[dm-devel] Need some help on dmraid RAID5

[Heinz Mauelshagen]

On Mon, May 29, 2006 at 04:19:55PM +0530, Srinivas, Vidya wrote:
> Hi,

Hi Vidya.

> 
> 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.

No.

> 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)

Yes, it's 'number of disks - 1' with a minimum of 3 disks.

> 
> 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?

If so, it would be wrong.
The _smallest() calculation identifies the smallest of the deveices
in a RAID set in order to make up sane mappings (i.e. all disks mapped
to in a target line must have this minumum size).

> 
> 
> 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.

Well, that number indicates the capacity being used for parity chunks
(i.e. 1 device).

> 
> 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. 

Can you send me your metadata (files created by 'dmraid -rD', tar'ed/bzip2'ed
into Srinivas_Vidya-raid5.tar.bz2), please.

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

Strange. Will lokk into this once I got your metadata.

> 
> Any suggestions would be helpful
> 
> Thanks
> Vidya
> 
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-- 

Regards,
Heinz    -- The LVM Guy --

*** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Storage Development                               56242 Marienrachdorf
                                                  Germany
Mauelshagen at RedHat.com                            PHONE +49  171 7803392
                                                  FAX   +49 2626 924446
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