Bug 423391

+++ This bug was initially created as a clone of Bug #147679 +++
RHEL5 clone

The dm device using stripe target need to set readahead of the dm device
properly (similarly to MD raid0 target).

Also there is currently ignored --readahead parameter in lvcreate/lvchange.

Computed or user defined readahead value should be propagated through
libdevmapper library to dm device (using GETRA ioctl).

Original bug report:

Description of problem:
LVM2 has a very bad reading performance using a larger disk stripe. Below is 
a single raw device out of a HP disk system (DS2405 to be exactly), connected 
via fibre channel (max 400 MB/s) to an HP DL145 (Opteron) machine:

--- snipp ---
[root@opteron ~]# time dd if=/dev/zero of=/dev/sdo bs=1024k count=10000
10000+0 records in
10000+0 records out

real    2m36.544s
user    0m0.017s
sys     0m16.983s
[root@opteron ~]#
[root@opteron ~]# time dd if=/dev/sdo of=/dev/null bs=1024k count=10000
10000+0 records in
10000+0 records out

real    2m20.435s
user    0m0.023s
sys     0m19.391s
[root@opteron ~]#
--- snapp ---

This results to 64 MB/s writing and 71 MB/s reading; that's okay.

Below is a disk stripe containing out of 14 HDDs with manual fibre channel 
multipath by LVM2 (at the same HP disk system):

--- snipp ---
[root@opteron ~]# time dd if=/dev/zero of=/dev/vg01/lv01 bs=1024k count=100000
100000+0 records in
100000+0 records out

real    4m44.290s
user    0m0.255s
sys     3m28.193s
[root@opteron ~]#
[root@opteron ~]# time dd if=/dev/vg01/lv01 of=/dev/null bs=1024k count=100000
100000+0 records in
100000+0 records out

real    9m32.262s
user    0m0.276s
sys     4m19.993s
[root@opteron ~]#
--- snapp ---

This results to 350 MB/s writing and 175 MB/s reading - a worse result for 
reading. If I use a HP-UX box, I get ~ 350/350 MB/s; a good result. So the 
problem is LVM2.

Version-Release number of selected component (if applicable):
lvm2-2.00.31-1.0.RHEL4

How reproducible:
Everytime, see above.

Actual results:
Only 50% reading performance compared with the measured writing speed.

Expected results:
Same reading speed like at writing or even better.