On Fri, Feb 19, 2010 at 8:14 AM, Edward Ned Harvey <[email protected]>wrote:
> > > If you have rsyncd running on the remote machine instead of mounting
> > > it as a remote filesystem on the local client, then the rsync local
> > > client will communicate with the remote daemon, and they will each
> > > calculate their own respective checksums, which can then be compared.
> >
> > Actually, I believe this happens anyway.
> >
> > The normal mode of operation, where you use ssh/rsh to connect to a
> > remote host, invokes { rsync --server --sender } on the remote side.
>
> This was my understanding as well. That when I use rsync over ssh, it
> would
> create a new single-use rsync server at the remote side, which should
> theoretically allow the local rsync to read the local file, and the remote
> rsync to read the remote file, so the two can be diff'd at DAS speed
> without
> transmitting the whole file across the network. However ...
>
> When I did this, my initial rsync to send the whole file took 30 minutes.
> Then I changed 1M in the middle of the sparse file, and did an incremental.
> I expected it to complete in 6-7 minutes. I waited an hour and cancelled
> the process because obviously it wasn't working. I tried several
> variations
> of switches, and consulted the rsync discussion list, google, man pages,
> etc. So far I haven't had any success at this...
>
> During those failed tests, which take 2x longer or more than doing a full
> image, I only checked the time. I did not monitor the network to see if
> the
> file was flying across the wire. So I don't know what it was doing or why
> it was taking so long.
>
> It seems to be worth while, to try updating to the latest rsync, and to try
> starting rsyncd at the remote side, to see if it will behave better than
> what I've seen. Thanks for the suggestions, I'll plan to give that a try.
>
Running rsync as a daemon would give you the same results as running it via
ssh, as you are right that the remote side runs a copy of rsync and they
talk to each other.
The other usage scenario Brad was talking about is if you had the remote
filesystem mounted via NFS or something on the local machine. If you were
then to run rsync in that scenario, rsync would be reading the remote file
as if it were a local file over NFS, and that would cause the whole file to
be transferred.
As I've already said, the goal of rsync is to get lower bandwidth use at the
expense of CPU and possibly time. If that is not your goal, then rsync may
not be the right tool for this.
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