Re: Hot-replace for RAID5

Hi Neil,

I decided to move backup file on other device. I stopped the array,
mdadm stopped it but wrote "mdadm: failed to unfreeze array". What
does it exactly mean? I dont want to proceed until I am sure it does
not signalize error.

I quickly checked sources and it seems to be related to some sysfs
resources, but I am not sure. But the array disappeared from
/sys/block/.

Thanks.

Patrik

On Sun, May 13, 2012 at 9:43 AM, Patrik Horník [off-list ref] wrote:

Hi Neil,

On Sun, May 13, 2012 at 1:19 AM, NeilBrown [off-list ref] wrote:

quoted

On Sat, 12 May 2012 17:56:04 +0200 Patrik Horník [off-list ref] wrote:

quoted

Neil,

Hi Patrik,
 sorry about the "--layout=preserve" confusion.  I was a bit hasty.
 -layout=left-symmetric-6" would probably have done what was wanted, but it
 is a bit later for that :-(

--layout=preserve is mentioned also in the md or mdadm
documentation... So is it not the right one?

quoted

quoted

so I further analyzed the behaviour and I found following:

- The bottleneck cca 1.7 MB/s is probably caused by backup file on one
of the drives, that drive is utilized almost 80% according to iostat
-x and its avg queue length is almost 4 while having await under 50
ms.

- The variable speed and low speeds down to 100 KB are caused by
problems on drive I suspected as problematic. Its service time is
sometimes going above 1 sec.. Total avg speed is about 0.8 MB/s. (I
tested the read speed on it by running check of array and it worked
with 30 MB/s. And because preserve should only read from it I did not
specifically test its write speed )

So my questions are:

- Is there a way I can move backup_file to other drive 100% safely? To
add another non-network drive I need to restart the server. I can boot
it then to some live distribution for example to 100% prevent
automatic assembly. I think speed should be couple of times higher.

Yes.
If you stop the array, then copy the backup file, then re-assemble the
array giving it the backup file in the new location, all should be well.
A reboot while the array is stopped is not a problem.

Should or will? :) I have 0.90, now 0.91, metadata, is everything
needed stored there? Should mdadm 3.2.2-1~bpo60+2 from
squeeze-backports work well? Or should I compile mdadm 3.2.4?

In case there is some risk involved I will need to choose between
waiting and risking power outage happening sometimes in the following
week (we have something like storm season here) and risking this...

Do you recommend some live linux distro installable on USB which is
good for this? (One that has newest versions and dont try assemble
arrays.)

Or will automatic assemble fail and it will cause no problem at all
for sure? (According to md or mdadm doc this should be the case.) In
that case can I use distribution on the server, Debian stable plus
some packages from squeeze, for that? Possibly with added
raid=noautodetect? I have LVM on top of raid arrays and I dont want to
cause mess. OS is not on LVM or raid.

quoted

quoted

- Is it safe to fail and remove problematic drive? The array will be
down to 6 from 8 drives in part where it is not reshaped. It should
double the speed.

As safe as it ever is to fail a device in a non-degraded array.
i.e. it would not cause a problem directly but of course if you get an error
on another device, that would be awkward.

I actually "check"-ed this raid array couple of times few days ago and
data on other drives were OK. Problematic drive reported couple of
reading errors, always corrected with data from other drives and by
rewriting.

About that, shoud this reshaping work OK if it encounter possible
reading errors on problematic drive? Will it use data from other
drives to correct that also in this reshaping mode?

Thanks.

Patrik

quoted

quoted

- Why mdadm did ignore layout=preserve? I have other arrays in that
server in which I need replace the drive.

I'm not 100% sure - what version of mdadm are you using?
If it is 3.2.4, then maybe commit 0073a6e189c41c broke something.
I'll add test for this to the test suit to make sure it doesn't break again.
But you are using 3.2.2 .... Not sure. I'd have to look more closely.

Using --layout=left-symmetric-6 should work, though testing on some
/dev/loop devices first is always a good idea.

NeilBrown

--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html