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?
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
