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meobeo u

Joined: Feb 14, 2019
Messages: 1
Hi, I'm new to QRecall. I've downloaded the software, and used it a couple of times to backup to an archive using trial keys. It seems very full-featured and robust. I am wondering, though, about whether QRecall detects or protects me in the event that a file goes bad on the source disk I'm backing up; I recall seeing some articles a few months back about "bit rot," the possibility that old, seldom accessed files will silently fail by flipping a bit here or there. Since these files aren't accessed in normal use, the corruption lies possibly undetected for a long time, until one day you access it and you get a disk error, or your JPEG shows artifacts. I know that btrfs and ZFS maintain integrity on source disks by storing extra redundant data; I'm hoping that QRecall might give me some of that robustness and peace of mind, albeit in slices of backups.

So, I was wondering if QRecall reads entire source files (all the blocks that comprise file data) during the backup process, or just file meta data? If it reads entire files (of all files, not just files whose meta data indicated a change), what does it do when it detects that the underlying block data has changed, but the meta data hasn't changed?

James Bucanek

Joined: Feb 14, 2007
Messages: 1568

As a rule, QRecall isn't designed to protect against changes to your source files, because source files change all the time.

QRecall protects against data corruption by preserving older versions of your files for as long as possible. Its block-level data de-duplication and compression help in that mission by storing your files in as little space as possible, allowing you to keep more history. So if you do discover that a file is corrupted, you have a better chance of recovering an earlier, valid, copy.

There are utilities that will calculate checksums of your file. You can then periodically use that information to detect if any data in your files has changed.

On the other side of the equation, it's a completely different story. QRecall archives have multiple layers of interlocking data integrity checks, so that even a single corrupt bit of data in an archive will be detected. These integrity checks are verified during every operation. It also has the option of storing redundant data so that modest amounts of data corruption can be corrected and recovered.

I hope that answers your questions.

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