online fstrim in ext4 for local qcow2 image

[ddio]

Hi,
I want to get fstrim working with no luck so far.
What I did so far:
1. Switched from virtio-blk to virtio-scsi (changed the disc to scsi0 and changed the emulated scsi-controller to VIRTIO)
2. Switched discard=on as parameter for the disk
3. mounted the ext4 filesystem with discard
It is working so far, that I can do fstrim -v /, and there is nothing to be trimmed, but the qcow2 file is increasing (double the size of the occupancy rate of the filesystem).

I got it only working with shutting down the vm and converting the disk as stated in Shrink_Qcow2_Disk_Files#Linux_Guest_Preparation in pve wiki (sry I am not allowed to post an link), but for that the vm has to be shutdown...
Am I missing sth?
Thanks!

 

[wolfgang]

Hi,
it is not necessary to activate trim inside a Vm if you use a image.
It is enough if you enable it on the host.
The effective io on the ssd will managed by the host kernel and not by the vm kernel.

here a good information on linux and ssd for the host

https://wiki.archlinux.org/index.php/Solid_State_Drives

 

[ddio]

Hi,
it is not necessary to activate trim inside a Vm if you use a image.
It is enough if you enable it on the host.
The effective io on the ssd will managed by the host kernel and not by the vm kernel.

here a good information on linux and ssd for the host

I have no trim support on the host (it has normal hdds), but I want to save disk space with trim (already overprovisioned).

Does this mean this is only possible if you have trim support for the storage on the hypervisor?

 

[gradinaruvasile]

TRIM inside the vm will overwrite the unused parts with zeroes, but you will have to compact the qcow2 image from outside (by creating a copy for example), it does not shrink by itself.

 

[LnxBil]

Maybe it works with newest thin-provisioned LVM volumes?

Or you can use ZFS.

 

[aderumier]

Note that you don't need to mount your fs with discard=on, to use fstrim.

if you use discard=on, trim will be done at each delete in your filesystem, and this can be quite slow.

It's better to simply add a cron, daily for example, which launch fstrim. (without any discard=on)