Let’s take a closer look at the cache write back mode in Proxmox. Bobcares, as a part of our Proxmox Support Services offers solutions to every query that comes our way.

The cache write back mode in Proxmox

The cache=writeback mode is pretty similar to a raid controller with a RAM cache. In this mode, qemu-kvm interacts with the disc image file or block device without using O_DSYNC or O_DIRECT  semantics. As a result, the host page cache is utilized.

The writes are reported as complete to the guest when placed there. And normal page cache management will handle commitment to the storage device. Additionally, the writeback cache is disclosed to the guest’s virtual storage adapter, so the guest would be expected to send down flush commands as necessary to maintain data integrity.

In this mode, the read-write cache is the host page cache. The write back is the guest disc cache mode in Proxmox. In the event of a power outage, the chance of losing data is high. If the Linux guest’s kernel is older than 2.6.37, we must use the barrier option in the fstab file to prevent FS corruption in the event of a power outage.

In the cache=writeback (unsafe) mode, it is as writeback, but disregards the guest’s commands to flush. There is no data integrity even if the guest is sending flush commands. It is not advised for use in production.

Configuring cache in Proxmox VE

We can configure the cache in PVE with the following steps:

1. Firstly, login to PVE web GUI.
2. Then go to Datacenter >> node name/cluster name >> VM >> Hardware.
3. Also, find the disk we want to enable/disable Cache for.
4. Save the settings.
5. Lastly, reboot the VM.

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Conclusion

Modern file systems fully support this typical storage design, which is common. However, it should be understood that this mode exposes the guest to data loss in the unlikely event of a host failure because there is a window of time between the time a write is reported as completed and that write is committed to the storage device.