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Archiving the archive  XML
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john hampson



Joined: 13-Apr-07 13:09
Messages: 22
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There's something in me which is always paranoid about accepting an archive which is in some binary format, i.e. is not immediately viewable as a collection of files. My worry is that archive corruption may cause me to lose data.

It would be great if you could burn a copy of the archive to DVD's, kind of like a restore to DVD. If the archive is greater than 4GB then it could split it up into a series of images. I'd be happy to see a collection of dmg files that could be burned to DVD.

Kind of like (for a 12Gb archive)

work_1.dmg
work_2.dmg
work_3.dmg

Then once a month I could make a permanent copy of the archive.

Is this a crazy idea?
James Bucanek



Joined: 14-Feb-07 10:05
Messages: 1548
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It's a great idea. I do something similar myself, occasionally making a copy of my important archives to a second hard disk.

There's nothing stopping you from splitting an archive across multiple CDs/DVDs. This could be accomplished via the built-in command-line tools, and I believe there are several free and commercial GUI applications that will do this too. For instance, Toast 8 has an automatic disc spanning feature.

Should you need to restore from your discs, just reassemble the archive and recover the files.

Having QRecall do this itself is a feature I've considered adding.

For the technically minded, a QRecall archive is actually a package containing several files. The only file that really needs to be preserved is the 'repository.data' file. All of the others can be recreated by reindexing the archive package.

One footnote: The data in an QRecall archive is probably safer than a loose collection of files on a volume. QRecall archives have multiple layers of integrity checks that regular volumes lack. If file data or directory structures on a volume are damaged, there is little in the way of sanity checks to detect the problem, and virtually no tools that can reliably reconstruct the original data. QRecall archives were designed with both of these goals in mind.

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Bruce Champ



Joined: 07-Feb-08 21:28
Messages: 1
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James Bucanek wrote:It's a great idea. I do something similar myself, occasionally making a copy of my important archives to a second hard disk.

There's nothing stopping you from splitting an archive across multiple CDs/DVDs. This could be accomplished via the built-in command-line tools, and I believe there are several free and commercial GUI applications that will do this too. For instance, Toast 8 has an automatic disc spanning feature.

Should you need to restore from your discs, just reassemble the archive and recover the files.


It would be nice to have a secondary archive stored on DVDs. It would make for easy offsite storage. The difficulty would be keeping it current. Certainly, Toast could split the archive among multiple DVDs. But one would have to start over each time you wanted to add to the DVD archive. This would lead to a proliferation of DVDs being used. It would be a drag to have to burn the full set of DVDs each time you wanted to create a complete offsite backup. This is one thing Retrospect handles fairly elegantly -- DVDs can be added to as if they are big floppies. This is the only thing that has kept me from abandoning Retrospect.

I can't think of a good way to implement this in QRecall, but it would be a nice feature.
sjk



Joined: 13-Feb-08 16:20
Messages: 6
Location: Eugene
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James Bucanek wrote:One footnote: The data in an QRecall archive is probably safer than a loose collection of files on a volume. QRecall archives have multiple layers of integrity checks that regular volumes lack. If file data or directory structures on a volume are damaged, there is little in the way of sanity checks to detect the problem, and virtually no tools that can reliably reconstruct the original data. QRecall archives were designed with both of these goals in mind.

I can see tradeoffs with QRecall's monolithic archive vs. loose collections of files, and appreciate QR being thoughtfully designed for the former. Without understanding all the technical implications, I'm curious if the QR archive might be compared to using a disk image file as far as damage vulnerability and recovery salvaging is concerned. Obviously redundancy is worthwhile to minimize irrecoverable data lossage, which is the main topic here.

Too soon to know how I'd handle that with QR and my current two-Mac home setup, though I suspect it would be easier if each system uses its own archives instead of a fancier shared archive configuration. For simple on-site redundancy, I'd previously kept copies of one system's disk image encapsulated backups on an external hard drive of the other system. With QR, I can imagine similarly copying redundancy-desired archives between systems. Question I'd have with spanning them across multiple DVDs is:

What happens if any of that media develops errors; could a usable archive still be reconstructed?

This message was edited 1 time. Last update was at 16-Feb-08 20:16

James Bucanek



Joined: 14-Feb-07 10:05
Messages: 1548
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sjk wrote:What happens if any of that media develops errors; could a usable archive still be reconstructed?

"Usable" is a subjective term. Basically, QRecall's Repair command can reconstruct whatever valid data remains in an archive.

The archive data structures were designed specifically with the goal of being able to recover as much valid data from a damaged archive as possible. There are also numerous integrity checks to ensure that all files recovered by the Repair command are valid.

For example, unlike a regular file system, each directory record in a QRecall archive store its complete path. This allows a directory to be reconstructed and placed in its proper place in the file system hierarchy even when all of the directory information for its parent directories has been lost.

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sjk



Joined: 13-Feb-08 16:20
Messages: 6
Location: Eugene
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James Bucanek wrote:"Usable" is a subjective term.

Was there a more objective one for describing what I meant?

Thanks for the explanation. That kind of information helps me better understand certain issues (e.g. data integrity) I'll be questioning while evaluating QRecall. I want confidence I can trust it enough for my intended usage, and most OS X backup/archive utilities fail to satisfy some necessary criteria.
James Bucanek



Joined: 14-Feb-07 10:05
Messages: 1548
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sjk wrote:
James Bucanek wrote:"Usable" is a subjective term.

Was there a more objective one for describing what I meant?

I have no idea.

When people say "usable" it's hard to know if they mean it in a technical way ("I can open and view the contents of an archive") or an subjective way ("I can recover all of my damaged files").

You're clearly a savvy enough user to know that if data in an archive has been lost due to media failure, there's simply no way of getting it back. But I can't always assume that. Some people might think that the Repair command should simply fix the damaged drive and recover all of the lost data!

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sjk



Joined: 13-Feb-08 16:20
Messages: 6
Location: Eugene
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I'm curious to know a bit more about what the Verify and Repair commands actually and will eventually start a new thread about that instead of taking this one any further off-topic.

This message was edited 1 time. Last update was at 21-Feb-08 08:27

 
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