[Dovecot] Howto add another disk storage

[Adrian M]

On Thu, Jul 5, 2012 at 7:37 AM, Stan Hoeppner <stan at hardwarefreak.com> wrote:
> On 7/4/2012 4:09 PM, Adrian Minta wrote:
>> On 07/04/12 23:22, J E Lyon wrote:
>>> On 4 Jul 2012, at 21:01, Adrian Minta wrote:
>>>
>>>> What is the best strategy to add another storage to an existing
>>>> virtual mail system ?
>>>> Move some domains to the new storage and create symlinks ?
>>>> Switch to dovecot hashing ? But in this case what is the easy-east
>>>> way to migrate ?
>>>>
>>>> Thanks for any suggestions or tips !
>>> Are you using Volume Management (VLM) on the system, or do you have
>>> regular partitions mounted? Is there any RAID or other factors to
>>> consider . . in fact, a few details about your system might help :)
>>>
>>> ~ James.
>>
>> No LVM and the RAID is done in the SAN appliance.
>
> It absolutely kills me every time I see a mail server admin display
> almost total lack of knowledge of his/her storage back end, or the
> inability to describe it technically, in an email...
>
>> My gut tells me that formatting a single huge 25TB partition is not the
>> best way to go.
>
> What is this statement supposed to convey to us?  It makes no sense.
>
>> That's why I consider having two different LUN's on two
>> different SAN's mounted together as two directories.
>
> And this doesn't make sense either.
>
>> A fsck and a
>> recovery will work faster.
>
> If you're having to run fsck on your filesystem on a regular basis,
> you've have larger problems than storage provisioning.  What filesystem
> are you using?
>
>> The only issue is distributing maildirs. across the two file systems.
>
> Here's the type of information and level of detail you need to provide
> for any of us to intelligently assist you with this issue:
>
> My SAN array is model X from company Y.  It has 12x 3TB drives
> configured as a RAID6 array.  I have created a 5TB virtual drive and
> exported it as LUN #X.  It houses the filesystem (extX/XFS/JFS) that
> contains the mail store, which is now getting full.  I have 25TB of the
> 30TB net array space unallocated.  How can I best use this space to
> increase storage space for Dovecot?
>
> To which I would answer:
>
> Unless your SAN array is an el cheap model, you should be able to simply
> increase the size of the current virtual drive by adding a portion of
> the 25TB of unallocated space.  Say you expand the virtual drive by
> 10TB.  The virtual drive is already exported as LUN #X, so the host to
> which the LUN is unmasked simply now sees /dev/sdX as being 15TB total
> size instead of 5TB.  At this point, simply grow the filesystem across
> the additional 10TB of the "disk".  This negates the need for multiple
> filesystems/namespaces and jumping through hoops to balance your maildir
> workload across two separate filesystems.
>
> This "on the fly" expansion capability is one of the biggest selling
> points of SAN technology.  Surely your unit has such capability.
>
> --
> Stan

Hi Stan,
I know how to add drives to the storage and how to grow the existing
filesystem, but such big filesystems are somehow new to mainstream
linux. Yes, I know some university out there already have pentabytes
filesystems, but right now stable linux systems have trouble
formatting ext4 partition over 16T.
All this is telling me that is safer to have two or tree smaller
filesystems than a big one. Dovecot has a nice feature for this
"Directory hashing" http://wiki.dovecot.org/MailLocation/

What I don't know is a nice way to migrate from a single directory no
hashing to more than one and hashing.