[Dovecot] Indexes to MLC-SSD

[Ed W]

On 03/11/2011 16:53, Patrick Westenberg wrote:
> Ed W schrieb:
>
>>> I'm using NexentaStor (Solaris, ZFS) to export iSCSI-LUNs and I was
>>> thinking about a SSD based LUN for the indexes. As I'm using multiple
>>> servers this LUN will use OCFS2.
>>
>> Given that the SAN always has the network latency behind it, might you
>> be better to look at putting the SSDs in the frontend machines?
>> Obviously this then needs some way to make users "sticky" to one machine
>> (or some few machines) where the indexes are stored?
>
> Storing the indexes on several machines?
> In this case I have to synchronize them.
>

See the "sticky" in my reply.  You use one of several techniques to
ensure that users always end up on the server with the indexes on.  That
way much of the IO is served from that local machine and you only access
the SAN for the (in theory much less frequent) access to the mail files
themselves.

Clearly if the machine with the indexes on dies then the load balancer
needs to pick a new machine and there will be delay/io/etc while the
indexes are regenerated.  Various techniques could mitigate this...

I don't have such a larger system - please ignore all my advice...  The
basis for the suggestion is that I understand file access (locking in
particular) is "expensive" on OCFS2/GFS.  Therefore I read here on this
list that others have found performance issues accessing maildir over
OCFS2?  It's also not hard to find benchmarks that show OCFS2/GFS are
"fast", but slower than accessing the same storage without using a
cluster filesystem - this makes sense.  Hence it seems like a trade
between convenience of storing everything on a central store and "some"
performance improvement from a more complex system...

I think if you search on benchmarks of DRBD vs OCFS2 and read here on
the list about the "director" and "proxy" services you can see the
point?  I'm just trying to help you see the effects you might want to
measure!  (I don't have a system large enough to know much about this
stuff from experience...)

Good luck!

Ed W