[PVE-User] Discard transmission between VM / LVM / mdadm layers ?

Alexandre DERUMIER aderumier at odiso.com
Fri Dec 13 13:15:37 CET 2019


When a VM sends a discard/trim command, is it sent to the SSD, LVM does 
not block the command? 

Hi, yes it's working with lvm (lvm thin only)

>>Or is it useless, because mdadm handles discard/trim in his own way? 

The trim command need to be send by the guest os.

on linux guest : /etc/fstab  discard option on mounpoint, or run fstrim command in cron,
on windows: it's a periodic schedule task (in "optimize disk" tool)



----- Mail original -----
De: "Frédéric Massot" <frederic at juliana-multimedia.com>
À: "proxmoxve" <pve-user at pve.proxmox.com>
Envoyé: Jeudi 12 Décembre 2019 17:00:47
Objet: [PVE-User] Discard transmission between VM / LVM / mdadm layers ?

Hi, 

I have a question about the discard/trim transmission between the VM / 
LVM / mdadm layers up to the SSD. 

I have a server with four disks mounted in a RAID 10 array with mdadm. 
On this RAID 10 array, there is a volume group with LVM, which contains 
multiple logical volumes for the hypervisor and VMs. fstrim is 
periodically launched on the hypervisor and VMs. 

I know that : 
- A VM can pass discard/trim commands to the lower layer if it uses a 
"Virtio SCSI" controller. 
- LVM (since 2.02.85 with issue_discards enabled) can pass discard/trim 
to the lower layer during remove or reduce operations of a logical volume. 
- mdadm supports discard/trim since kernel 3.7. 

When a VM sends a discard/trim command, is it sent to the SSD, LVM does 
not block the command? 
Or is it useless, because mdadm handles discard/trim in his own way? 


Regards. 
-- 
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| FRÉDÉRIC MASSOT | 
| http://www.juliana-multimedia.com | 
| mailto:frederic at juliana-multimedia.com | 
| +33.(0)2.97.54.77.94 +33.(0)6.67.19.95.69 | 
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