Re: error injection

On Wed, 28 Sep 2011 17:59:49 -0700 Jojy Varghese [off-list ref]
wrote:

Thanks Neil. I tried setting my sda7 partition to generate write
errors every 40 bytes(writing 1 byte at a time). I did :

md doesn't see byte writes.  It sees sectors or more - usually whole pages or
groups of pages.

1. Create a array with:
mdadm -C /dev/md/me0 -l faulty -n1 /dev/sda7

-C will write a superblock to /dev/sda7 which you don't really want.  It
doesn't hurt, but I always used -B (--build) to avoid any metadata.

After this step I can see /dev/md127 and when i do a mdadm -D /dev/md127, i get:

/dev/md127:
        Version : 1.2
  Creation Time : Wed Sep 28 17:35:50 2011
     Raid Level : faulty
     Array Size : 969410424 (924.50 GiB 992.68 GB)
   Raid Devices : 1
  Total Devices : 1
    Persistence : Superblock is persistent

    Update Time : Wed Sep 28 17:35:50 2011
          State : clean
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : eng-dev16.lab.local:me0  (local to host eng-dev16.lab.local)
           UUID : 96f4be10:312f9574:f40107aa:d9f278ba
         Events : 0

    Number   Major   Minor   RaidDevice State
       0       8        7        0      active sync   /dev/sda7


2. Set write fault level with:

mdadm -G /dev/md/me0 --layout=wp40



  After this when i write > 40 bytes into /dev/md127, i dont get any
I/O errors. I am sure i am doing something wrong here.

When you write to /dev/md127 it will just go into the page cache and
eventually be flushed to the device in one write.
Use O_DIRECT or O_SYNC and it will be flushed out more quickly, but always
write at least 512  bytes at a time.

NeilBrown

Any help is much appreciated.

Thanks
Jojy