[PATCH v2 docs] storage: note that qcow2 internal snapshots are inefficient

[Fiona Ebner <f.ebner@proxmox.com>]

It's a commonly reported issue, often also in the enterprise support,
that taking or removing snapshots of large qcow2 files on file-based
network storages can take a very long time. Add a note about this
limitation.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v2:
* add recommendation about doing the operations offline

 pvesm.adoc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/pvesm.adoc b/pvesm.adoc
index c634f6b..97feca0 100644
--- a/pvesm.adoc
+++ b/pvesm.adoc
@@ -88,6 +88,12 @@ block device functionality.
 
 ^2^: On file based storages, snapshots are possible with the 'qcow2' format,
 either using the internal snapshot function, or snapshots as volume chains^4^.
+Creating and deleting internal 'qcow2' snapshots will block a running VM and
+is not an efficient operation. The performance is particularly bad with network
+storages like NFS. On some setups and for large disks (multiple hundred GiB or
+TiB sized), these operations may take several minutes, or in extreme cases, even
+hours. If your setup is affected, create and remove snapshots while the VM is
+shut down, expecting a long task duration.
 
 ^3^: It is possible to use LVM on top of an iSCSI or FC-based storage.
 That way you get a `shared` LVM storage
-- 
2.47.3