On 2016-05-25 07:07, Hugo Mills wrote:
On Wed, May 25, 2016 at 04:00:00AM -0700, H. Peter Anvin wrote:
On 05/25/16 02:29, Hugo Mills wrote:
On Wed, May 25, 2016 at 01:58:15AM -0700, H. Peter Anvin wrote:
Hi,
I'm looking at using a btrfs with snapshots to implement a generational
backup capacity. However, doing it the naïve way would have the side
effect that for a file that has been partially modified, after
snapshotting the file would be written with *mostly* the same data. How
does btrfs' COW algorithm deal with that? If necessary I might want to
write some smarter user space utilities for this.
Sounds like it might be a job for one of the dedup tools
(deupremove, bedup), or, if you're writing your own, the safe
deduplication ioctl which underlies those tools.
I guess I would prefer if data wasn't first duplicated and then
deduplicated if possible. It sounds like I ought to write a "smart
copy-overwrite" tool for this.
I _think_ rsync --in-place may help here. IIRC, it'll only
overwrite the sections of files that have changed, rather than write
and replace the whole file. (I may be wrong about that, though. I
haven't tested it at that level).
This is absolutely correct, and I actually use rsync instead of cp on a
regular basis partly for this reason.
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