From: Brad Nicholson <bnichols@ca.afilias.info>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Scott Carey <scott@richrelevance.com>
Cc: Michael March <mmarch@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Completely un-tuned Postgresql benchmark results: SSD vs desktop HDD
Date: Tue, 10 Aug 2010 13:57:20 -0400
Message-ID: <4C619300.3080400@ca.afilias.info> (raw)
In-Reply-To: <4C617C80.9080509@2ndquadrant.com>
References: <AANLkTikXLYc32Jyr5Fb8VeVNt6R86mCcF+0CkTHALd8W@mail.gmail.com>
<BAE01AAD-6C69-48C7-BD2E-77E463D49903@richrelevance.com>
<AANLkTi=km+H+mZ=yyx0_HTFZ-yU2sh85D+p5cs+ZcUSF@mail.gmail.com>
<A1640F99-A7C5-4F04-9A25-782C46BE7DAB@richrelevance.com>
<4C617C80.9080509@2ndquadrant.com>
On 8/10/2010 12:21 PM, Greg Smith wrote:
> Scott Carey wrote:
>> Also, the amount of data at risk in a power loss varies between
>> drives. For Intel's drives, its a small chunk of data ( < 256K).
>> For some other drives, the cache can be over 30MB of outstanding writes.
>> For some workloads this is acceptable
>
> No, it isn't ever acceptable. You can expect the type of data loss
> you get when a cache fails to honor write flush calls results in
> catastrophic database corruption. It's not "I lost the last few
> seconds"; it's "the database is corrupted and won't start" after a
> crash. This is why we pound on this topic on this list. A SSD that
> fails to honor flush requests is completely worthless for anything
> other than toy databases. You can expect significant work to recover
> any portion of your data after the first unexpected power loss under
> heavy write load in this environment, during which you're down. We do
> database corruption recovery at 2ndQuadrant; while I can't talk about
> the details of some recent incidents, I am not speaking theoretically
> when I warn about this.
>
What about putting indexes on them? If the drive fails and drops writes
on those, they could be rebuilt - assuming your system can function
without the index(es) temporarily.
--
Brad Nicholson 416-673-4106
Database Administrator, Afilias Canada Corp.
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To: pgsql-performance@postgresql.org
Cc: bnichols@ca.afilias.info, greg@2ndquadrant.com, scott@richrelevance.com, mmarch@gmail.com
Subject: Re: Completely un-tuned Postgresql benchmark results: SSD vs desktop HDD
In-Reply-To: <4C619300.3080400@ca.afilias.info>
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