On 8/6/2012 2:53 PM, Bob Friesenhahn wrote:
On Mon, 6 Aug 2012, Stefan Ring wrote:
Intel's brief also clears up a prior controversy of what types of
data are actually cached, per the brief it's both user and system
data!
So you're saying that SSDs don't generally flush data to stable medium
when instructed to? So data written before an fsync is not guaranteed
to be seen after a power-down?
If that -- ignoring cache flush requests -- is the whole reason why
SSDs are so fast, I'm glad I haven't got one yet.
Testing has shown that many SSDs do not flush the data prior to
claiming that they have done so. The flush request may hasten the
time until the next actual cache flush.
Honestly, I don't think this last point can be emphasized enough. SSDs
of all flavors and manufacturers have a track record of *consistently*
lying when returning from a cache flush command. There might exist
somebody out there who actually does it across all products, but I've
tested and used enough of the variety (both Consumer and Enterprise) to
NOT trust any SSD that tells you it actually flushed out its local cache.
ALWAYS insist on some form of power protection, whether it be a
supercap, battery, or external power-supply. That way, even if they
lie to you, you're covered from a power loss.
-Erik
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