[linux-lvm] Re: [PATCH] 64 bit scsi read/write

[Jonathan Lundell]

At 4:04 PM -0600 2001-07-13, Andreas Dilger wrote:
>**) As I said in my previous posting, it depends on if/how MD RAID does
>    write ordering of I/O to the data sector and the parity sector.  If
>    it holds back the parity write until the data I/O(s) are complete, and
>    trusts the data over parity on recovery, you should be OK unless you
>    have multiple failures (i.e. bad disk + crash).  If it doesn't do this
>    ordering, or trusts parity over data, then you are F***ed (I doubt it
>    would have this problem).

That wouldn't help, would it, if >1 data sectors were being written.

The fault mode of a sector simply not being written seems like a real 
weak point of both RAID-1 and RAID-5. Not that RAID-5 parity ever 
gets checked, I think, under normal circumstances, nor RAID-1 mirrors 
get compared, but if they were check and there was an parity or 
mirror-compare error and no other indication of a fault (eg CRC), 
there's no way to recover correct data.
-- 
/Jonathan Lundell.