Re: [Doubt] how is a disk marked faulty in RAID5

[Anuj Goel <agoel@cs.stonybrook.edu>]

Hi folks,
Can someone please help me with below doubts.
Thanks in advance !!

On Tue, Apr 24, 2012 at 7:36 PM, Anuj Goel <agoel@cs.stonybrook.edu> wrote:
> Hi,
>
> I have been looking into the RAID5 code, but unable to find how a disk
> is marked faulty in a RAID5 array.
> I consider the case when we try to read, say 2 sectors within a chunk
> and the read fails. My understanding so far is as below:
>
> The status of the read operation is returned in the call back function
> "raid5_align_endio" registered in chunk-aligned_read().
> If there was an error in the read, it LIFO adds the original bio to
> the retry list and wakes up the raid5d thread.
> This thread will remove the bio from the retry list and send it to
> retry_aligned_read().
>
> In retry_aligned_read(), we first compute the disk number and sector
> offset within the disk using raid5_compute_sector().
>
> 1. Then we do some stripe operations, but I cannot see where the
> actual read from the disk is scheduled.
> 2. Also, if the sector on the disk is found unreadable, according to
> the RAID5 design, it should be recomputed using parity and the disk
> marked FAULTY. Can you please point me to the code/functions I should
> look into to understand how this is being done.
> 3. After one disk failure, if another disk fails, I think the RAID5
> array cannot be used anymore. How is the second disk failure reported
> ?
>
> This is my first tryst with Linux code, (specifically software RAID),
> so I am not sure how to debug and understand the code flow.
> Is code reading the only way to understand the flow, or is there some
> documentation giving a high level overview of the implementation of
> software RAID ?
>
> Any suggestions will be highly appreciated !!
>
> --
> Best Regards,
> Anuj Goel
> Experimental Computer Science Lab
> Stony Brook University.



-- 
Best Regards,
Anuj Goel
Experimental Computer Science Lab
Stony Brook University.