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@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
