Mirroring Solution

[Eric W. Bates]

----- Original Message ----- 
From: "Scott Gerhardt" <scott at g-it.ca>
To: <freebsd-isp at freebsd.org>
Sent: Saturday, June 07, 2003 2:46 PM
Subject: Mirroring Solution


> 
> Does anyone know of a good way to mirror two servers running Apache, PHP and
> MySQL on FreeBSD 4.8?
> 
> The ideal solution would be real-time (synchronous) or near-real-time
> mirroring.  The failover transfer is already taken care of.
> 
> I think Rsync run from cron would do a good job of syncing the relatively
> static files (configs, php scripts etc.).  I don't think Rsync would be a
> good way of mirroring the MySQL DB though.  For that, I'm thinking of using
> MySQL's built in replication.

You are correct.  Syncing live mysql data files is a bad idea. At any given instant they cannot really be considered to be in a consistent state because so much information is in memory.  If you want to physically copy files, mysqlhotcopy does a pretty good job.  However, we have found that using replication (v 4's circular replication is a big improvement) allows us to run mysqlhotcopy on the backup machine rather than the live one (we do a hot dump every 15 minutes). If you have to restore from tape, you will need a dump.

NB: replication is clear-text. (the mysql v4 docs say "no ssl for replication yet")  We use stunnel to obscure things.

There is a problem with rsync when the list of files to replicate grows too long.  We never really nailed it down; but if you attempt to rsync /usr, it will probably fail.  What we see is the sending version of rsync eventually timing out while waiting for the receiving copy; meanwhile the receiving copy ends up hung with no visible errors.  Our work-around was a little meta script with a config file listing which trees should be synced between which machines; and it runs smaller rsync jobs in sequence.  Oddly enough, we ran into similar failure modes with rdist and unison.

> I think dumping the MySQL data and importing into the slave would be a bit
> clumsy and slow especially when the DB grows quite large.
> 
> Does anyone on the list have any recommendations or experience with
> mirroring of this type?
> 
> Any other recommendations besides Rsync + MySQL replication?
> 
> 
> 
> Thanks,
> 
> 
> -- 
> Scott Gerhardt, P.Geo.
> Gerhardt Information Technologies [G-IT]
> 
> 
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