Re: Linear device of two arrays

[NeilBrown <neilb@suse.com>]

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On Sat, Jul 08 2017, Stan Hoeppner wrote:

> On 07/08/2017 05:26 AM, Veljko wrote:
>> On Sat, Jul 8, 2017 at 12:52 AM, Stan Hoeppner <stan@hardwarefreak.org> wrote:
>>> On 07/07/2017 03:26 PM, Veljko wrote:
>>>> I just noticed that I replied to Wol insted to list.
>>>>
>>>> On Wed, Jul 5, 2017 at 8:07 PM, Wols Lists <antlists@youngman.org.uk>
>>>> wrote:
>>>>> On 05/07/17 17:42, Roman Mamedov wrote:
>>>>>> On Wed, 5 Jul 2017 17:34:09 +0200
>>>>>> Veljko <veljko3@gmail.com> wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> I have a RAID10 device which I have formated using the mkfs.xfs
>>>>>>> defaults (Stan helped me with this few years back). I reached 88%
>>>>>>> capacity and it is time to expand it. I bought 4 more drives to create
>>>>>>> another RIAD10 array. I would like to create linear device out of
>>>>>>> those two and grow XFS across the 2nd device. How can this be done
>>>>>>> without loosing the existing device's data? I would also like to add a
>>>>>>> spare HDD. Do I have to have a separate spare HDD for each array or
>>>>>>> one can be used by both of them?
>>>>>> Why make another RAID10? With modern versions of mdadm and kernel you
>>>>>> should
>>>>>> be able to simply reshape the current RAID10 to increase the number of
>>>>>> devices used from 4 to 8.
>>>>>>
>>>>>>
>>>>> I was thinking of replying, but isn't that not possible for some
>>>>> versions of RAID-10?
>>>>>
>>>>> My feeling was, if you can't just add drives to the existing raid 10,
>>>>> create a new one which you can expand, migrate the fs across (btrfs
>>>>> would let you do that live, I believe, so xfs probably can too), then
>>>>> you can scrap the old raid-10 and add the drives into the new one.
>>>>>
>>>>> Cheers,
>>>>> Wol
>>>>
>>>> Thanks for your input, Roman and Wol.
>>>>
>>>> Expanding existing RAID is one of the options, but I was advised by
>>>> Stan Hoeppner to do it this way and I tend to believe him on this
>>>> subject. With my metadata heavy backup workload, this will provide
>>>> better performance.
>>>>
>>>> So my question is still, how can an existing array be added to linear
>>>> device, and it's file system expanded over the second array.
>>> For this to work the existing RAID10 array must already be a member of a
>>> linear device with one component device.  If this linear array already
>>> exists then you could add another RAID10 array to the linear device.  If you
>>> currently have an XFS filesystem sitting atop a 'bare' RAID10 then I don't
>>> believe the linear option will work for you.  Thus I'd tend to agree with
>>> others that reshaping your current RAID10 is the best option.
>>>
>>> My apologies if I wasn't clear in my previous advice.
>>>
>>> Stan
>> Here is your previous advice:
>> "Do not use LVM.  Directly format the RAID10 device using the mkfs.xfs
>> defaults.  mkfs.xfs will read the md configuration and automatically
>> align the filesystem to the stripe width.
>>
>> When the filesystem reaches 85% capacity, add 4 more drives and create
>> another RAID10 array.  At that point we'll teach you how to create a
>> linear device of the two arrays and grow XFS across the 2nd array."
>>
>>  From this I concluded that it is possible to create linear device
>> using existing array, but since it is not the case, I'll just have to
>> create new array, move data to it, than add first array to a new
>> linear device (1 member), copy my data to it, and than join second
>> device to it. I wanted to avoid all this copying but will have to do
>> it. Than I will add one drive to spare-group both arrays will be a
>> members of, as advised by Andreas. Does this sound OK?
>
> Yes.  Although, I was hoping Neil Brown would chime in here. I'm not 
> entirely sure that you can't do this the way I originally mentioned.

I wasn't clear to me that I needed to chime in..  and the complete lack
of details (not even an "mdadm --examine" output), meant I could only
answer in vague generalizations.
However, seeing you asked.
If you really want to have a 'linear' of 2 RAID10s, then
0/ unmount the xfs filesystem
1/ backup the last few megabytes of the device
    dd if=/dev/mdXX of=/safe/place/backup bs=1M skip=$BIGNUM
2/ create a linear array of the two RAID10s, ensuring the
   metadata is v1.0, and the dataoffset is zero (should be default with
   1.0)
    mdadm -C /dev/mdZZ -l linear -n 2 -e 1.0 --data-offset=0 /dev/mdXX /dev/mdYY
3/ restore the saved data
    dd of=/dev/mdZZ if=/safe/place/backup bs=1M seek=$BIGNUM
4/ grow the xfs filesystem
5/ be happy.

I cannot comment on the values of "few" and "$BUGNUM" without seeing
specifics.

NeilBrown

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