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Re: SSD + RAID

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2010-02-22 05:39:06
Message-ID: 4B82187A.7060604@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
Ron Mayer wrote:
> Bruce Momjian wrote:
>   
>> Agreed, thought I thought the problem was that SSDs lie about their
>> cache flush like SATA drives do, or is there something I am missing?
>>     
>
> There's exactly one case I can find[1] where this century's IDE
> drives lied more than any other drive with a cache:

Ron is correct that the problem of mainstream SATA drives accepting the 
cache flush command but not actually doing anything with it is long gone 
at this point.  If you have a regular SATA drive, it almost certainly 
supports proper cache flushing.  And if your whole software/storage 
stacks understands all that, you should not end up with corrupted data 
just because there's a volative write cache in there.

But the point of this whole testing exercise coming back into vogue 
again is that SSDs have returned this negligent behavior to the 
mainstream again.  See 
http://opensolaris.org/jive/thread.jspa?threadID=121424 for a discussion 
of this in a ZFS context just last month.  There are many documented 
cases of Intel SSDs that will fake a cache flush, such that the only way 
to get good reliable writes is to totally disable their writes 
caches--at which point performance is so bad you might as well have 
gotten a RAID10 setup instead (and longevity is toast too).

This whole area remains a disaster area and extreme distrust of all the 
SSD storage vendors is advisable at this point.  Basically, if I don't 
see the capacitor responsible for flushing outstanding writes, and get a 
clear description from the manufacturer how the cached writes are going 
to be handled in the event of a power failure, at this point I have to 
assume the answer is "badly and your data will be eaten".  And the 
prices for SSDs that meet that requirement are still quite steep.  I 
keep hoping somebody will address this market at something lower than 
the standard "enterprise" prices.  The upcoming SandForce designs seem 
to have thought this through correctly:  
http://www.anandtech.com/storage/showdoc.aspx?i=3702&p=6  But the 
product's not out to the general public yet (just like the Seagate units 
that claim to have capacitor backups--I heard a rumor those are also 
Sandforce designs actually, so they may be the only ones doing this 
right and aiming at a lower price).

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com   www.2ndQuadrant.us

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