On Tue, 30 Mar 2010 16:52:13 +0200
Markus Hochholdinger [off-list ref] wrote:

Hello,

Am 28.06.2008 um 01:41 Uhr schrieb Neil Brown [off-list ref]:

quoted

On Friday June 27, Markus@hochholdinger.net wrote:

quoted

Am Freitag, 27. Juni 2008 08:51 schrieb NeilBrown:

quoted

From: Chris Webb <redacted>
Allow /sys/block/mdX/md/rdY/size to change on running arrays, moving
the superblock if necessary for this metadata version. We prevent the
available space from shrinking to less than the used size, and allow it
to be set to zero to fill all the available space on the underlying
device.

I'm very happy of this new feature. But I'm a little confused how to use
it correctly.
Can md now recognize the change by itself and I only have to run mdadm
--grow? Or have I manually update /sys/block/mdX/md/rdY/size and
afterwards run mdadm --grow?

No, md does not recognise the change by itself.
Currently you need to update ..../size yourself before using
   "mdadm --grow"
This should probably be incorporated into mdadm at some stage, but it
hasn't yet.

it's some time ago but today i'm able to test this (because now i have a xen 
kernel which recognizes size changes).
I tested with metadata version 0.9, 1.0 and 1.1 but didn't get it to work. I 
create a raid1:
mdadm --create /dev/md1 --level=1 --raid-disks=2 --metadata=1.0 /dev/xvda1 /dev/xvdb1

Then I change the size of xvda1 and get the following kernel message:
[ 3281.905317] Setting capacity to 4194304
[ 3281.905317] xvda1: detected capacity change from 1073741824 to 2147483648

After this i do:
echo "0" > /sys/block/md1/md/rd0/size
No kernel message so far and
  cat /sys/block/md1/md/rd0/size
still shows 1048570.

That is very odd.  The number printed out in the "detected capacity change"
line is exactly the name that should be used when you write to "md/rd0/size".

You could try
   strace echo 0 > /sys/block/md1/md/rd0/size

to be sure there is no error return.
And maybe
   blockdev --getsz /dev/xvda1
and
   blockdev --getsz /dev/xvda

to double check that the block size looks right.

Are you sure the message didn't say "xvda: detected capacity change...", and
you still need to change the size of the partition?

NeilBrown

Then the same for xvdb1.

And then I tried to grow md1 in the hope it will somehow detect the new size 
correct, but i only get a very little grow:
[ 3345.133882] md1: detected capacity change from 1073729536 to 1073735680
[ 3345.140087] md: resync of RAID array md1
[ 3345.140098] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[ 3345.140105] md: using maximum available idle IO bandwidth (but not more 
than 200000 KB/sec) for resync.
[ 3345.140121] md: using 128k window, over a total of 1048570 blocks.
[ 3345.140129] md: resuming resync of md1 from checkpoint.
[ 3345.140412] md: md1: resync done.
[ 3345.160064] RAID1 conf printout:
[ 3345.160071]  --- wd:2 rd:2
[ 3345.160076]  disk 0, wo:0, o:1, dev:xvda1
[ 3345.160081]  disk 1, wo:0, o:1, dev:xvdb1

Especially "md1: detected capacity change from 1073729536 to 1073735680" makes 
me wonder.


I also tried to set values to /sys/block/md1/md/rd1/size, but all i got was
  echo: write error: Invalid argument

And the output of /sys/block/md1/md/rd1/size never changes.

That is very odd.

quoted

quoted

To be on the safe side I'd first lvresize one disk of the raid1, then do
mdadm --grow to let md update/move the superblock of this disk. And after
this is successful, lvresize the other disk and do mdadm --grow. So in
case of a failure i wouldn't loose the whole raid1!?
Am I correct or am I missing something?

You don't want to "mdadm --grow" until everything has been resized.
First lvresize one disk, then write '0' to the .../size file.
Then do the same for the other disk.
Then "mdadm --grow /dev/mdX --size max".

I tried mdadm version 3.0.3 and 3.1.1. Is it possible that something has 
changed inside mdadm?
I used kernel 2.6.32-4-xen-amd64 (debian squeeze).

Are am I doing something wrong? Has something changed so i'm on the wrong way? 
Is there another way to do online grow of an raid1 without full resync? 
Anyone any ideas?

Many thanks in advance.