Re: SSD + RAID

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