Re: Best Practice: Add new device to RAID1 pool

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-07-24 07:27, Cloud Admin wrote:
> Hi,
> I have a multi-device pool (three discs) as RAID1. Now I want to add a
> new disc to increase the pool. I followed the description on https://bt
> rfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices and
> used 'btrfs add <device> <btrfs path>'. After that I called a balance
> for rebalancing the RAID1 using 'btrfs balance start <btrfs path>'.
> Is that anything or should I need to call a resize (for example) or
> anything else? Or do I need to specify filter/profile parameters for
> balancing?
> I am a little bit confused because the balance command is running since
> 12 hours and only 3GB of data are touched. This would mean the whole
> balance process (new disc has 8TB) would run a long, long time... and
> is using one cpu by 100%.

Based on what you're saying, it sounds like you've either run into a 
bug, or have a huge number of snapshots on this filesystem.  What you 
described is exactly what you should be doing when expanding an array 
(add the device, then run a full balance).  The fact that it's taking 
this long isn't normal, unless you have very slow storage devices.