On 10/25/2010 3:03 AM, Johan De Meersman wrote:
> SSD may still be useful if you have a lot of writes, though.
Only if by "a lot" you mean "a minority".
A lone 2 TB rotating disk will beat a top-of-the-line SSD for linear
writes, and you can beat an SSD for linear reads with a pair of disks in
RAID-0 or -1, or four disks in RAID-10. (Or, I suppose, some huge
number of spindles in RAID-5 or -6, but I've never seen such an array
big enough to be called fast at writes.) SSDs have a clearer advantage
for random I/O, a useful property for databases, but still, you
shouldn't ignore the fact that SSD writes are expensive.
Therefore, you get the SSD speed benefit only if writes are rare enough
that more data is coming off the drive at any given time than is being
written, or if your current disk subsystem is bottlenecked by rotating
disk head seek time, or some combination.
Since the original poster is using RAID-10, it's definitely not a sure
deal that replacing that array with a single SSD will help.
However, it might be entertaining to benchmark it against 4 SSDs in
RAID-10. Or 8. :)
> Incidentally, i'm not aware of how SSD plays with hard/software RAID setups
> - anyone know more about this ?
Some software RAID and RAID-like systems are gaining SSD awareness so
they can intentionally place frequently-accessed data on the SSD.