It is now possible to pre-compute delta copying state for large files when taking over existing backups with Bvckup 2.

The way the delta copying works is that it relies on block hashes saved from the last file update to detect which parts of the file changed since then and copy over only these modified blocks.

This means that the actual delta part of the copying kicks in only on the second update, because when we are copying file for the first time, we don't have the block hashes yet. Once the file is copied, we have the hashes and all set to do the delta updating.

This inability to detect block changes on the first run is not a big deal. Most of the times we are adding new files to a backup and these need to be copied in full anyway, so we do just that and initialize their delta state at the same time.

However, when taking over existing backups, it presents a slight problem.

Consider the case when you spent a day copying a very large file over a slow link to an off-site storage. Then, you created a backup job to handle all further updates.

If you are to touch the file now and run the backup, there'll be no block hashes yet, so the file will be re-copied in full. If only we could somehow initialize the delta copying state without copying a file...

Enter delta state pre-comp

Release 79.13 adds support for pre-computing block hashes for all files in the backup that qualify for delta copying but not yet have the delta state.

That is, if you put an existing backup under Bvckup's control, you can now flip a switch, do a quick run and then have all files ready for delta copying on their first update.

Quick here assumes that your source is local, reads are cheap and writes are expensive, which is how the setup should be for making full use of delta copying benefits.

The screencap above shows taking over a Debian VM backup that is initially created by just copy-pasting VM folder in Windows Explorer.

Running a backup executes delta pre-comp for .vmdk files and patches up "created" times on backup items - all without writing a single byte to destination.
Made by IO Bureau in Switzerland

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