> journald does do a rewrite of the log file on rotation, so you're paying that IO anyway even if you ignore the dumb hash table updates.
You linked copy_file_atomic_at_full(), why? That function is not in the normal rotation path for journald, it's only used in a workaround when clearing FS_NOCOW_FL fails.
Rotation does not rewrite the log file normally, but there is a hole-punching operation though for reclaiming unused space.
> it's only used in a workaround when clearing FS_NOCOW_FL fails.
Clearing FS_NOCOW_FL doesn't work on btrfs for non-empty files. So what do you think journald is doing when it notices that it can't clear the flag?
When did this become a discussion limited to journald on btrfs?
and that seems like something btrfs should fix at some point
So, yes, journald does in fact do bulk copies of log files on rotate. btrfs is hardly some fringe FS and its COW-flag behavior is documented and well-known. I'd expect extensive work on journald's storage engine to have uncovered this behavior at some point.
> When did this become a discussion limited to journald on btrfs?
btrfs is in the HN thread title.
> and that seems like something btrfs should fix at some point
Amazing. The Linux kernel should change to work around journald's inflexibility?
What someone should fix at some point is journald's strange IO patterns and hard-coded "helpful" attribute changes. I'd rather it just rename the file and let me do any defrag/compression/flag-setting I want than do anything with chattr behind my back in ways I can't even configure.
> btrfs is in the HN thread title.
as is ext4
So write amplification on btrfs doesn't matter?