On Jan 5, 2009, at 2:38 PM, Chris Anderson wrote:
On Mon, Jan 5, 2009 at 11:32 AM, Damien Katz <[email protected]>
wrote:
1) delayed commit (what you did last night)
2) fsync() commit (what I suspect Couch did on and around 0.8)
3) optional F_FULLSYNC commit, on OS X and any other platform that
provides this level of commit
If necessary and possible, we'll patch the Erlang VM. But if a
platform
doesn't support proper flushing, then it's not a platform that can
support
an ACID database.
If I follow correctly, you're saying that on OS X, 2 == 1, so we use 3
when we want to be ACID. On Linux, as far as we know, 2 will support
ACID. There's only really two options, ACID commit, and delayed
commit. It's up to CouchDB / Erlang / the OS to make sure that the
ACID option isn't false advertising. Makes sense.
If fsync doesn't work on a platform (like previously on OS X), then
neither the delayed commit or fully flushed commit can work.
The way the delayed commit works, the database on disk is never in an
inconsistent state. The new data may be partially written and the
header in memory points to it, but the header on disk still points to
the old consistent data. So this new option isn't like OS X before the
F_FULLFSYNC patch, because that could actually cause corruptions
(header points to incomplete structures) and you'd lose previously
committed data. This new patch might lose the last uncommitted writes
if it crashes before the final flush, but the previously committed
data is always consistent and instantly available, no fixup needed.
-Damien
Definitely, I think commit options should be settable per-database.
But for
now I was just wanting to address the slowdown, especially for
replication
and the tests, to keep everyone productive. More commit features
and options
is lower priority work for now, I was just addresses the most serious
slowdown.
The config system is so flexible and easy to hook into, that I'm not
worried about adding these settings. Should be a breeze once the
software itself is settled.
+1 to the whole thing.
--
Chris Anderson
http://jchris.mfdz.com
