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

[Dave Wysochanski]

On Tue, 2006-11-21 at 19:20 +0800, Kalpak Shah wrote:
> 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.
> 
It depends how you create the logical volume on which you put the
filesystem.

> 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?
> 

That sounds like you created a logical volume with stripe size (in
device mapper it's termed "chunk size") of 8KB.
What is the output of "dmsetup table" for the volume in question?

The default stripe size is 64KB (128 sectors) but you can change it when
you create the lv.  An example on my setup I got this for one lv I
created (notice the 16 in the output):
# lvcreate -L 296M -i 2 --stripesize 8K -n lv0 vg0
  Logical volume "lv0" created
# dmsetup table
vg0-lv0: 0 606208 striped 2 16 7:3 384 7:4 384

The format for the striped target is:
/*
 * Construct a striped mapping.
 * <number of stripes> <chunk size (2^^n)> [<dev_path> <offset>]+
 */