On 11/02/11 11:39 AM, Thomas Strunz wrote:
> For database I assume random read and writes are by way the most
> important thing and any recent ssd is orders of magnitude faster in
> that are compared to HDD even the "slow" Intel drives.
actually, SSD's have issues with committed small block (8K) random
writes such as databases do a lot of. the SSD has a rather large block
size that has to be written all at once, so what they tend to do is
accumulate random writes in a buffer, then write them all at once to a
contiguous block (remapping the logical LBA sector address to an actual
block/offset address).
as a test at work, I compared a 2 x 100GB SAS enterprise SSD RAID0 with
a 20 x 146GB SAS 15k HD RAID10, both raids using a HP p410 hardware raid
controller with 1Gb cache, and both using XFS. Both file systems are
approximately the same in sustainable random writes from postgres, up
around 12000 wr/sec during heavy sustained pgbench activity (scaling
factor of 500, 48 clients, 24 threads, on a 12 core 24 thread dual xeon
e5660 48gb server). The HD raid is faster at sustained large block
writes from iozone (1.2GB/sec vs 800MB/sec for the SSD). of course, the
HD raid10 is 1.3TB of data, while the SSD raid0 is 200GB of data.
--
john r pierce N 37, W 122
santa cruz ca mid-left coast
