Testing exabyte-sized filesystems using qcow2 and guestfish

You can use qcow2 backing files as a convenient way to test what happens when you try to create exabyte-sized filesystems. Just to remind you, 1 exabyte is a million terabytes, or a pile of ordinary hard disks stacked 8 miles high.

There is a bug in qemu that prevents you from creating very large disks unless you adjust the cluster_size option (thanks Kevin Wolf):

$ qemu-img create -f qcow2 huge.qcow2 \
      $((1024*1024))T -o cluster_size=2M
Formatting 'huge.qcow2', fmt=qcow2 size=1152921504606846976 encryption=off cluster_size=2097152 lazy_refcounts=off 

After that you can just attach the disk to guestfish and start playing with huge filesystems.

[I should note that virt-rescue is probably a better choice of tool here, especially for people who need to experiment with unusual filesystem or LVM options]

$ guestfish -a huge.qcow2

Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

><fs> run
><fs> blockdev-getsize64 /dev/sda
1152921504606846976
><fs> part-disk /dev/sda gpt

Ext4 (according to Wikipedia) is supposed to support 1 exabyte disks, but I couldn’t get that to work, possibly because there was not enough RAM:

><fs> mkfs ext4 /dev/sda1
libguestfs: error: mkfs: ext4: /dev/sda1: mke2fs 1.42.5 (29-Jul-2012)
/dev/sda1: Not enough space to build proposed filesystem while setting up superblock

XFS could create a filesystem, but I didn’t let it run to completion because it would need about 5 petabytes to store the filesystem metadata:

><fs> mkfs xfs /dev/sda1
[ disks churn for many minutes while qcow2 file grows
and grows and grows ... ]

LVM2 PVs are possible, but creating a VG requires us to adjust the extent size:

><fs> pvcreate /dev/sda1
><fs> vgcreate VG /dev/sda1
libguestfs: error: vgcreate:   PV /dev/sda1 too large for extent size 4.00 MiB.
  Format-specific setup of physical volume '/dev/sda1' failed.
  Unable to add physical volume '/dev/sda1' to volume group 'VG'.
><fs> debug sh "vgcreate -s 1G VG /dev/sda1"
  Volume group "VG" successfully created
><fs> lvcreate LV VG 1000000000
><fs> lvs-full
[0] = {
  lv_name: LV
[...]
  lv_size: 1048576536870912
}

Previously …

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