Getting the balance right …

October 24, 2013

Ying and yang … the Chinese had it just about right when they coined that phrase (and yes, I used google to check that in fact it is Chinese – though I’m sure some smart soul will tell me otherwise) …

Had several customers recently who have been reading about hybrid RAID. This is where you can mix an SSD and a HDD on an Adaptec Series 6, 7 or 8 and make a mirror out of those two drives. While this sounds crazy it is pretty simple. Writes go to both drives (which is nicely buffered by controller cache so you get good speed), but all reads are directed from the SSD – giving lightning read speed.

Sounds good … and people use the old story of “don’t need RAID 5 so lets go for an entry-level card”. In this case, the 6405E. So far, so good. However … the 6405E is pcie2 x 1 – which limits the throughput to 500mb/sec. That’s probably not going to be too much of a problem on a RAID 1 – one SSD will go close to saturating that but it will be close.

However … make a RAID 10, with 2 x SSD and 2 x HDD and you are starting to stretch the friendship a bit. Reading a relatively large file off that RAID should in theory saturate the pcie2 x 1 bus, making the card the bottleneck. So in this case you need to go to the 6405 controller, not the “E”, which has a pcie2 x 8 connector and can easily handle the throughput of the SSDs.

So yes, you only need an entry level card for a RAID 1 or RAID 10, but if you are doing a hybrid RAID then you probably need to consider the theoretical speed of the SSD you are using and make sure there is no bottleneck in the way of their performance.





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