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Re: raid10 write performance

From: Matthew Wakeling <matthew(at)flymine(dot)org>
To: Ivan Voras <ivoras(at)freebsd(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: raid10 write performance
Date: 2010-06-23 12:46:13
Message-ID: alpine.DEB.2.00.1006231331580.2534@aragorn.flymine.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Wed, 23 Jun 2010, Ivan Voras wrote:
> On 06/23/10 14:00, Florian Weimer wrote:
>> Barrier support on RAID10 seems to require some smallish amount of
>> non-volatile storage which supports a high number of write operations
>> per second, so a software-only solution might not be available.
>
> If I understand you correctly, this can be said in general for all
> spinning-disk usage and is not specific to RAID10. (And in the case of
> high, constant TPS, no amount of NVRAM will help you).

No. Write barriers work fine with a single disc, assuming it is set up 
correctly. The barrier is a command telling the disc to make sure that one 
piece of data is safe before starting to write another piece of data.

However, as soon as you have multiple discs, the individual discs do not 
have a way of communicating with each other to make sure that the first 
piece of data is written before the other. That's why you need a little 
bit of non-volatile storage to mediate that to properly support barriers.

Of course, from a performance point of view, yes, you need some NVRAM on 
any kind of spinning storage to maintain high commit rates.

Matthew

-- 
 I wouldn't be so paranoid if you weren't all out to get me!!

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