Meaning of struct statx->stx_blksize field

[Zeno Endemann <zeno.endemann@mailbox.org>]

Hi,

The man page of statx(2) describes the stx_blksize like so:

 > The "preferred" block size for efficient filesystem I/O.
 > (Writing to a file in smaller chunks may cause an
 > inefficient read-modify-rewrite.)


However, from observation it rather looks like that field
is set to the "block size" (meaning the allocation unit) of
the file system the given file resides on - at least for
ext4 and vfat.

To my understanding that value is fairly unrelated to a good
IO transfer size and for the most part not really relevant
for userspace apps. For avoiding inefficient read-modify
rewrite the actual relevant thing is writing in multiples of
the page size (unless the page cache is bypassed, in which
case I guess stx_dio_offset_align is the value to use).

Can someone confirm my interpretation, so the man page can be
corrected? Or do I misunderstand or overlook something?

Thanks,

Zeno Endemann