qm: guest trim: add note mentioning issue with ext4

It is rather unexpected and seems worth mentioning. Reported in the
community forum [0] and the explanation found by Alwin [1].

[0]: https://forum.proxmox.com/threads/123819/
[1]: https://serverfault.com/questions/1113127/fstrim-is-very-slow-on-xfs-and-always-return-same-value-unlike-ext4/1113129#1113129

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

diff --git a/qm.adoc b/qm.adoc
index bd535a2926975761e5acbe3f82942efed305c0d5..3ba2a3c932466fedc14ca6932917909e4c5a0c3a 100644 (file)
--- a/qm.adoc
+++ b/qm.adoc
@@ -1063,6 +1063,12 @@ operations that have the potential to write out zeros to the storage:
 
 On a thin provisioned storage, this can help to free up unused space.
 
+NOTE: There is a caveat with ext4 on Linux, because it uses an in-memory
+optimization to avoid issuing duplicate TRIM requests. Since the guest doesn't
+know about the change in the underlying storage, only the first guest-trim will
+run as expected. Subsequent ones, until the next reboot, will only consider
+parts of the filesystem that changed since then.
+
 Troubleshooting
 ^^^^^^^^^^^^^^^