[dm-devel] Breaking of large IO requests into small bios by device-mapper

[Kalpak Shah]

Hi,

Hi I am checking if device-mapper breaks down the reads/writes which are
sent by the file system. For example if an FS like Lustre sends a 1MB
read/write whether the request goes to the disk as a single request or
is it broken down into many small requests.

I have put some printks in __map_bio, __split_bio, etc. It seems that
even though 1 MB writes are sent to the device mapper the bios that are
sent to generic_make_request in __map_bio are at max 8k in size. In
function max_io_len(), ti->split_io is just 16 sectors at max, hich is
why the requests get broken down.

Why is this ->split_io variable set to 16 sectors? Can it  be more (arnd
2048 sectors) without affecting the stability?

Thanks,
Kalpak.