Re: [tlug] Raid5 box & backup

[Curt Sampson <cjs@example.com>]

On 2008-05-26 09:39 +0900 (Mon), Edmund Edgar wrote:

> > So a TrueCrypt volume is just layered over top of a
> > standard filesystem,
> 
> Yes - it looks like a file until you mount it with TrueCrypt.
> ...
> No - it looks like a file full of junk to someone not using TrueCrypt.
> Loop-mounting the file full of junk....

Ok, this is just the usual Linux semantic confusion, probably caused by
the usual "I don't understand operating systems" approach to designing
Linux userland tools.

What appears to be the case (correct me if I'm wrong) is this: A
TrueCrypt volume is *not* layered over a standard filesystem; it's just
another filesystem, like ext2fs or whatever, that's layered over a
*block device*. The so-called "loopback mount" is actually a standard
mount operation preceeded by a completely orthogonal creation of a block
device backed by a file. (These are separate operations in NetBSD, for
obvious reasons. E.g., what if the file is the image of a hard disk
containing several partitions?)

This has direct implications for how one would build the kind of thing
we were talking about, by the way. At the very least, it might have
you creating an unnecessary filesystem and block device between your
base block device and the encrypted FS, with at least some effect on
performance.

> 'Cos if I've understood you right, conceptually it's what you're
> describing here: http://www.tlug.jp/ML/0805/msg00336.html

Yes, it appears to be, though note that in my proposal you do not use
a loopback mount for the encrypted FS. (That's why I was asking if it
was a standard filesystem (i.e., runs over a block device) or something
odd.)

The layering of what I proposed is the following:

    encrypted FS	(filesystem created on/mounted from block device below)
    Client iSCSI device	(virtual block device backed by iSCSI mount)

    iSCSI server	(servers blocks from device below over network)
    virtual disk	(block device backed by file below)
    file		(in filesystem below)
    ext2fs		(filesytsem, mounted on server's disk below)
    Server's disk	(block device)

cjs
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Curt Sampson       <cjs@example.com>        +81 90 7737 2974   
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