Re: What to do about the 2TB limit on HDIO_GETGEO ?

[Theodore Tso]

On Tue, Mar 25, 2008 at 12:47:50PM -0400, Theodore Tso wrote:
> 
> Well 2TB, assuming a 4k blocksize, means a block bitmap is 512 megs.
> So at least for ext3, 4GB should be just enough, unless you hit
> certainly really nasty complicated corruptions (i.e. large number of
> blocks claimed by more than one inode, which can happen if an inode
> table is written to the wrong location on disk --- on top of some
> other portion of the inode table), or if the filesystem has a large
> number of files with hard links (such as the case with certain backup
> programs).

Whoops, screwed up my math.  The block bitmap for a 2TB filesystem is
64 megs, not 512 megs.  2*41 / 2**12 / 2**3 == 2**26, or 64mb.  E2fsck
in the worst case will allocate 5 inode bitmaps and 3 block bitmaps,
plus various arrays for directory blocks and keeping track of
refcounts (which are optimized for counnts of 0 and 1, so lots of hard
links will blow up your memory usage, although we do have a tdb option
which helps in that particular case).  So I'd say that most of the
time 3GB of address space should really be enough for a 2TB raid
array, unless you get really pathalogical corruption cases.

							- Ted
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