On 01/07/2011 10:50, Robin Hill wrote:
On Fri Jul 01, 2011 at 09:23:43 +0200, David Brown wrote:
On 30/06/2011 23:28, NeilBrown wrote:
On Thu, 30 Jun 2011 16:21:57 +0200 Karsten Römke<k.roemke@xxxxxx> wrote:
Hi Phil
If your CPU has free cycles, I suggest you run raid6 instead of raid5+spare.
Phil
I started the raid 6 array and get:
Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid6 sde5[4] sdd5[3] sdc5[2] sdb2[1] sda3[0]
13759296 blocks level 6, 64k chunk, algorithm 2 [5/5] [UUUUU]
[=================>...] resync = 87.4% (4013184/4586432) finish=0.4min speed=20180K/sec
^^^^^^
Note: resync
when I started the raid 5 array I get
md0 : active raid5 sdd5[4] sde5[5](S) sdc5[2] sdb2[1] sda3[0]
13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
[=>...................] recovery = 6.2% (286656/4586432) finish=0.9min speed=71664K/sec
^^^^^^^^
Note: recovery.
so I have to expect a three times less write speed - or is this calculation
to simple ?
You are comparing two different things, neither of which is write speed.
If you want to measure write speed, you should try writing and measure that.
When you create a RAID5 mdadm deliberately triggers recovery rather than
resync as it is likely to be faster. This is why you see a missed device and
an extra spare. I don't remember why it doesn't with RAID6.
What's the difference between a "resync" and a "recovery"? Is it that a
"resync" will read the whole stripe, check if it is valid, and if it is
not it then generates the parity, while a "recovery" will always
generate the parity?
From the names, recovery would mean that it's reading from N-1 disks,
and recreating data/parity to rebuild the final disk (as when it
recovers from a drive failure), whereas resync will be reading from all
N disks and checking/recreating the parity (as when you're running a
repair on the array).
The main reason I can see for doing a resync on RAID6 rather than a
recovery is if the data reconstruction from the Q parity is far slower
that the construction of the Q parity itself (I've no idea how the
mathematics works out for this).
Well, data reconstruction from Q parity /is/ more demanding than
constructing the Q parity in the first place (the mathematics is the
part that I know about). That's why a two-disk degraded raid6 array is
significantly slower (or, more accurately, significantly more
cpu-intensive) than a one-disk degraded raid6 array.
But that doesn't make a difference here - you are rebuilding one or two
disks, so you have to use the data you've got whether you are doing a
resync or a recovery.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
