How is a cache pool of disparately-sized drives handled?

 

I currently have a 60GB SSD and a 120GB SSD pooled for my cache.

The result is listed at 90GB of cache space.

 

That seems wrong for either RAID 0 or RAID 1 in my limited understanding.

This post seems like it details alternate options for this configuration, but doesn't shed light of the default behavior.

 

(I was considering swapping the 120GB drive out for a matching 60GB drive to use the larger drive elsewhere, but I want to understand exactly how the current system is working before I make that decision.)

Edited April 5, 20188 yr by Ruthalas

http://carfax.org.uk/btrfs-usage/

6 hours ago, johnnie.black said:

http://carfax.org.uk/btrfs-usage/

 

And, according to that calculator, the RAID 0 capacity is 120 GB and the RAID 1 capacity is 60 GB, as you might expect. So why is the OP seeing 90 GB reported? Is it a BTRFS bug or an unRAID bug? I vaguely remember this cropping up before and I'm surprised it hasn't been fixed yet.

 

@Ruthalas Would you mind posting your diagnostics so we can see what BTRFS thinks is going on?

 

2 minutes ago, John_M said:

So why is the OP seeing 90 GB reported? Is it a BTRFS bug or an unRAID bug? I vaguely remember this cropping up before and I'm surprised it hasn't been fixed yet.

It's a btrfs limitation:

https://lime-technology.com/forums/topic/46802-faq-for-unraid-v6/?do=findComment&comment=480420

 

For my understanding- that means I have 60GB of usable space, regardless of the reported size, yes?

 

In that case I should swap to the matching-size drive, as no benefit in conferred by having one larger drive.

 

How does the process of changing a cache disk out differ when you have 2 disks, if at all?

1 minute ago, Ruthalas said:

For my understanding- that means I have 60GB of usable space, regardless of the reported size, yes?

Correct.

 

2 minutes ago, Ruthalas said:

How does the process of changing a cache disk out differ when you have 2 disks, if at all?

You can use this one instead, much simpler and quicker for a pool.

 

Ah, excellent. That's perfect. Thank you!

I am having an issue, I believe due to the fact that I am attempting to move to a smaller drive.

 

I receive this error:
BTRFS: error (device sdj1) in btrfs_create_pending_block_groups :10269: errno=-28 No space left

 

Any advice on how to approach that?

Edit: I have backed up the full contents of the cache pool.

Edited April 6, 20188 yr by Ruthalas

1 hour ago, Ruthalas said:

I believe due to the fact that I am attempting to move to a smaller drive.

You can't replace a device with a smaller one using that procedure, it warns so in the notes.

Can I rebuild the entire pool, and then copy the contents back on? 

If the pool still mounts use this procedure to backup, rebuild pool, restore, if it doesn't post current diags.

I am attempting to follow those instructions but am running into one issue- The system share won't move.

 

I have set it to 'Yes', but a single file (system\libvirt\libvirt.img) won't move when I invoke the mover.

 

Am I missing something, or is there a way to force that move?

The obvious reason would be the file being in use, did you shutdown the VM service?

I did: Under Settings>Vm Manager>Enable VMs is set to "No"

 

Is there something else that might be using that?

 

Edit: (And if I didn't follow the proper method to disable VMs, let me know!)

Edited April 6, 20188 yr by Ruthalas

Probably not shutting down correctly, leave it disable, reboot and run the mover.

A restart did not allow that file to be moved.

 

(afterwards) I toggled the VM enable back on and off as well, with no success.
 

Would I be able to see errors that it may be throwing in the diagnostics download?

Edited April 6, 20188 yr by Ruthalas

Can I simply back that file up, perform the swap, and restore the file?

Can I simply back that file up, perform the swap, and restore the file?

Yes, if successfully copied, there's a chance it's not being moved because it's corrupt, diagnostics would show.

 

 

I was able to ignore that single file and copy it over manually after a rebuild.

 

Thanks for your help man!