Resuming mdadm reshape on a different system

Resuming mdadm reshape on a different system

[Jbum List]

My current situation:

1. Raspberry Pi 4 w/ 4 disk RAID 5 array
2. PC for general development and test.

I had a 5th disk I wanted to add to the array and in the interest of making things go faster, I decided to temporarily hook up the raid array to my PC.  I brought over the existing mdadm.conf settings from the Pi and the array was brought up successfully without any issue on my PC.

I started the grow/reshape operation after adding the new disk and everything is going well.  However, I noticed that the rebuild speed isn't that much better than what I'm used to see on the Pi.  So rather than wait for the operation to complete (in a few days), I wanted to move the array over to the Pi and continue there.

Can I pause/halt the reshape that's currently running on my PC and resume on the Pi?  I know you can pause/halt and resume on the same system it was started on but wasn't sure if that's possible across systems when both have the same configuration settings for the raid array.

Thanks for your help.

Re: Resuming mdadm reshape on a different system

[Roman Mamedov]

On Thu, 5 Dec 2024 00:09:02 +0000 (UTC)
Jbum List <jbumslist@yahoo.com> wrote:

> My current situation:
> 
> 1. Raspberry Pi 4 w/ 4 disk RAID 5 array
> 2. PC for general development and test.
> 
> I had a 5th disk I wanted to add to the array and in the interest of making things go faster, I decided to temporarily hook up the raid array to my PC.  I brought over the existing mdadm.conf settings from the Pi and the array was brought up successfully without any issue on my PC.
> 
> I started the grow/reshape operation after adding the new disk and everything is going well.  However, I noticed that the rebuild speed isn't that much better than what I'm used to see on the Pi.  So rather than wait for the operation to complete (in a few days), I wanted to move the array over to the Pi and continue there.
> 
> Can I pause/halt the reshape that's currently running on my PC and resume on the Pi?  I know you can pause/halt and resume on the same system it was started on but wasn't sure if that's possible across systems when both have the same configuration settings for the raid array.

Should be no problem. The reshape state is not saved in the OS, it's on the
actual array drives.

Before moving it back, you could first try:

  echo 1000000 > /sys/devices/virtual/block/mdX/md/sync_speed_min

and see if this makes it faster.

I would also expect the PC to be faster, at least if you connect the drives to
its onboard fully independent SATA ports with enough PCIe bandwidth to the
controller, and not e.g. the same USB enclosure.

-- 
With respect,
Roman

Re: Resuming mdadm reshape on a different system

[Jbum List]

> On Wednesday, December 4, 2024 at 04:26:22 PM PST, Roman Mamedov <rm@romanrm.net> wrote:
>
> On Thu, 5 Dec 2024 00:09:02 +0000 (UTC)
>
> Jbum List <jbumslist@yahoo.com> wrote:
>
>> My current situation:
>>
>> 1. Raspberry Pi 4 w/ 4 disk RAID 5 array
>> 2. PC for general development and test.
>>
>> I had a 5th disk I wanted to add to the array and in the interest of making things go faster, I decided to temporarily hook up the raid array to my PC.  I brought over the existing mdadm.conf settings from the Pi and the array was brought up successfully without any issue on my PC.
>>
>> I started the grow/reshape operation after adding the new disk and everything is going well.  However, I noticed that the rebuild speed isn't that much better than what I'm used to see on the Pi.  So rather than wait for the operation to complete (in a few days), I wanted to move the array over to the Pi and continue there.
>>
>> Can I pause/halt the reshape that's currently running on my PC and resume on the Pi?  I know you can pause/halt and resume on the same system it was started on but wasn't sure if that's possible across systems when both have the same configuration settings for the raid array.
>
> Should be no problem. The reshape state is not saved in the OS, it's on the
actual array drives.

Awesome.  That's music to my ears.  Thanks for the confirmation.

>
> Before moving it back, you could first try:
>
>  echo 1000000 > /sys/devices/virtual/block/mdX/md/sync_speed_min
>

I tried this but I didn't see any noticeable difference.  It could be due to other settings I had already tweaked (read ahead, stripe cache size, etc.).

> and see if this makes it faster.
>
> I would also expect the PC to be faster, at least if you connect the drives to
> its onboard fully independent SATA ports with enough PCIe bandwidth to the
> controller, and not e.g. the same USB enclosure.

My "PC" is really a laptop so I'm using the same multi-bay drive enclosure.  I suspect the USB interface is what's mostly limiting the speed.  Appreciate the suggestion though.

Thanks, Roman.