When comparing SSD against Hard drive performance it really makes
me cross when folk think comparing an array of SSD running on 3GBits/sec to
hard drives running on 6GBits/second is somehow valid. In a paper from DELL (http://www.dell.com/downloads/global/products/pvaul/en/PowerEdge-PowerVaultH800-CacheCade-final.pdf)
on increasing database performance using the DELL PERC H800 with Solid State
Drives they compare four SSD drives connected at 3Gbits/sec against ten 10Krpm
drives connected at 6Gbits [Tony slaps
forehead while shouting DOH!].
It is true in the case of hard drives it probably doesn’t
make much difference 3Gbit or 6Gbit because SAS and SATA are both end to end protocols
rather than shared bus architecture like SCSI, so the hard drive doesn’t share
bandwidth and probably can’t get near the 600MiBytes/second throughput that
6Gbit gives unless you are doing contiguous reads, in my own tests on a single 15Krpm
SAS disk using IOMeter (8 worker threads, queue depth of 16 with a stripe size
of 64KiB, an 8KiB transfer size on a drive formatted with an allocation size of
8KiB for a 100% sequential read test) I only get 347MiBytes per second
sustained throughput at an average latency of 2.87ms per IO equating to 44.5K
IOps, ok, if that was 3GBits it would be less – around 280MiBytes per second,
oh, but wait a minute [...fingers tap
desk]
You’ll struggle to find in the commodity space an SSD that
doesn’t have the SATA 3 (6GBits) interface, SSD’s are fast not only low latency
and high IOps but they also offer a very large sustained transfer rate,
consider the OCZ Agility 3 it so happens that in my masters dissertation I did
the same test but on a difference box, I got 374MiBytes per second at an
average latency of 2.67ms per IO equating to 47.9K IOps – cost of an 240GB Agility
3 is £174.24 (http://www.scan.co.uk/products/240gb-ocz-agility-3-ssd-25-sata-6gb-s-sandforce-2281-read-525mb-s-write-500mb-s-85k-iops),
but that same drive set in a box connected with SATA 2 (3Gbits) would only yield
around 280MiBytes per second thus losing almost 100MiBytes per second
throughput and a ton of IOps too.
So why the hell are “enterprise” vendors still only connecting
SSD’s at 3GBits? Well, my conspiracy states that they have no interest in you
moving to SSD because they’ll lose so much money, the argument that they use
SATA 2 doesn’t wash, SATA 3 has been out for some time now and all the
commodity stuff you buy uses it now.
Consider the cost, not in terms of price per GB but price
per IOps, SSD absolutely thrash Hard Drives on that, it was true that the
opposite was also true that Hard Drives thrashed SSD’s on price per GB, but is
that true now, I’m not so sure – a 300GByte 2.5” 15Krpm SAS drive costs £329.76
ex VAT (http://www.scan.co.uk/products/300gb-seagate-st9300653ss-savvio-15k3-25-hdd-sas-6gb-s-15000rpm-64mb-cache-27ms)
which equates to £1.09 per GB compared to a 480GB OCZ Agility 3 costing £422.10
ex VAT (http://www.scan.co.uk/products/480gb-ocz-agility-3-ssd-25-sata-6gb-s-sandforce-2281-read-525mb-s-write-410mb-s-30k-iops)
which equates to £0.88 per GB.
Ok, I compared an “enterprise” hard drive with a “commodity”
SSD, ok, so things get a little more complicated here, most “enterprise” SSD’s
are SLC and most commodity are MLC, SLC gives more performance and wear, I’ll
talk about that another day.
For now though, don’t get sucked in by vendor marketing,
SATA 2 (3Gbit) just doesn’t cut it, SSD need 6Gbit to breath and even that SSD’s
are pushing. Alas, SSD’s are connected using SATA so all the controllers I’ve
seen thus far from HP and DELL only do SATA 2 – deliberate? Well, I’ll let you
decide on that one.
