On Sat, 18 Jul 2009 10:08:10 +1000 Neil Brown <neilb@xxxxxxx> 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. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
