Re: limits on raid

On Saturday June 16, david@dgreaves.com wrote:

Neil Brown wrote:

quoted

On Friday June 15, wakko@animx.eu.org wrote:
 

quoted

                                                  As I understand the way
raid works, when you write a block to the array, it will have to read all
the other blocks in the stripe and recalculate the parity and write it out.

Your understanding is incomplete.

Does this help?
[for future reference so you can paste a url and save the typing for code :) ]

http://linux-raid.osdl.org/index.php/Initial_Array_Creation

David



Initial Creation

When mdadm asks the kernel to create a raid array the most noticeable activity 
is what's called the "initial resync".

The kernel takes one (or two for raid6) disks and marks them as 'spare'; it then 
creates the array in degraded mode. It then marks spare disks as 'rebuilding' 
and starts to read from the 'good' disks, calculate the parity and determines 
what should be on any spare disks and then writes it. Once all this is done the 
array is clean and all disks are active.

This isn't quite right.
Firstly, it is mdadm which decided to make one drive a 'spare' for
raid5, not the kernel.
Secondly, it only applies to raid5, not raid6 or raid1 or raid10.

For raid6, the initial resync (just like the resync after an unclean
shutdown) reads all the data blocks, and writes all the P and Q
blocks.
raid5 can do that, but it is faster the read all but one disk, and
write to that one disk.

NeilBrown

Neil Brown wrote:

This isn't quite right.

Thanks :)

Firstly, it is mdadm which decided to make one drive a 'spare' for
raid5, not the kernel.
Secondly, it only applies to raid5, not raid6 or raid1 or raid10.

For raid6, the initial resync (just like the resync after an unclean
shutdown) reads all the data blocks, and writes all the P and Q
blocks.
raid5 can do that, but it is faster the read all but one disk, and
write to that one disk.

How about this:

Initial Creation

When mdadm asks the kernel to create a raid array the most noticeable activity 
is what's called the "initial resync".

Raid level 0 doesn't have any redundancy so there is no initial resync.

For raid levels 1,4,6 and 10 mdadm creates the array and starts a resync. The 
raid algorithm then reads the data blocks and writes the appropriate 
parity/mirror (P+Q) blocks across all the relevant disks. There is some sample 
output in a section below...

For raid5 there is an optimisation: mdadm takes one of the disks and marks it as 
'spare'; it then creates the array in degraded mode. The kernel marks the spare 
disk as 'rebuilding' and starts to read from the 'good' disks, calculate the 
parity and determines what should be on the spare disk and then just writes to it.

Once all this is done the array is clean and all disks are active.

This can take quite a time and the array is not fully resilient whilst this is 
happening (it is however fully useable).





Also is raid4 like raid5 or raid6 in this respect?