Jeremy Cole on MySQL ReplicationPeter Zaitsev
Jeremy Cole recently posted very nice post about MySQL Replication Performance. There are however few points I should comment on.
- Jeremy Speaks about MyISAM and Innodb in the same terms, in fact they are quite different for Replication. As you probably know MyISAM uses buffered IO so if master crashes you’re out of sync anyway – whatever sync_binlog option was set to. Well it is still a bit better with sync_binlog enabled as slave could have consistent data after it catches up but master may have some of its tables out of sync with binary log and it is not easy to find which.
- I’m surprised to read enabling binary log drops update performance. I guess Jeremy speaks about enabling binary log and setting sync_binlog option which is good for safe replication. But it is setting which is off by default so you should not see such effect just by enabling log_bin.
- Innodb tables also have to flush logs on transaction commit unless you disable it so you should not be getting 1500 update transactions per second from the single thread without battery backed up cache. If you do it is quite possible Operation system is faking fsync() for you. In certain cases it only does so for sequential writes this is why sync_binlog changes things. Two logs are synced now which requires seeks which make certain disks to flush their cache.
- Watch out for MySQL 5.0 – if you enabled Binary log with MySQL 5.0 you loose group commit which can dramatically increase commit rate for multiple user load. See this post by Vadim for benchmark results.
- Solution with Battery Backed up RAID is great one and is not that expensive these days. Other two probably just create conditions for Operation System to fake fsync. You need to get data to the disk and you can’t physically get data to the disk surface at 1500 operarions per second. Jeremy is speaking about seeks but it is only part of the latency. Disk rotation is another big contibutor. Even if we stay on the same track we’ll be only able to do single disk write per rotation, which is about 250 for 15000 RPM drive. If you’re getting more without battery backed up cache something is likely faking it.
I also should mention this covers only Master overhead of Replication. Slave is where you can expect more problems, especially on high end systems. Slave serializes all operations to single stream so group commit can’t work. Furthermore even if you run with innodb_flush_logs_at_trx_commit=2 you still do not solve all problems. You still will practically have only One CPU to handle replication load. Furthermore if you have multiple disks they will not be used effectively as replication SQL thread will typically submit IO requests to the disk one by one so only one drive will do the work and other will be idle. You mostly really affected by this if you have a lot of writes to large database size.
This is one more reason why you might want to go with scale out and keep single server a box with few CPUs and few hard drives while running MySQL.