Message-ID: <4B822DFF.50501@tweakers.net>
Date: Mon, 22 Feb 2010 08:10:55 +0100
From: Arjen van der Meijden <acmmailing@tweakers.net>
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To: pgsql-performance@postgresql.org
Subject: Re: SSD + RAID
References: <201002211410.o1LEAht05941@momjian.us>
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On 22-2-2010 6:39 Greg Smith wrote:
> 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).

That's weird. Intel's SSD's didn't have a write cache afaik:
"I asked Intel about this and it turns out that the DRAM on the Intel 
drive isn't used for user data because of the risk of data loss, instead 
it is used as memory by the Intel SATA/flash controller for deciding 
exactly where to write data (I'm assuming for the wear 
leveling/reliability algorithms)."
http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3403&p=10

But that is the old version, perhaps the second generation does have a 
bit of write caching.

I can understand a SSD might do unexpected things when it loses power 
all of a sudden. It will probably try to group writes to fill a single 
block (and those blocks vary in size but are normally way larger than 
those of a normal spinning disk, they are values like 256 or 512KB) and 
it might loose that "waiting until a full block can be written"-data or 
perhaps it just couldn't complete a full block-write due to the power 
failure.
Although that behavior isn't really what you want, it would be incorrect 
to blame write caching for the behavior if the device doesn't even have 
a write cache ;)

Best regards,

Arjen