Need to shrink md component disk partitions - what is the best method?

Need to shrink md component disk partitions - what is the best method?

[Ram Ramesh]

I shrunk md device to suit my needs and now I need to shrink the 
partition of each component device so that I can reuse the unused space 
on the component disks. Every google search that I encountered only 
outlines a method where you fail, remove, re-partition, add, sync each 
disk. With bitmap, I guess each sync will be very fast as data is really 
not being changed. Since I have raid6, I should be able to work on one 
disk at a time without much risk also.

However, is this the recommended method? Why not simply boot on a rescue 
system, do not assemble the array, simply change all partitions and 
reboot the original system so that the array will get assembled the 
normal way? Is this too risky or inferior?

While we are at it, mdadm -E says
>  Avail Dev Size : 11720780943 (5588.90 GiB 6001.04 GB)
>      Array Size : 12348030976 (11776.00 GiB 12644.38 GB)
>   Used Dev Size : 6174015488 (2944.00 GiB 3161.10 GB)
What is Used Dev Size? Is is the last sector used or the number of 
sectors used? If my current partition(s) start at 2048, does it mean I 
should end the md partitions on or after 6174015488 or (6174015488+2048)?

How do I make mdadm scrub the entire array even though bit map says 
there is no need?

Ramesh

Re: Need to shrink md component disk partitions - what is the best method?

[Ram Ramesh]

On 07/10/2017 09:27 PM, Ram Ramesh wrote:
> I shrunk md device to suit my needs and now I need to shrink the 
> partition of each component device so that I can reuse the unused 
> space on the component disks. Every google search that I encountered 
> only outlines a method where you fail, remove, re-partition, add, sync 
> each disk. With bitmap, I guess each sync will be very fast as data is 
> really not being changed. Since I have raid6, I should be able to work 
> on one disk at a time without much risk also.
>
> However, is this the recommended method? Why not simply boot on a 
> rescue system, do not assemble the array, simply change all partitions 
> and reboot the original system so that the array will get assembled 
> the normal way? Is this too risky or inferior?
>
> While we are at it, mdadm -E says
>>  Avail Dev Size : 11720780943 (5588.90 GiB 6001.04 GB)
>>      Array Size : 12348030976 (11776.00 GiB 12644.38 GB)
>>   Used Dev Size : 6174015488 (2944.00 GiB 3161.10 GB)
> What is Used Dev Size? Is is the last sector used or the number of 
> sectors used? If my current partition(s) start at 2048, does it mean I 
> should end the md partitions on or after 6174015488 or (6174015488+2048)?
>
> How do I make mdadm scrub the entire array even though bit map says 
> there is no need?
>
> Ramesh
>
My attempt to remove a drive repartition and readd did not work. mdadm 
refused to readd the device. Any one know why? This is what I did.

sudo mdadm /dev/md0 --fail /dev/sdb1
sudo mdadm /dev/md0 --remove /dev/sdb1
sudo gdisk /dev/sdb
< shrunk the partition size so that /dev/sdb1 is  2048 - 6442452991 and 
created /dev/sdb2 6442452992-11721045134>

    New partition table:
    Number  Start (sector)    End (sector)  Size       Code  Name
        1            2048      6442452991   3.0 TiB     FD00  Linux RAID
        2      6442452992     11721045134   2.5 TiB     FD00  Linux RAID

sudo mdadm /dev/md0 --re-add /dev/sdb1
***mdadm: --re-add for /dev/sdb1 to /dev/md0 is not possible***

Why? I have enabled bit maps and /proc/mdstat shows that.

Any way, I simply added the drive and it is rebuilding. I thought I knew 
what will happen. You live and learn.

Now I am happy that I did not redo the partition on more than one drive. 
I will wait for expert advice before proceeding.

Ramesh

Re: Need to shrink md component disk partitions - what is the best method?

[Adam Goryachev]

On 11/07/17 14:56, Ram Ramesh wrote:
> On 07/10/2017 09:27 PM, Ram Ramesh wrote:
>> I shrunk md device to suit my needs and now I need to shrink the 
>> partition of each component device so that I can reuse the unused 
>> space on the component disks. Every google search that I encountered 
>> only outlines a method where you fail, remove, re-partition, add, 
>> sync each disk. With bitmap, I guess each sync will be very fast as 
>> data is really not being changed. Since I have raid6, I should be 
>> able to work on one disk at a time without much risk also.
>>
>> However, is this the recommended method? Why not simply boot on a 
>> rescue system, do not assemble the array, simply change all 
>> partitions and reboot the original system so that the array will get 
>> assembled the normal way? Is this too risky or inferior?
>>
>> While we are at it, mdadm -E says
>>>  Avail Dev Size : 11720780943 (5588.90 GiB 6001.04 GB)
>>>      Array Size : 12348030976 (11776.00 GiB 12644.38 GB)
>>>   Used Dev Size : 6174015488 (2944.00 GiB 3161.10 GB)
>> What is Used Dev Size? Is is the last sector used or the number of 
>> sectors used? If my current partition(s) start at 2048, does it mean 
>> I should end the md partitions on or after 6174015488 or 
>> (6174015488+2048)?
>>
>> How do I make mdadm scrub the entire array even though bit map says 
>> there is no need?
>>
>> Ramesh
>>
> My attempt to remove a drive repartition and readd did not work. mdadm 
> refused to readd the device. Any one know why? This is what I did.
>
> sudo mdadm /dev/md0 --fail /dev/sdb1
> sudo mdadm /dev/md0 --remove /dev/sdb1
> sudo gdisk /dev/sdb
> < shrunk the partition size so that /dev/sdb1 is  2048 - 6442452991 
> and created /dev/sdb2 6442452992-11721045134>
>
>    New partition table:
>    Number  Start (sector)    End (sector)  Size       Code  Name
>        1            2048      6442452991   3.0 TiB     FD00  Linux RAID
>        2      6442452992     11721045134   2.5 TiB     FD00  Linux RAID
>
> sudo mdadm /dev/md0 --re-add /dev/sdb1
> ***mdadm: --re-add for /dev/sdb1 to /dev/md0 is not possible***
>
> Why? I have enabled bit maps and /proc/mdstat shows that.
>
> Any way, I simply added the drive and it is rebuilding. I thought I 
> knew what will happen. You live and learn.
>
> Now I am happy that I did not redo the partition on more than one 
> drive. I will wait for expert advice before proceeding.
>
> Ramesh 

I think the md signature is at the end of the device, thus it can't be 
found after you shrink the partition. You would need to use a version of 
the MD label which is at the beginning of the partition to make this work.

AFAIK...

Regards,
Adam

-- 
Adam Goryachev Website Managers www.websitemanagers.com.au
-- 
The information in this e-mail is confidential and may be legally privileged.
It is intended solely for the addressee. Access to this e-mail by anyone else
is unauthorised. If you are not the intended recipient, any disclosure,
copying, distribution or any action taken or omitted to be taken in reliance
on it, is prohibited and may be unlawful. If you have received this message
in error, please notify us immediately. Please also destroy and delete the
message from your computer. Viruses - Any loss/damage incurred by receiving
this email is not the sender's responsibility.