Query re: rolling checksum algorithm of rsync

Wayne Davison wayned at samba.org
Thu Feb 10 08:40:12 GMT 2005

On Tue, Feb 08, 2005 at 11:31:59PM -0800, Naveen Athresh wrote:
> Does this rolling for every byte addition and removal process slow
> down the speed of rsync and cause any sort of a latency in incremental
> backups

The purpose of the rsync algorithm is to trade increased CPU use and
increased local disk I/O (on the receiving side) in exchange for
reducing the data sent over the socket.  As long as the net connection
is the bottleneck, the rsync algorithm should result in a faster
transfer.  When it is not, disabling the algorithm (using --whole-file)
can be faster.

Certainly checksumming new data that has no match in the basis file uses
CPU with no reduction in the data sent, but rsync has no way of knowing
there will be no matches without actually doing the checksums.  The
speed of the transfer can thus suffer if the computers involved start to
run low on available CPU cycles.

I'm not sure what you mean by "latency in incremental backups", though,
so I don't know if that fully answers your question.


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