Question about ext4 conversion and leaf size

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

Conversion from ext4 works really well and is an important step for
adoption. After recently converting a large-ish device I noticed
dodgy performance, even after defragment & rebalance; noticeably
different from the quite good performance of a newly-created btrfs
with 16k leaf size, as is the default since recently.

So I went spelunking and found that the btrfs-convert logic indeed
uses the ext4 block size as leaf size (from #2220):
https://git.kernel.org/cgit/linux/kernel/git/mason/btrfs-progs.git/tree/btrfs-convert.c#n2245

This is typically 4096 bytes and explains the observed performance.

So while I'm basically familiar with btrfs's design, I know nothing
about the details of the conversion (I'm amazed that it works so well,
including rollback!) but can/should this not be updated to the new default
of 16k, or is there a strong necessary correlation between the ext4 block
size and the newly created btrfs?

thanks!
Holger