Re: How to handle >16TB devices on 32 bit hosts ??

[Andreas Dilger <adilger@sun.com>]

On Jul 21, 2009  23:59 -0700, Andrew Morton wrote:
> On Sat, 18 Jul 2009 10:08:10 +1000 Neil Brown <neilb@suse.de> wrote:
> I expect that the VFS could be made to work with 64-bit pgoff_t fairly
> easily.  The generated code will be pretty damn sad.
> 
> radix-trees use a ulong index, so we would need a new
> lib/radix_tree64.c or some other means of fixing that up.
> 
> The bigger problem is filesystems - they'll each need to be checked,
> tested, fixed and enabled.  It's probably not too bad for the
> mainstream filesystems which mostly bounce their operations into VFS
> libarary functions anyway.

I don't think this is a primary concern for most filesystems even today.
Filesystems that work correctly > 16TB on 64-bit platforms should continue
to work correctly on 32-bit platforms.  ext4 and XFS will be fine, and
we can slap a "refuse to mount > 16TB filesystem on 32-bit" check in
*_fill_super() for the other filesystems, ext3 included.  Maintainers can
veto that if they think it will work, and for the rest I don't think
anyone will even notice.

> There's perhaps a middle ground - support >16TB devices, but not >16TB
> partitions.  That way everything remains 32-bit and we just have to get
> the offsetting right (probably already the case).
> 
> So now /dev/sda1, /dev/sda2 etc are all <16TB.  The remaining problem
> is that /dev/sda is >16TB.  I expect that we could arrange for the
> kernel to error out if userspace tries to access /dev/sda beyond the
> 16TB point, and those very very few applications which want to touch
> that part of the disk will need to be written using direct-io, (or
> perhaps sgio) or run on 64-bit machines.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.