[Dovecot] SSD drives are really fast running Dovecot

[Stan Hoeppner]

Rick Romero put forth on 1/14/2011 8:29 PM:

> And that's assuming a platter squeezing in 1TB of data at 7200RPMs doesn't
> get a comparable performance improvement to a higher rotational speed on a
> lower volume platter...  

Size and density are irrelevant.  Higher density will allow greater streaming
throughput at the same spindle speed, _however_ this does nothing for seek
performance.  Streaming performance is meaningless for transaction servers.
IOPS performance is critical for transaction servers.  Seek performance equals
IOPS performance.  The _only_ way to increase mechanical disk IOPS is to
increase the spindle speed the or the speed of the head actuator.  If you've
watched mechanical drive evolution for the past 20 years you've seen that
actuator speed hasn't increased due to the physical properties of voice coil
drive actuators.

> Hell for the price of a single 250gb SSD drive,
> you can RAID 10 TEN 7200 RPM 500GB SATAs.  

I think your pricing ratio is a bit off but we'll go with it.  You'd get 50,000
4KB random IOPS from the SSD and only 750 IOPS from the RAID 10.  The SSD could
handle 67 times as many emails per second for 10 times the cost.  Not a bad trade.

> So while, yes, my 10 drive SATA RAID 10 ONLY performs 166MB/sec with a
> 'simplistic' dd test, In reality I just don't think Joe User is going to
> notice the difference between that and the superior performance of a
> single SSD drive when he POPs his 10 3k emails.

But Joe User _will_ notice a difference if this server with the RAID 10
mentioned above is supporting 5000 concurrent users, not just Joe.  Responses
will lag.  With the SSD you can support 10000 concurrent users (assuming the
rest of the hardware is up to that task and you have enough RAM) and responses
for all of them will be nearly instantaneous.  This is the difference SSD makes,
and why it's worth the cost in many situations.  However, doing so will require
an email retention policy that doesn't allow unlimited storage--unless you can
afford than much SSD capacity.

You can get 240,000 4k random IOPS and 1.9TB of capacity from two of these in a
software RAID0 for $6,400 USD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227665

That's enough transactional IOPS throughput to support well over 50,000
concurrent IMAP users, probably far more.  Of course this would require a server
likely on the order of at least a single socket G34 AMD 12 core Magny Cours
system w/2GHz cores, 128GB of RAM, and two free PCIe X4/X8 slots for the SSD
cards, based on a board such as this SuperMicro:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813182240
(Actually this is the perfect board for running two of these RevoDrive X2 cards)

-- 
Stan