A space-efficient guest filesystem for grow-as-needed virtual disks?

Question

A common practice is to use non-preallocated virtual disks.

Since they only grow as needed, it makes them perfect for fast backup, overallocation and creation speed.

Since file systems are usually based on physical disks they have the tendency to use the whole area available¹ in order to increase the speed² or reliability³.

I'm searching a filesystem that does the exact opposite : try to touch the minimum blocks need by an aggressive block reuse.

I would happily trade some performance for space usage.

There is already a similar question, but it is rather general. I have very specific goal : space-efficiency.

_{1. Like page caching uses all the free physical memory}
_{2. Canonical example : online defragmentation}
_{3. Canonical example : snapshotting}

I would think that this approach (at least under vmware) would cause your redo logs to grow at a larger than normal rate — Jim B, May 07 '10 at 17:28
Are you talking about the filesystem on the host or in the guest? — Kamil Kisiel, May 07 '10 at 20:27
I'm talking about the guest filesystem (as the title implied) — Steve Schnepp, May 08 '10 at 07:15

user38808 · Answer 1 · 2010-05-07T17:09:47.990

If you don't mind being on the edge, you can do this with the new btrfs filesystem. The backup target device will occupy no more space than is used by the copied data.

https://btrfs.wiki.kernel.org/index.php/Main_Page

Create a partition (using LVM logical volume or regular)
Format: mkfs -t btrfs
Mount it: mount -t btrfs /mnt/btrfs
Create a brts sub-volume: btrfsctl -S home_template /mnt/btrfs
Umount /mnt/btrfs and mount the subvol: mount -t btrfs -o subvol=home_template /mnt/template
Populate /mnt/template with files/folders if needed in the backup target
Make a snapshot of the home_template subvol for each user. This occupies 0 space until mounted and written to. btrfsctl -s /mnt/joeblow /mnt/template 8 Mount the snapshot as needed and let the backups begin: mount -t btrfs -o subvol=joeblow /mnt/backup

Is it for the guest fs also ? – Steve Schnepp May 15 '10 at 07:13 — Steve Schnepp, May 15 '10 at 07:13

score -1 · Answer 2 · answered May 06 '10 at 11:45

-1

you can also use LVM volumes, adding PVs as necessary to a VG in order to grow it, and using lvextend to grow LVs dedicated to VMs

answered May 06 '10 at 11:45

dyasny

18,482
6
48
63

But none of those is a filesystem. – Ignacio Vazquez-Abrams May 06 '10 at 11:47
That's why the word "also" is used. This approach allows better grained space management. The actual FS is in the VM anyway, not necessarily on the host – dyasny May 06 '10 at 12:32
the main purpose of the *grow-as-needed* is specifically to avoid this kind of space micro-management. – Steve Schnepp May 06 '10 at 17:04
actually, vdsm does exactly that - lvextend as required. – dyasny May 06 '10 at 18:59

A space-efficient guest filesystem for grow-as-needed virtual disks?

2 Answers2