[H-GEN] ext2 or ext3

[David Jericho]

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On Wed, 2002-06-19 at 22:09, Jason Henry Parker wrote:
> As a quick note, as far as I know, no filesystem will save you from a
> disk crash---only RAID (level 5?  Someone help me out) will *really*
> save you from a disk wearing out.

Not directed at you Jason, but Redundant Array of Inexpensive Disks my
tea cosy.

At least the Free Online Dictionary of Computing has the sense to define
it as Independent.

RAID suffers from many problems, and it's merely a way of delaying the
inevitable. People go whacking in RAID 5 arrays because they hear you
can loose a disk and still keep the data. No hot spare to be seen
anywhere.

Given that drives from a particular batch typically tend to have similar
life times, if one drive fails, chances are the others are soon to go.
Most arrays are constructed of drives from the same batch.

I also see people prattle on about RAID 0+1 (or is that 1+0) and few
people realise once you've lost a drive, you've got 1/(n/2) chances of
losing data.

Hotspares are the name of the game, and plenty of. Plenty of RAID
processor grunt to accomodate the RAID 5 checksums, and mirror, mirror,
mirror.

Idealy you'd run small RAID 5+1 arrays joined via some form of volume
managment with a collection of hotspares. But the problem is RAID 5
checksums can slow down writes quite a fair bit. 

Copyying data to a RAID array does not make it a backup. RAID exists to
ensure continuance of data access in a degraded mode until full disk
operation can be restored.

RAID definitions :-
RAID 0: Straight bit/byte/block striping. Data blocks are sequentially 
	stored across drives in the array. Lose one drive, lose the 	data.
RAID 1: Mirroring. Each drive has at least 1 active mirror. Need to
	lose both drives in the mirror set to lose data.
RAID 4: Basically RAID 0 with a parity drive. Lose a drive from the 
	RAID 0 set, or the parity drive, and the data stays valid.
RAID 5: Striped parity. Need at least 3 drives, and you can lose one
	drive in the array.
hotspare: A hotspare is a drive that exists to take the place of a  
          failed drive. The data has to be rebuilt from a straight
          copy (RAID 1), or a parity rebuild (RAID 4,5)

-- 
David Jericho
Senior Systems Administrator, Bytecomm Pty Ltd

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