Re: two quick conceptual questions RE: rsync (and rsyncing snapshots)

From: Chuck Swiger (cswiger_at_mac.com)
Date: 11/01/05

  • Next message: user: "Re: two quick conceptual questions RE: rsync (and rsyncing snapshots)" 
    Date: Tue, 01 Nov 2005 16:15:50 -0500
To: user <user@dhp.com>

    user wrote:
    > First, how does rsync respond to, and perform, when the source filesystem
    > is under very heavy change ? If I have a filesystem that I want to rsync
    > up to a backup server, but that filesystem is _very busy_ with the
    > creation, destruction and changing of files, how well does rsync perform,
    > and how much does it interfere with the performance of the underlying
    > filesystem that it is sending up to the backup server ?

    rsync complains when the filesystem changes underneath it, but it will continue
    to run. On the other hand, rsync is not going to safely maintain the
    referential integrity of a complex file like a live database, but it's okay for
    most other things including mbox's.

    Rsync imposes a significant workload if you are syncronizing a large tree of
    stuff which changes a lot, but it's efficient considering the size of the task.

    > Related: it occurs to me that perhaps it would be better to snapshot the
    > filesystem, mount the snapshot, and then rsync the snapshot. On the other
    > hand, the filesystem is continuously altering the snapshot as files are
    > destroyed or changed ... so perhaps this does not gain anything. Is
    > rsyncing a snapshot of a busy filesystem always, ever or never easier than
    > rsyncing the busy filesystem itself ?

    rsync'ing a snapshot is a fine idea.

    > Finally, am I correct that there are _only two_ rsync comparison methods -
    > the default checksum method, and the --size-only method ? Am I correct
    > that rsync _always_ looks at the timestamp first, and then applies either
    > checksum or size comparison ONLY IF the timestamps are different ?

    No, rsync checks both timestamp and size or checksum. 

    -- 
-Chuck
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