Re: swidth with mdadm and RAID6

[Timothy Shimmin <tes@sgi.com>]

Shailendra Tripathi wrote:
> 
> Hi Tim,
> 
>> 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?
>       I agree with you that for operational raid since there would not 
> be any faulty disks, active disks should the number of disks. However, I 
> am just concerned that active disks tracks live disks (not failed 
> disks). If we ever used these commands when the system has faulty drive, 
> the information returned wouldn't be correct. Though, from XFS 
> perspective, I can't think of where it can happen.
>       I would still say that lets rely more on raid_disks to be more 
> conservative, just my choice.

I see your point.
I can just change md_get_subvol_stripe(): s/nr_disks/raid_disks/
I just liked the idea of removing the switch statement which could
potentially get out of date in the future. Too bad :)

--Tim