Re: User space RAID-6 access

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

> Look in the mdadm source code, particularly at restripe.c
> 
> Also
>    make test_stripe
> 
> make a program the the test suite uses for verify data correctness.
> 
> That should give you enough hints to get you started.

Hi Neil,

I'm trying to use the "test_stripe" binary, just to
confirm the operation, in test mode (no save nor restore).

Unfortunately, it seems not work properly, in my hands.

I created a 4 disks RAID-6 (/dev/loop[0-3]), with:

mdadm -C /dev/md111 -l 6 -n 4 --chunk=64 /dev/loop[0-3]

Filled the array from urandom:

dd if=/dev/urandom of=/dev/md111

And tried:

./test_stripe test file_test.raw 4 65536 6 2 65536 $[ 65536 * 3 ] /dev/loop[0-3]

This returns:

0->0
1->1
P(2) wrong at 1
Q(3) wrong at 1
0->3
1->0
P(1) wrong at 2
Q(2) wrong at 2
0->2
1->3
P(0) wrong at 3
Q(1) wrong at 3

The array filled with "0" does not return any error.

Am'I missing something or the code has problems?

An other question, I noticed the code uses an array of
"offsets" which seem to be filled with "0" and never
changed.
Is this really wanted?
Is the offset information the one found per component
using "mdadm -E ..." or /sys/class/block/mdX/md/rdX/offset?
What's the relation with the "start" parameter of "test_stripe"?

Thanks in advance for any hints or suggestions.

The md device is:

/dev/md111:
        Version : 1.2
  Creation Time : Sat Feb  5 17:48:49 2011
     Raid Level : raid6
     Array Size : 524160 (511.96 MiB 536.74 MB)
  Used Dev Size : 262080 (255.98 MiB 268.37 MB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Sat Feb  5 18:19:45 2011
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           Name : lazy.lzy:111  (local to host lazy.lzy)
           UUID : b794fb80:c4006853:47761570:3d97a1d2
         Events : 42

    Number   Major   Minor   RaidDevice State
       0       7        0        0      active sync   /dev/loop0
       1       7        1        1      active sync   /dev/loop1
       2       7        2        2      active sync   /dev/loop2
       3       7        3        3      active sync   /dev/loop3

bye,

-- 

piergiorgio