Re: RAID and SSD configuration question

Hi,

On 10/20/2015 04:33 PM, Scott Marlowe wrote:
>
> We're running LSI MegaRAIDs at work with 10 SSD RAID-5 arrays, and we
> can get ~5k to 7k tps on a -s 10000 pgbench with the write cache on.
>
> When we turn the write cache off, we get 15k to 20k tps. This is on
> a 120GB pgbench db that fits in memory, so it's all writes.

I'm not really surprised that the performance increased so much, as the
SSDs have large amounts of DRAM on them - with 10 devices it may easily
be 10GB (compared to 1 or 2GB, which is common on RAID controllers). So
the write cache on the controller may be a bottleneck.

But the question is how disabling the write cache (on the controller)
affects reliability of the whole RAID array.

The write cache is there not only because it improves performance, but
also because it protects against some failure modes - you're mentioned
RAID-5 which is vulnerable to "write hole" problem.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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