On February 13, 2009 1:10:55 PM -0500 Miles Nordin <car...@ivy.net> wrote:
"fc" == Frank Cusack <fcus...@fcusack.com> writes:
fc> If you're misordering writes
fc> isn't that a completely different problem?
no. ignoring the flush cache command causes writes to be misordered.
oh. can you supply a reference or if you have the time, some more
explanation? (or can someone else confirm this.)
my understanding (weak, admittedly) is that drives will reorder writes
on their own, and this is generally considered normal behavior. so
to guarantee consistency *in the face of some kind of failure like a
power loss*, we have write barriers. flush-cache is a stronger kind
of write barrier.
now that i think more, i suppose yes if you ignore the flush cache,
then writes before and after the flush cache could be misordered,
however it's the same as if there were no flush cache at all, and
again as long as the drive has power and you can quiesce it then
the data makes it to disk, and all is consistent and well. yes?
whereas if you drop a write, well it's gone off into a black hole.
fc> Even then, I don't see how it's worse than DROPPING a write.
fc> The data eventually gets to disk, and at that point in time,
fc> the disk is consistent. When dropping a write, the data never
fc> makes it to disk, ever.
If you drop the flush cache command and every write after the flush
cache command, yeah yeah it's bad, but in THAT case, the disk is still
always consistent because no writes have been misordered.
why would dropping a flush cache imply dropping every write after the
flush cache?
fc> In the face of a power loss, of course these result in the
fc> same problem,
no, it's completely different in a power loss, which is exactly the point.
If you pull the cord while the disk is inconsistent, you may lose the
entire pool. If the disk is never inconsistent because you've never
misordered writes, you will only lose recent write activity. Losing
everything you've ever written is usually much worse than losing what
you've written recently.
yeah, as soon as i wrote that i realized my error, so thank you and i
agree on that point. *in the event of a power loss* being inconsistent
is a worse problem.
-frank
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