linux-kernel - Re: What to do about the 2TB limit on HDIO

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Message-ID: <20080325205117.GP16358@mit.edu>
Date:	Tue, 25 Mar 2008 16:51:17 -0400
From:	Theodore Tso <tytso@....EDU>
To:	Ric Wheeler <ric@....com>, Matthew Wilcox <matthew@....cx>,
	Mark Lord <lkml@....ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Jeff Garzik <jgarzik@...ox.com>,
	Tejun Heo <htejun@...il.com>, Greg KH l <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: What to do about the 2TB limit on HDIO_GETGEO ?

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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