Re: [pve-devel] [PATCH docs] lvm: add note about requirements of snapshot-as-volume-chains

["DERUMIER, Alexandre via pve-devel" <pve-devel@lists.proxmox.com>]

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From: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
To: "pve-devel@lists.proxmox.com" <pve-devel@lists.proxmox.com>
Subject: Re: [pve-devel] [PATCH docs] lvm: add note about requirements of snapshot-as-volume-chains
Date: Sun, 27 Jul 2025 12:03:39 +0000
Message-ID: <d06d26c9f0c5a4c3a23057d2854c76a96714288b.camel@groupe-cyllene.com>

about discard, maybe we should add a note about enabling
issue_discards=1 in lvm.conf. (don't known if it could be enabled by
default for new install ?)


-------- Message initial --------
De: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Répondre à: Proxmox VE development discussion <pve-
devel@lists.proxmox.com>
À: pve-devel@lists.proxmox.com
Objet: [pve-devel] [PATCH docs] lvm: add note about requirements of
snapshot-as-volume-chains
Date: 25/07/2025 11:52:08

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
---

Notes:
    not sure whether we want to also add a note suggesting to (heavily)
    overprovision the LUN/backing device on the storage side, so that
there is
    enough "space" for creating snapshots?

 pve-storage-lvm.adoc | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/pve-storage-lvm.adoc b/pve-storage-lvm.adoc
index 6928330..c508fe1 100644
--- a/pve-storage-lvm.adoc
+++ b/pve-storage-lvm.adoc
@@ -57,8 +57,7 @@ snapshot's name and starts a new volume backed by the
snapshot.
 +
 A volume based on a snapshot references its parent snapshot volume as
its
 backing volume and records only the differences to that backing
volume.
-Snapshot volumes are currently thick-provisioned LVM logical volumes,
but the
-underlying block storage may provide thin provisioning.
+Snapshot volumes are currently thick-provisioned LVM logical volumes.
 +
 This design avoids issues with native LVM snapshots, such as
significant
 input/output (I/O) penalties and unexpected, dangerous behavior when
running out
@@ -74,6 +73,11 @@ functionality.
 The snapshot functionality is managed by the PVE storage system.
 +
 Enabling or disabling this flag only affects newly created virtual
disk volumes.
++
+For efficient support of `snapshot-as-volume-chain`, the backing
storage must
+support thin-provisioning and discard. Each snapshot will appear to
use the
+full volume size on the PVE side, but the actual space usage on the
underlying
+storage will be smaller if those requirements are met.
 
 .Configuration Example (`/etc/pve/storage.cfg`)
 ----

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