Re: Max theoretical XFS filesystem size in review

[Matthew Wilcox <willy@infradead.org>]

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.