Protection Against Silent Data Corruption / Bit Rot

[ChrisH]

Inspired by http://forum.bittorrent.com/topic/30920-active-distribuited-data-integrity-protection-raid-and-other-backup-obsolete/ but he explicitly stated there that this was not what he wanted:

 

BTSync could periodically re-hash the files in the shared folders. If the current hash differs from the hash in the database, but the file change date is still the original, this indicates a corrupted file (i.e. a bit flipped somewhere in the storage subsystem). Files found corrupted should then be re-synced from any online peer, provided that their hash still matches the original.

 

This should be a per-folder option. The re-hash interval can be configured and set quite high, like maybe once a month or so.

Use case: Archived files that are almost exclusively read-only (photos, movies, MP3s, ISOs) but are kept on disk for months and years, thus having a bigger risk of bit rot.

 

http://en.wikipedia.org/wiki/Data_rot

[piotrnik]

I also think this would be a really cool feature, especially since many of the items I've synced are exactly that - photos, movies, music, disk images, etc - that rarely change.  Due to cpu usage, I'd prefer a user-settable range, and I'd probably choose to do re-check every 2-3 months (probably up to 6 months, depending on the exact content).

 

I would also agree that it would be best per-folder, so that you could re-check important files more often, as well as not having to suddenly re-hash several TB of data at once. 

[AcostaJA]

Good to see you finally understand my feature request, do not need to duplicate.

Now you realize what is data integrity protection (bit rot is just an scenery). Now you understand too why it's so important to keep file versions all along.

[ChrisH]

Good to see you finally understand my feature request, do not need to duplicate.

No, I still don't understand what you want and frankly I don't care anymore.

 

Now you understand too why it's so important to keep file versions all along. 

File versions have nothing to do with this request.

[AcostaJA]

[this post has been removed]

Edited August 10, 2014 by GreatMarko
Post removed - personal attack against another forum member

[GreatMarko]

@AcostaJA - personal attacks against other contributors to these forums is not tolerated.

 

You feel that @ChrisH's suggestion is identical to your own, he feels differently. You are both entitled and welcome to contribute to these forums.

 

If moderators/administrators detect threads in this Feature Request forum that are identical or near identical to an existing thread, they may be merged accordingly.

 

@ChrisH specifically "forked" your original thread as he felt that his suggestion was distinctly different from your own.

 

Both of your contributions are welcome, and If we feel it necessary to merge them at some stage we will.

 

However, if they continue to descend into personal attacks against each other/each other's posts, they risk both being deleted all together.

[AcostaJA]

@AcostaJA - personal attacks against other contributors to these forums is not tolerated.

You feel that @ChrisH's suggestion is identical to your own, he feels differently. You are both entitled and welcome to contribute to these forums.

If moderators/administrators detect threads in this Feature Request forum that are deemed identical or near identical to an existing thread, they may be merged accordingly.

@ChrisH specifically "forked" your original thread as he felt that his suggestion was distinctly different from your own.

Both of your contributions are welcome, and If we feel the need to merge them at some stage we will.

However, if they continue to descend into personal attacks against each other/each other's posts, they risk both being deleted all together.

I don't want to start a battle here, it's good to read moderator are not absent.

[mrmachine]

Getting back to the feature request, is there any chance the BitTorrent Sync could propagate bit rot to other clients?

 

I have a ZFS NAS where I store most things, so I feel safe that it is protected against bit rot. But I want to sync some of its files to an iMac and a MacBook, which run JHFS+ file systems, which are susceptible to bit rot.

 

If my iMac or MacBook do suffer from bit rot, is there any way that BitTorrent Sync will detect that as a legitimate change and sync the corrupted file it back to the ZFS NAS and all other devices, thereby negating the bit rot protection provided by ZFS?

 

[trevellyan]

Unfortunately, this is entirely possible. Imagine a file has been silently corrupted on your iMac. You make a small change to that file on your iMac and don't notice the bit rot. When you save your changes, the corrupted file will sync back to your NAS.

[mrmachine]

So there's basically nothing that can be done about it... All devices need to have protection against bit rot for the pool to be safe?

[trevellyan]

Yes, and this is one reason why no synchronization solution should be seen as a backup solution. In your case one approach would be to backup the most important content from your NAS to another bit-rot resistant destination.

 

Note that Sync will not spontaneously propagate bit rot, it will only happen if you modify a file and trigger Sync to transfer it.

[mrmachine]

I don't use Sync as backup. I have CCC to an external bootable HDD and Arq (to the ZFS NAS) for that. But if Sync will propagate bitrot to the ZFS NAS, because ultimately all the machines I actually use daily don't have any protection against bitrot, then it's still a manual process of discovering the rot to begin with, and then searching incremental backups for the last good copy.

 

Doesn't Sync periodically index all its shares to determine if it is missing any files from the pool and sync them? I seem to recall with BitTorrent in general, if you modified some local data the torrent app would detect the block was not right and re-download it?

 

If during a reindex, Sync detected a mismatch, how would it know whether or not to propagate the change to other clients, or re-download the file from other clients?

 

For example, say I shut down Sync on one device, modified a file but left its file size and mtime the same. When I started Sync, I assume it would re-index all the shares. Would it "fix" my altered file by replacing it with data from other devices, or distribute the modified file to other devices?

[trevellyan]

Glad to hear you don't consider Sync to be a backup solution, my comment about that was meant to be general, not accusatory.

 

As with most synchronization and backup solutions, when Sync periodically indexes a share, it only looks for metadata changes (times, sizes). Anything else would be too resource intensive. Sync only looks at the content when it's actually preparing to propagate changes. As long as bit rot doesn't affect the metadata, it won't propagate spontaneously.

 

There are other solutions to the bit-rot problem involving the periodic generation of hashes on filesystem content - which is what ZFS has built-in.

[mrmachine]

Would you mind sharing what those other solutions are? I've recently talked with the creator of CCC and it offers some protection against bitrot on the destination volume, but still has the same problem of propagating corruption on the source to the backups.

[trevellyan]

I haven't used any myself, but there are lots listed here: http://en.wikipedia.org/wiki/Comparison_of_file_verification_software

[palobo]

I'd love to see this type of feature come to BT Sync. I'm sure that if it did, it would certainly rock and blow the competition away!

[rocket]

This is quite an important feature. I would not feel comfortable synchronising all that data across devices if a bit rot error were to propagate.

 

Even I would like to get in the source and make a fork just for this, but there is no source code as far as I know, only dev api.

[kotor610]

i know this thread has been dead for a while but i'd like to revive it. it would be a game changer if resilio sync incorporated a checksumming feature. right now the only way to ensure bit rot doesn't propagate is to have every device connected running a file system with built in redundancy (ZFS, BTRFS, ReFS). or some third party checksumming tool. while this is tolerable on personal machines that you oversee, its much more difficult to control on family, friends, and colleagues devices. even if we do all the heavy lifting of setting up a resilient file system it can all be undone a corrupted file on a peer.

 

this could be solved by an optional checksum option ("file integrity check") in resilio sync. so if a file is corrupted it can get fixed by a peer with the correct copy. a checksum file would be very small in comparison to the actual data,