On Fri, Mar 15, 2024 at 02:48:27AM +0000, Matthew Wilcox wrote: > On Fri, Mar 15, 2024 at 12:14:05PM +1100, Dave Chinner wrote: > > On Thu, Mar 14, 2024 at 05:12:22PM -0700, Luis Chamberlain wrote: > > > Joining two 8 EB files with device-mapper seems allowed: > > > > > > truncate -s 8EB /mnt-pmem/sparse-8eb.1; losetup /dev/loop1 /mnt-pmem/sparse-8eb.1 > > > truncate -s 8EB /mnt-pmem/sparse-8eb.2; losetup /dev/loop2 /mnt-pmem/sparse-8eb.2 > > > > > > cat /home/mcgrof/dm-join-multiple.sh > > > #!/bin/sh > > > # Join multiple devices with the same size in a linear form > > > # We assume the same size for simplicity > > > set -e > > > size=`blockdev --getsz $1` > > > FILE=$(mktemp) > > > for i in $(seq 1 $#) ; do > > > offset=$(( ($i -1) * $size)) > > > echo "$offset $size linear $1 0" >> $FILE > > > shift > > > done > > > cat $FILE | dmsetup create joined > > > rm -f $FILE > > > > > > /home/mcgrof/dm-join-multiple.sh /dev/loop1 /dev/loop2 > > > > > > And mkfs.xfs seems to go through on them, ie, its not rejected > > > > Ah, I think mkfs.xfs has a limit of 8EiB on image files, maybe not > > on block devices. What's the actual limit of block device size on > > Linux? > > We can't seek past 2^63-1. That's the limit on lseek, llseek, lseek64 > or whatever we're calling it these days. If we're missing a check > somewhere, that's a bug. Thanks, I can send fixes, just wanted to review some of these things with the community to explore what a big fat linux block device or filesystem might be constrained to, if any. The fact that through this discussion we're uncovering perhaps some missing checks is already useful. I'll try to document some of it. Luis
