Hi,
I have three equal machines with Pentium(R) D CPU 3.20GHz, 2GiB RAM, FreeBSD
8, Erlang R13B04 (erts-5.7.5) [source] [64-bit] [smp:2:2] [rq:2]
[async-threads:0] [hipe] [kernel-poll:false], and CouchDB 1.0.0.
I would like to replicate documents between the three (even more machines
later) in a fully meshed replica agreement (every node replicates from/to
every other to ensure that there is no SPoF and every document gets to
others ASAP). The nodes would store small, but quickly changing documents
(application no. 1) and larger (from several kBs to several GBs) binary
attachments (application no. 2). The applications are not mixed on the same
CouchDB instance (even the machines).
I've experimented with the first and noticed that no matter how fast insert
documents (BTW, I could achieve about 230 inserts per second, parallel
connections, no bulk inserts) the traffic between the machines doesn't go
beyond about 500 kBps and the replicas lag behind the written node (a lot!).
Based on this, I've started another test, now with smaller binary
attachments. The first run did this:
for i in `jot 128`
do
curl -X PUT http://localhost:5984/testdb/$i/file -H "Content-Type:
application/octet-stream" --data-binary @bin1
done
That is, it uploads 128 MB of data (bin1 is 1MB of size).
Without replication, it runs in 8.64 seconds (14.81 MBps, not that fast
either, but hey, it's erlang :). If I run it with background curl processes
(maximum 128 parallel uploads), the script runs in 6.74s (18.99 MBps).
Now if I make a one way replica to another node (connected with gigabit
ethernet), the run time slightly increases to 7.04s on the master node, but
it takes 42 seconds (3.04 MBps) for all the 128 documents to reach the slave
node.
Things get worse when I make a two way replication between the two nodes,
this time the upload on the "master" node takes 7.4 seconds, but 75 seconds
are needed for the two nodes to become consistent. The erlang processes on
both sides eat more resources, so this slowdown is completely visible, not
network bound (of course).
If I make two one way replications (A->B, A->C node), the times look like
this: time needed to upload on the master (A) node: 6.52s, time needed for
the slave (B, C) nodes to get consistent with A: 44s (A->B), 39s (A->C).
BTW, I calculate this from the start of the script (I'm not writing the data
on A and then set up replication).
With the following replications defined: A<->B, A<->C, I get these:
uploading to A: 7.34s, A->B consistency: 72s , A->C consistency: 72s
During the process I saw this on node A:
PID USERNAME THR PRI NICE SIZE RES STATE C TIME WCPU COMMAND
15427 couchdb 11 109 0 217M 149M CPU0 0 14:44 135.94%
beam.smp
and this was after the upload has been done, so this is what CouchDB eats
when doing bilateral replication towards two nodes.
And now the full mesh (A<->B, A<->C, B<->C):
CouchDB resource usage tops:
15427 couchdb 11 110 0 270M 202M CPU1 1 18:44 140.14%
beam.smp
and the consistency times also: A->B: 125s, A->C: 107s.
BTW, the upload lasted for 7.59s.
Summary: it seems unilateral replication is consistent in it's resource
usage, and it's pretty slow (7s on localhost write vs. 42s of replication to
the remote node). If I define a bilateral replication it slows down further,
nearly to the half. Every bilateral agreement introduces this slowdown, so
one unilateral: 42s, one bilateral: 72s, two bilaterals: 125s.
I'm sure it's not about waiting for the network or disk, it seems to be pure
resource usage problem. Is this known? Will it be fixed?
Thanks,
