Re: replication consistency checking - Mailing list pgsql-admin

On Fri, Jun 5, 2015 at 4:56 PM, Scott Ribe <scott_ribe@elevated-dev.com> wrote:

On Jun 5, 2015, at 8:42 AM, Igor Neyman <ineyman@perceptron.com> wrote:
>
> The problem I see with “checksum utility” is that for it to work both compared servers should be “static”:  not transactions while it does its job.

Indeed, and that was brought up before and OP seems to be ignoring it. What magic does MySQL (supposedly) use to compare databases without interfering with updates?

One could imagine a built-in feature in PG which depends on using MVCC and having both sides look at the same snapshot. (Which would require repeatable reads.)

But for an external utility, that's quite a bit harder. One suggestion which would involve minimal interruption to processing: if you have a snapshottable file system, shut down master, take snapshot, shut down replica, bring master back up, snapshot replica, bring it back up. You *still* have the issue of making sure that at the moment you take master down all changes have been streamed to replica, and the (easier) issue of making sure replica has applied them before taking its snapshot... But if you can manage to pull that off, then you can checksum & compare as much as you want, *IF* you can actually pull that off correctly ;-)

Hello,
I wasn't talking about a static check, indeed I was referring to an online tool.

I haven't read the sources for the MySQL tool yet, however it computes the checksum on the master, writes the operation to the binary log and while using statement replication, the slave computes the checksum - then those can be compared. Not all data in the table are checksummed at once, but smaller chunks are used instead.

As Igor mentioned before, that tool can also fail and is not 100%. But I suppose if it would be misbehaving, chances are the checksums will differ and you will notice it. It will probably not by accident compute the same crc/md5 whatever.

I have done some tests with it, comparing data after setting up replication, all went fine, then I changed some integer on the slave, it really computed different checkum as expected. I also did a logical dump compare, it really seems to be doing its job fine.