Re: Spares and partitioning huge disks

[Frank van Maarseveen <frankvm@frankvm.com>]

On Sat, Jan 08, 2005 at 05:49:32PM +0100, maarten wrote:
> 
> > For one second I thought it's a clever trick but gut feeling tells
> > me the odds of losing the entire array won't change (simplified --
> > because the increased complexity creates room for additional errors).
> 
> No.  It is somewhat more complex, true, but no different than making, for 

Got it.

> However, IF during that 
> resync one other drive has a read error, it gets kicked too and the array 
> dies.  The chances of that happening are not very small;

Ouch! never considered this. So, RAID5 will actually decrease reliability
in a significant number of cases because:

-	>1 read errors can cause a total break-down whereas it used
	to cause only a few userland I/O errors, disruptive but not foobar.
-	disk replacement is quite risky. This is totally unexpected to me
	but it should have been obvious: there's no bad block list in MD
	so if we would postpone I/O errors during reconstruction then
	1:	it might cause silent data corruption when I/O error
		unexpectedly disappears.
	2:	we might silently loose redundancy in a number of places.

I think RAID6 but especially RAID1 is safer.

A small side note on disk behavior:
If it becomes possible to do block remapping at any level (MD, DM/LVM,
FS) then we might not want to write to sectors with read errors at all
but just remap the corresponding blocks by software as long as we have
free blocks: save disk-internal spare sectors so the disk firmware can
pre-emptively remap degraded but ECC correctable sectors upon read.

-- 
Frank