> Sorry to waste bandwith with this only partially relevant topic, but I
> have some questions regarding MD:
> Assuming MD exists between the device layer and the file system layer,
> how do you ensure that important information (like copies of
> superblock) is stored across the disks? The way you (S.Shapiro)
> describe it, if you store a copy of the superblock every 2^n blocks,
> there is a fair chance all the superblocks get put on *one drive only*
> with striping, and when this drive gets...uh...run over by a truck or
> something, salvaging the file system is going to be difficult. Is
> some measure taken to prevent this?
Hmm. I have pretty extensive experience with the 'md' driver on suns.
They recommend at least three copies of the 'critical' data be instituted
on at least three different spindles.
This 'meta-meta-data' contains the state of the meta-disks. Does the 'MD'
driver do something similar?
At boot-time, if at least two copies of the meta-meta-data do not agreen
the md is assumed to be hopelessly corrupted and will not mount. There
are ways to force a recovery (tricks with dd).
> There's also the performance issue, but that appears to be slightly
> more complex, so I won't even try :-)
Just my 2c.
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