hi,
On Mon, Mar 25, 2002 at 09:50:14PM -0600, Derek James Witt wrote:
> Hey all.
>
> I'm going through the kernel source for XFS and I'm trying to find out
> how many superblocks a partition usually has and the maximum it's
> allowed to have.
There is one superblock at the head of every allocation group.
xfs_info/xfs_db/... will show you how many allocation groups a
filesystem has. There is one primary superblock, in the first
AG (at offset zero); and any number of secondary superblocks -
the number of AGs is a mkfs time option.
The primary superblock is the one the kernel uses at all times.
The secondary superblocks are usually only written at mkfs time,
and are read by xfs_repair when reconstructing the primary SB.
> So far in xfs_mount.h, I'm seeing that we are allowed
> two superblocks. I am proposing that we have two consecutive primary
> superblocks (duplicates) in use. In other words, mirrored blocks.
I'll leave it for others to judge the relative merits of this,
my first thought is that it seems overly complex when the answer
should be "don't overwrite the SB in the first place".
> So, if lilo or any other boot loader comes along and overwrites a part
> of the first superblock, the mounting code can still get at the 2nd sb.
I think the operational model here should remain "run xfs_repair"
once you've blown your foot off... but, just my 2c.
> But, the problem is that the size of the superblock I've seen varies
> from 512 bytes to 65535 bytes. In lilo's sources, we got blocks of size
> 1024 and sector sizes of 512. Any ideas on what to do on this?
The XFS superblock fits within 512 bytes.
cheers.
--
Nathan
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