Messages in this thread | | | Date | Tue, 21 Jul 2009 23:59:04 -0700 | From | Andrew Morton <> | Subject | Re: How to handle >16TB devices on 32 bit hosts ?? |
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On Sat, 18 Jul 2009 10:08:10 +1000 Neil Brown <neilb@suse.de> 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 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.
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.
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