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On Thu, Sep 13, 2007 at 02:27:21PM +0100, Tom Potts wrote: > On Thursday 13 September 2007 08:53, Robin Cornelius wrote: > > Hi, does anyone know anything about/recommend/warn away from services > > like rsync.net for off site backups. It appears pretty flexible and > > allows you to use various encryption etc to protect your data. > > > > Another question is there something i can extend rsync with or some > > other backup tool so that it knows what local data has changed since > > last backup, eg it maintains its own local list. My reasoning is that > > it seems crazy comparing GBs of files over the net when often only a > > few MBs have changed and a simple local list could save a lot of time > > and bandwidth. > Not sure but I think it uses times and sizes so if they don't change..... According to the technical report (/usr/share/doc/rsync/tech_report.tex.gz), it splits the file into blocks, checksums each block, then compares the checksums of each file; then only the parts of the file that don't exist on the receiving host are sent over the network (plus some metadata that tells the receiving host how to construct the new file from the old file and the newly-sent pieces). Also, if you specify the `--update` option, it will ignore files with the same or newer modification time. I don't know if it pays any attention to time and size otherwise (the file could be completely different but still have the same size, for example). -- Benjamin A'Lee <bma@xxxxxxxxxxxxxx> http://subvert.org.uk/~bma/ "The Constitution gives every American the inalienable right to make a damn fool of himself." - John Ciardi
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