It was brought to my attention that commits on OS X were very slow
with the latest releases of Erlang. After I upgraded to the most
recent version, I found them to be indeed slow, slowing the tests down
to point it was painful to run them. It appears, any disk sync from
Erlang now takes somewhere between 50 and 100ms, up from the previous
times of ~5ms. This is almost certainly due to the F_FULLFSYNC flag
the erlang file handling now uses on darwin based systems, but it's
surprising how bad performance on OS X. A little investigation had
shown other database engines have similar issues on OS X.
To address this problem, I implemented delayed commit functionality.
We had always intended to implement delayed commit for performance
reasons, but hadn't had the need until now. This makes updates much
faster in the general case, but with the caveat they aren't flushed
completely to disk right way. If you can't tolerate the possible loss
of recent updates, you can use the "full commit" option for ACID
commits.
For full acid commit, add a header field to the doc PUT or _bulk_docs
POST like this:
X-Couch-Full-Commit:true
Then couchdb will completely commit the change before returning.
Also, if you have several delayed updates and you want to make sure
they all made it to disk, you can invoke POST /db/_ensure_full_commit
and all outstanding commits are flushed to disk.
The view engine has been already modified to deal with delayed commits
too, it ensures it never fully commits it's own indexes to disk if the
documents indexed aren't already committed to disk.
The last remaining work item is db server crash detection, so that
clients can detect when a server has crashed and potentially lost
updates. This is pretty simple, each db server just needs a unique ID
generated at it's startup. Client retrieve this value at the beginning
of the writes and then checks that the value is the same once down a
flushed to disk. If not, we know we maybe have lost some updates and
we redo the replication from the last known good commit.
Right now the default is to delay the commits, because I think that
will be the most common use case but I'm really not sure. I definitely
want the commits delayed for the test suite, to keep things running
fast.
-Damien
