Re: unreadable drives can be synchronized?

[Bill Davidsen <davidsen@tmr.com>]

Colin McCabe wrote:
> Hi all,
>
> I am running software RAID on Linux 2.6.21.
>
> While experimenting with adding and removing devices from the RAID 
> array, I
> noticed something very troubling. I have a bad drive (let's call it 
> drive B)
> which gets random read errors. I also have a good drive, call it drive A.
>
> B can synchronize with A. But then, if I remove A from the raid array, A
> cannot be re-added. This is because the bad drive, B, cannot be read 
> from.
>
> Basically, B appears to be "write-only"; it will never return an error 
> on a
> write, but just try to read from it, and you will be sorry.
>
You may be able to recover from this (why would you do such a thing?) by 
stopping the array and reassembling the array with only the "good" drive 
and the other as failed. Caution, I made this up, it should work but I 
have no bad drive to use for a test, we have a good recycling system in 
my area.
> Writing is fine:
> [root@cmccabe-devel root]# dd if=/dev/zero of=/dev/sdb bs=524288
> dd: writing `/dev/sdb': No space left on device
> 114464+0 records in
> 114463+0 records out
>
> Reading is not:
> [root@cmccabe-devel root]# dd if=/dev/sdb of=/dev/null bs=524288
> ata1.00: exception Emask 0x0 SAct 0x3 SErr 0x0 action 0x2 frozen
> ata1.00: cmd 60/00:00:00:b0:01/01:00:00:00:00/40 tag 0 cdb 0x0 data 
> 131072 in
> [ ... copious errors ... ]
>
> I have disabled write caching using hdparm -W0.
> Both drives are: Fujitsu MHV2060BH, 60 GB, Serial ATA
> The SATA controller is: ICH6
>
> My problem is that even though B gets into the synchronized state, it 
> is no
> good at all. This is potentially misleading, and if someone removes A 
> after
> synchronizing B, the system will probably crash, since there will be 
> no good
> drives left.
>
> I wonder if anyone else is interested in a "paranoid recovery" mode 
> where the
> md layer tests the data that has been written. Even if this doubles the
> recovery time, I think that it would be desirable for many applications.


-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979