important information on btrfs getting lost in wiki & wrong vm disk defaults with btrfs storage

[RolandK]

via google search for btrfs and o_direct i found this wiki page

https://pve.proxmox.com/wiki/Special:WhatLinksHere/Storage:_BTRFS

that mentions the folloging important information

"BTRFS will honor the O_DIRECT flag when opening files, meaning VMsshould not use cache mode none, otherwise there will be checksum errors."

this indeed is a known problem with btrfs, and not resolved.

https://bugzilla.redhat.com/show_bug.cgi?id=1914433

furthermore, when using compression, O_DIRECT renders compression useless ( https://marc.info/?l=linux-btrfs&m=171053186915054&w=2 )

i can confirm this problem, because i just experienced such errors and started searching for the reason.

two questions:

1. why does this important page/information seem to get lost in the wiki ?

2. why does a virtual disk of a VM created on top of btrfs still default to cache=none ?

 

[RolandK]

meanwhile, i have found that there are quirks in action:

https://forum.proxmox.com/threads/virtual-disk-default-no-cache-settings-weirdness.143430/

https://pve.proxmox.com/wiki/Storage:_BTRFS
"BTRFS will honor the O_DIRECT flag when opening files, meaning VMs should not use cache mode none, otherwise there will be checksum errors."

I think it's not easy understandable this way.

When that page contents being relinked/trasnformed/moved - i would recommend something like this for replacement:

"BTRFS can do DirectIO, that means files can be opened with O_DIRECT flag, avoiding Linux pagecache. Unfortunately there are issues with virtual machines using this setting ( https://bugzilla.redhat.com/show_bug.cgi?id=693530 ) , so proxmox currently is applying a quirk to circumvent this (removing cache=none, which makes underlying qemu use cache=writeback)

there are even more severe issues with DirectIO and btrfs:
https://lore.kernel.org/linux-btrfs...24ae7146606c.1676684984.git.boris@bur.io/T/#u