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[dm-devel] Re: How to handle >16TB devices on 32 bit hosts ??

On Jul 18, 2009  10:08 +1000, Neil Brown wrote:
>  It has recently come to by attention that Linux on a 32 bit host does
>  not handle devices beyond 16TB particularly well.
>  In particular, any access that goes through the page cache for the
>  block device is limited to a pgoff_t number of pages.
>  As pgoff_t is "unsigned long" and hence 32bit, and as page size is
>  4096, this comes to 16TB total.
>  I suppose we could add a CONFIG option to make pgoff_t be 
>  "unsigned long long".  Would the cost/benefit of that be acceptable?

I think the point is that for those people who want to use > 16TB
devices on 32-bit platforms (e.g. embedded/appliance systems) the
choice is between "completely non-functional" and "uses a bit more
memory per page", and the answer is pretty obvious.

For users who don't want to support this, they don't have to (just
like CONFIG_LBD or whatever), and for 64-bit systems it is irrelevant.
I think years ago we had the idea that it would be 64-bit everywhere
by now, and while that is true for many systems, embedded/appliance
systems will probably continue to be 32-bit for as long as they can.

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

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