Disable filesystem-level COW and checksumming for volatile-dirty.img in file-reflink

[iamahuman]

How to file a helpful issue

The problem you're addressing (if any)

BTRFS incurs extra bookkeeping overhead for durability (e.g., copy-on-write and checksumming) by default unless No_COW attribute (chattr +C) is specified. However, volatile AppVM volumes do not need such guarantees.

The solution you'd like

Set No_COW attribute on volatile-dirty.img in file-reflink storage backend.

The value to a user, and who that user might be

Reducing COW overhead improves I/O performance for swap and temporary files.

Related issues

• Store volatile.img in separate location #1527

[rustybird]

This came up on qubes-users. It's already possible (although a little clumsy - #1527 which you linked would make it straightforward) to set it up, but:

Disabling CoW and hence checksums [...] means losing protection against on-disk bit rot. But storing data on the volatile volume doesn't mean it is unimportant or even short-lived: It's not that unusual to have a long-running VM with weeks of uptime. Corruption in its swapped memory (or in diverged 'root' volume data, which too* is stored on the 'volatile' volume) could be devastating.

* If the root volume is read-only. By default it isn't, but see #6625. (Google Groups' anti-spam filter would not let me post this correction at the time.)