Backing up large unchanging files

[Tim Bishop <tim-lists@bishnet.net>]

I'm backing up some large unchanging files (web server logs). Aside from
the current log, they mostly are unchanging on a daily basis. As per
recommendations I've not compressed these files which gives Tarsnap the
best chance to deduplicate and compress.

But, the problem is that Tarsnap is reading these files every day in
their entirety. I guess it has to so it can identify changed blocks, but
this is making the backup take a long time and creates a fair amount of
I/O. And aside from the monthly log rollover, these files haven't
changed from one day to the next.

Are there any useful tips for how to handle this situation? I've
pondered looking at the nodump flag on the rotated log files, and making
sure that Tarsnap usually skips these files, except after they've
rotated. Or I could just compress them - they'd compress really well and
speed up the backups significantly, but I'd lose the benefits of
Tarsnap's deduplication as a result.

Tim.