From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: How to handle >16TB devices on 32 bit hosts ?? Date: Tue, 21 Jul 2009 23:59:04 -0700 Message-ID: <20090721235904.42e6cd35.akpm@linux-foundation.org> References: <19041.4714.686158.130252@notabene.brown> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-fsdevel@vger.kernel.org, linux-raid@vger.kernel.org, dm-devel@redhat.com, linux-kernel@vger.kernel.org To: Neil Brown Return-path: In-Reply-To: <19041.4714.686158.130252@notabene.brown> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com List-Id: linux-fsdevel.vger.kernel.org On Sat, 18 Jul 2009 10:08:10 +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 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.