[Box Backup-dev] Re: [Box Backup] bbackupd - read errors on database files

G. boxbackup-dev at fluffy.co.uk
Wed Apr 11 01:54:57 BST 2007


Chris,

>> I don't think that's the way to go. MD5s not only provide enhanced 
>> change detection, but also remote store content verification on every 
>> backup cycle.

>I'm afraid not, that's a different problem. We would store checksums of 
>the raw data, not the encrypted data, and so it would have to be 
>downloaded and decrypted by the client (just like compare -a) to verify it 
>properly. Since this is what compare -a already does, there would be no 
>benefit here.

The plain-text MD5 checksum in question (I mean one MD5 checksum per file, not one MD5 checksum per block) could become a part of a file's attribute stream, which is already decrypted and used for comparison of file attributes. It could also be used to generate/compare stronger folder-level checksum. So, you can just use such an MD5 directly to verify whether last-known file or folder content on the server matches file or folder content locally. Much less work than compare -aq, since there is no need to compare checksums on block-by-block basis, and there is no need to re-download MD5s in the first place (they are all already in-memory after a backup cycle, and could be preserved by StoreObjectInfoFile; if not, bbackupd already downloads all file attribute stream objects anyway the first time around).

(... do I remember this correctly, or did I screw up again...?) ;)

> Checksums of the encrypted data have been discussed, but that would 
> require new commands on the server to return the IV and checksum of each 
> block, which is more complex again.

Yup, you are absolutely right. I should have been more clear; I meant "remote checksum to local checksum" compare, not "remote checksum to remote disk content" (bbstored RAID-style integrity check) compare.

Gary






       
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