Looking for advice on rsync

From: Tony Lawrence (pcunix_at_gmail.com)
Date: 10/16/04 

Date: 16 Oct 2004 04:51:44 -0700

Yesterday I was at a client site where they explained that 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 confusing.

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).

Comments? 

-- 
Tony Lawrence
http://aplawrence.com
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