[dm-devel] Need some help on dmraid RAID5

[Srinivas, Vidya]

Hi,

I am checking dmraid functionality for RAID5 with 3 disks. I am using
the latest available patch on
http://people.redhat.com/heinzm/sw/dm/dm-raid45/
I have some confusion in the same.

The target length in the function raid_ctr in dm-raid4-5.c is being
displayed to me as the smallest disk sectors. However, I understand that
the target length should be the total sectors (of all available disks)
available for data (which in my case should be 2 disk sectors since 1
disk sectors would be utilized for parity)

What I understand is, in the user level dmraid, in
lib/activate/activate.c, under dm_raid45, sectors value is calculated
by:

Sectors = _smallest(lc,rs,0)

And, the same sectors value is passed to _dm_raid45_bol.

Whatever goes from this _dm_raid45_bol directly goes to the kernel
dm-raid4-5.c and under raid_ctr gets printed as ti->len right? So, the
smallest sectors (of all the devices) goes to ti->len. In my case also,
the smallest disk sectors is being printed as ti->len and not the total
data sectors. Am I wrong?


Also, I had another doubt in dm-raid4-5.c, under static struct raid_type
raid_types[], values for raid5_ls is:

{Raid5_ls. "RAID5 (left symmetric), 1, 3, raid5, left_sym}, where the
number of parity devices (being the 3rd parameter) is taken as 1.

But, I feel, since there isn't any dedicated parity device for RAID5,
shouldn't this be taken as 0. When I take this as 1, I am getting the
error, target length not divisable by number of devices. 

When I take it as 0, my RAID volume is being created and things seem to
work.

Any suggestions would be helpful

Thanks
Vidya