XFS reflink vs ThinLVM

[Gionatan Danti <g.danti@assyoma.it>]

Hi all,
as RHEL/CentOS 8 finally ships with XFS reflink enabled, I was thinking 
on how to put that very useful feature to good use. Doing that, I 
noticed how there is a significant overlap between XFS CoW (via reflink) 
and dm-thin CoW (via LVM thin volumes).

I am fully aware that they are far from identical, both in use and 
scope: ThinLVM is used to create multiple volumes from a single pool, 
with volume-level atomic snapshot; on the other hand, XFS CoW works 
inside a single volume and with file-level atomic snapshot.

Still, in at least one use case they are quite similar: single-volume 
storage of virtual machine files, with vdisk-level snapshot. So lets say 
I have a single big volume for storing virtual disk image file, and 
using XFS reflink to take atomic, per file snapshot via a simple "cp 
--reflink vdisk.img vdisk_snap.img".

How do you feel about using reflink for such a purpose? Is the right 
tool for the job? Or do you think a "classic" approach with dmthin and 
lvm snapshot should be preferred? On top of my head, I can thin about 
the following pros and cons when using reflink vs thin lvm:

PRO:
- xfs reflink works at 4k granularity;
- significantly simpler setup and fs expansion, especially when staked 
devices (ie: vdo) are employed.

CONS:
- xfs reflink works at 4k granularity, leading to added fragmentation 
(albeit mitigated by speculative preallocation?);
- no filesystem-wide atomic snapshot (ie: various vdisk files are 
reflinked one-by-one, at small but different times).

Side note: I am aware of the fact that a snapshot taken without guest 
quiescing is akin to a crashed guest, but lets ignore that for the moment.

Am I missing something?
Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
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