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Re: WAL in RAM

From: "Tomas Vondra" <tv(at)fuzzy(dot)cz>
To: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
Cc: "Marcus Engene" <mengpg2(at)engene(dot)se>, pgsql-performance(at)postgresql(dot)org
Subject: Re: WAL in RAM
Date: 2011-10-28 18:26:19
Message-ID: 70afb7c488aca8f564f4b569f857747b.squirrel@sq.gransy.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 28 Říjen 2011, 18:11, Merlin Moncure wrote:
> On Fri, Oct 28, 2011 at 10:28 AM, Marcus Engene <mengpg2(at)engene(dot)se> wrote:
>> Hi list,
>>
>> Every now and then I have write peaks which causes annoying delay on my
>> website. No particular reason it seems, just that laws of probability
>> dictates that there will be peaks every now and then.
>>
>> Anyway, thinking of ways to make the peaks more bareable, I saw the new
>> 9.1
>> feature to bypass WAL. Problems is mainly that some statistics tables
>> ("x
>> users clicked this link this month") clog the write cache, not more
>> important writes. I could live with restoring a nightly dump of these
>> tables
>> and loose a days worth of logs.
>>
>> Though not keen on jumping over to early major versions an old idea of
>> putting WAL in RAM came back. Not RAM in main memory but some thingie
>> pretending to be a drive with proper battery backup.
>>
>> a) It seems to exist odd hardware with RAM modules and if lucky also
>> battery
>> b) Some drive manufactureres have done hybird ram-spindle drives
>> (compare
>> with possibly more common ssd-spindle hybrides).
>>
>> b) sounds slightly more appealing since it basically means I put
>> everything
>> on those drives and it magically is faster. The a) alternatives also
>> seemed
>> to be non ECC which is a no-no and disturbing.
>>
>> Does anyone here have any recommendations here?
>>
>> Pricing is not very important but reliability is.
>
> Have you ruled out SSD?  They are a little new, but I'd be looking at
> the Intel 710.  In every case I've seen SSD permanently ends I/O
> issues.  DRAM storage solutions I find to be pricey and complicated
> when there are so many workable flash options out now.

Are you sure SSDs are a reasonable option for WAL? I personally don't
think it's a good option, because WAL is written in a sequential manner,
and that's not an area where SSDs beat spinners really badly.

For example the Intel 710 SSD has a sequential write speed of 210MB/s,
while a simple SATA 7.2k drive can write about 50-100 MB/s for less than
1/10 of the 710 price.

I'm not saying SSDs are a bad thing, but I think it's a waste of money to
use them for WAL.

Tomas


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