Using rsync for machine replication?
Yesterday I was at a client site where they explained they
wanted to keep a stand-by server up to date and ready to take over
in case of main system failure. Fine, lots of people do that, and
currently they are doing it by restoring backups every morning.
What they were asking about was using rsync or some other mechanism
to keep the machines more current.
My first reaction was to question them about their app: it's
apparently a mess of Basic programs that work on hundreds of
different data files 24 x 7. I asked if they shut down the users
for backup. They don't. I explained that the backup can't really be
guaranteed consistent if users are writing data to files because
obviously files are going to be backed up while they are being
written to. As the files are related to each other (A/R header and
detail files, indexes, etc.) you can have inconsistent versions on
the backup media. . Somebody told them (obviously incorrectly) that
rsync could prevent this. There are databases that will let you
replicate data while you run, but that's application level, and
this app has no such ability. Rsync can't do any better than a
backup for that.
I talked about snapshots, and asked if they could shut down
users for the very brief time it takes to do that. Nope. Can't ever
stop the flow of data. I explained that rsync can't answer that
problem any better than a tape backup can: files and indexes may be
inconsistent with each other.
I also wonder if rsync's rolling checksum might even make things
better or worse. On the one hand we get less data transferred as
opposed to just a rcp or whatever, but open files that may be
getting writes during the checksum make things even more
They can't change the app. They can't shut down users. Their
bank says they have to have better disaster recovery. These things
seem impossible to reconcile. My feeling is that if the banks
demands have to be met, then the user community HAS to put up with
periods where they can't use the app. If they use snapshots, that
period can be brief, otherwise its going to be fairly long (13 GB
of data to transfer).
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