Downsizing cache pool without moving data

[IamSpartacus]

I currently have four Intel 730 480GB SSDs in my cache pool that Im planning to move to a new server and I'll be replacing them with two other 480GB SSDs.

 

Of the roughly 1TB of usable space I'm only using 250GB currently so my plan is to swap two of the Intel 730s out for the two new SSDs, bring the pool back up, then bring it down again and pull the last two 730s out leaving only the two new SSDs.  Would this method work for keeping all my data in place?

[JorgeB]

Yes, but because the way btrfs raid1 works, I believe you should remove one SSD at a time:

 

1-remove one SSD, start array and wait for pool to balance (when there’s no more cache reads/writes)

2-remove other SSD, start array and balance

3-add both new SSDs, start array.

 

Possibly steps 2 and 3 could be done a same time, but I’m not sure as I never tested.

 

Old SSDs have to be physically disconnect (or formatted), trying to start array with them unassigned will result in unmountable cache pool.

 

Always good idea to backup cache just in case.

 

[IamSpartacus]

Yes, but because the way btrfs raid1 works, I believe you should remove one SSD at a time:

 

1-remove one SSD, start array and wait for pool to balance (when there’s no more cache reads/writes)

2-remove other SSD, start array and balance

3-add both new SSDs, start array.

 

Possibly steps 2 and 3 could be done a same time, but I’m not sure as I never tested.

 

Old SSDs have to be physically disconnect (or formatted), trying to start array with them unassigned will result in unmountable cache pool.

 

Always good idea to backup cache just in case.

 

So the process would go:

 

1.  Remove SSD1, start array, wait to balance

2.  Remove SSD2, start array, wait to balance

3.  Install 2 new SSDs, start array, wait to balance

4.  Remove SSD3, start array, wait to balance

5.  Remove SSD4, start array

 

And yes, of course I will have a backup of my data.

[JorgeB]

Yes, forgot you want to remove the other 2 also.

[IamSpartacus]

Yes, forgot you want to remove the other 2 also.

 

Sounds good.  What's the best way to determine when the pool is fully balanced after a change?

[JorgeB]

Wait for cache read/writes to stop, then check the cache page, if it reports the correct number of disks it’s done.