Dan Williams wrote:
On Thu, Jun 12, 2008 at 6:05 AM, Hubert Verstraete <hubskml@xxxxxxx> wrote:
Neil Brown wrote:
On Wednesday June 11, hubskml@xxxxxxx wrote:
By the way and FYI, with my configuration, all disks on the same
controller, internal bitmap, v1 superblock, ... the initial RAID-5
synchronization duration is the same whether I'm using the option --force or
not.
For this to be a valid test, you need to fill one drive up with
garbage to ensure that a resync is no a no-op.
If you don't use the "--force" option, then the recovery process will
read from N-1 drives and write to 1 drive, all completely sequentially
so it will go at a predictable speed.
When you use "--force" it will read from N drive and check parity.
When it finds an error it will re-write that parity block.
So if the parity blocks happen to be all correct (as probably was the
case in your experiment), it will run nice and fast. If the parity
blocks happen to all be wrong (as is likely when first creating an
array on drives that weren't an array before) it will be much slower.
I've just filled all the drives with /dev/zero and am currently building a
new array. Is this a valid test or should I fill the drives with /dev/random
?
No, /dev/zero will not work for this test since the xor sum of zeroed
blocks is zero. The check operation has the following stages: read
the parity from disk, verify it is correct, and if it is not correct
calculate / writeback correct parity. So, you need to ensure that the
parity check fails. Using /dev/urandom should be sufficient. Do not
use /dev/random as it will exhaust the entropy pool and then block.
Confirmed, I got the same resync time after zeroing the hard drives.
Nevertheless, let's say I build the arrays on brand new disks.
If I can assume these disks are full of zero, I can freely choose my
initial resync method :)
And it seems my new Seagate disk is made of zero (currently running a
diff with /dev/zero).
Thanks for explanations.
Hubert
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