Re: swidth with mdadm and RAID6

Timothy Shimmin
2006-09-19T05:12:09Z

Hi Shailendra and Steve,

Shailendra Tripathi wrote:
> Hi Steve,
> Your guess appears to be correct. md_ioctl returns nr which
> is total number of disk in the array including the spare disks. However,
> XFS function md_get_vol_stripe does not take spare disk into account. It
> needs to subtract spare_disks as well.
> However, md.spare_disks returned by the call returns spare + parity
> (both). So, one way could be substract spare_disks directly. Otherwise,
> the xfs should rely on md.raid_disks. This does not include spare_disks
> and nr.disks should be changed for that.
> When I run my program md_info on raid5 array with 5 devices and 2
> spares, I get
> [root@ga09 root]# ./a.out /dev/md11
> Level 5, disks=7 spare_disks=3 raid_disks=5
>
> Steve can you please compile the pasted program and run on your system
> with md prepared. It takes /dev/md<no> as input.
> In your case, you should get above line as:
> Level 6, disks=11 spare disks=3 raid_disks=10
>
> nr=working=active=failed=spare=0;
> ITERATE_RDEV(mddev,rdev,tmp) {
> nr++;
> if (rdev->faulty)
> failed++;
> else {
> working++;
> if (rdev->in_sync)
> active++;
> else
> spare++;
> }
> }
>
> info.level = mddev->level;
> info.size = mddev->size;
> info.nr_disks = nr;
> ....
> info.active_disks = active;
> info.working_disks = working;
> info.failed_disks = failed;
> info.spare_disks = spare;
>
> -shailendra

I'm not that au fait with RAID and md, but looking at what you wrote,
Shailendra, and the md code, instead of your suggestions
(what I think are your suggestions:) of:

(1) subtracting parity from md.raid_disk (instead of md.nr_disks)
where we work out parity by switching on md.level
or
(2) using directly: (md.nr_disks - md.spares);

that instead we could use:
(3) using directly: md.active_disks

i.e.
*swidth = *sunit * md.active_disks;
I presume that active is the working non spares and non-parity.

Does that make sense?

--Tim