When I look at promox virtual disk settings , the default for virtual disk is „Default (No cache)“.
The wiki page at https://pve.proxmox.com/wiki/Performance_Tweaks is telling :
„cache=none seems to be the best performance and is the default since Proxmox 2.X.“
The file section in the kvm/qemu process (via ps -ef), looks like this, when „Default (No cache)“ is selected :
-drive file=/btrfs-hdd-zstd/vms-raw/images/110/vm-110-disk-0/disk.raw,if=none,id=drive-scsi0,aio=threads,format=raw,detect-zeroes=on
We see, that there is no cache=…. setting in the qemu command.
Setting „Default (No cache)“ seems to be different from setting „No cache“. When „No cache“ is being selected, then cache=none is explicitly added to the process/commandline.
The qemu manpage at https://www.qemu.org/docs/master/system/qemu-manpage.html is telling:
„The default mode is cache=writeback.“
doesn't that mean, if "no cache setting is applied to the -drive option/commandline, then „writeback“ should be active !?
I found this weirdness while playing around with the settings, as I gut csum corruption on BTRFS, which does apparently happen when using direct-I/O with btrfs.
When I set „Default (No cache)“ I get no csum errors on virtual disk migration, when is set „No cache“ explicitly, then I get csum errors.
that means setting "Default (No cache)" and "No cache" is not the same!?
The wiki page at https://pve.proxmox.com/wiki/Performance_Tweaks is telling :
„cache=none seems to be the best performance and is the default since Proxmox 2.X.“
The file section in the kvm/qemu process (via ps -ef), looks like this, when „Default (No cache)“ is selected :
-drive file=/btrfs-hdd-zstd/vms-raw/images/110/vm-110-disk-0/disk.raw,if=none,id=drive-scsi0,aio=threads,format=raw,detect-zeroes=on
We see, that there is no cache=…. setting in the qemu command.
Setting „Default (No cache)“ seems to be different from setting „No cache“. When „No cache“ is being selected, then cache=none is explicitly added to the process/commandline.
The qemu manpage at https://www.qemu.org/docs/master/system/qemu-manpage.html is telling:
„The default mode is cache=writeback.“
doesn't that mean, if "no cache setting is applied to the -drive option/commandline, then „writeback“ should be active !?
I found this weirdness while playing around with the settings, as I gut csum corruption on BTRFS, which does apparently happen when using direct-I/O with btrfs.
When I set „Default (No cache)“ I get no csum errors on virtual disk migration, when is set „No cache“ explicitly, then I get csum errors.
that means setting "Default (No cache)" and "No cache" is not the same!?
