent across
path changes, but /dev/sdX id's aren't.  It seems that when a path
change happens, multipathd increments the /dev/sdX value, so what was
perhaps /dev/sdc is now /dev/sdd, and this just causes LVM a world of
pain.  Mount points go read-only and you've got to unmount, vgscan,
vgchange -a y, and then remount.

If I've just missed the boat, and someone knows a flag to pass LVM to
recognized physical drives by the /dev/disk-by-id/ entry, I would _love_
to hear it, because I've got a ton of LVM on multi-path storage, and am
busy scheming how to migrate it with a minimum of pain.

--
Tom Golson
Senior Lead Systems Engineer
Open Systems Group
Computing & Information Services
Texas A&M University

William Stearns wrote:
> Good morning, all,
> 
> On Thu, 16 Oct 2008, Steve Hillman wrote:
> 
>> ----- "John Fulton" <fultonj@lafayette.edu> wrote:
>>
>>> Has anyone expanded a Zimbra volume with LVM or does anyone have
>>> multiple primary and secondary mail stores on the same server?
>>
>> take out our redologs. My only real concern about this is how much (if
>> any) overhead LVM adds -- the responsiveness of the redolog volume
>> seems to be the most critical in terms of affecting the user
>> experience (makes sense when you consider that Zimbra does the Right
>> Thing and fsync's all transactions to the redolog before returning an
>> "ok" to the user.)
> 
>     My best understanding is that LVM contributes essentially no
> overhead at all.  Converting a logical block to a physical block is a
> lookup in memory that's completely dwarfed by reading from, writing to,
> and/or seeking to the actual sector.
>     Cheers,
>     - Bill
> 
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