Re: SSD + RAID

Merlin Moncure wrote:
> On Sat, Nov 14, 2009 at 6:17 AM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> lots of ram doesn't help you if:
>>> *) your database gets written to a lot and you have high performance
>>> requirements
>> When all the (hot) data is cached, all writes are sequential writes to
>> the WAL, with the occasional flushing of the data pages at checkpoint.
>> The sequential write bandwidth of SSDs and HDDs is roughly the same.
>>
>> I presume the fsync latency is a lot higher with HDDs, so if you're
>> running a lot of small write transactions, and don't want to risk losing
>> any recently committed transactions by setting synchronous_commit=off,
>> the usual solution is to get a RAID controller with a battery-backed up
>> cache. With a BBU cache, the fsync latency should be in the same
>> ballpark as with SDDs.
>
> BBU raid controllers might only give better burst performance. If you
> are writing data randomly all over the volume, the cache will overflow
> and performance will degrade.

We're discussing a scenario where all the data fits in RAM. That's what
the large amount of RAM is for. The only thing that's being written to
disk is the WAL, which is sequential, and the occasional flush of data
pages from the buffer cache at checkpoints, which doesn't happen often
and will be spread over a period of time.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com