Re: Tuning the configuration

On 12/12/14 11:36, Eric Pierce wrote:
>
> ________________________________________
> From: pgsql-performance-owner(at)postgresql(dot)org <pgsql-performance-owner(at)postgresql(dot)org> on behalf of Evgeniy Shishkin <itparanoia(at)gmail(dot)com>
> Sent: Thursday, December 11, 2014 7:11 AM
> To: Andrea Suisani
> Cc: mfatticcioni(at)mbigroup(dot)it; pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] Tuning the configuration
>
>> On 11 Dec 2014, at 15:02, Andrea Suisani <sickpig(at)opinioni(dot)net> wrote:
>>
>> On 12/10/2014 11:44 AM, Maila Fatticcioni wrote:
>>> 2- I would like to use the two SDD to store the wal file. Do you think
>>> it is useful or how should I use them?
>>
>> I definitely would give it a try.
>>
>
>
>> I don't understand the logic behind using drives,
>> which are best for random io, for sequent io workloads.
>
>> Better use 10k sas with BBU raid for wal, money wise.
>
> Very much agree with this. Because SSD is fast doesn't make it suited for certain things, and a streaming sequential 100% write workload is one of them. I've worked with everything from local disk to high-end SAN and even at the high end we've always put any DB logs on spinning disk. RAID1 is generally sufficient. SSD is king for read heavy random I/O workload.
>

Mind you wal is a little different - the limiting factor is (usually)
not raw sequential speed but fsync latency. These days a modern SSD has
fsync response pretty much equal to that of a card with BBU + spinners -
and has "more" high speed storage available (cards usually have only a
1G or so of RAM on them).

regards

Mark