Different size cache drives in a pool?

[ziggie216]

Does all the cache drive have to be the same size? I have a 140Gb, 240Gb, and a 250Gb SSD. The 250Gb is currently being use as a cache drive and it's almost full due to the VMs that's I'm running on it. If not, any chance there is a way to mount the 240Gb without being part of the array?

[SSD]

Does all the cache drive have to be the same size? I have a 140Gb, 240Gb, and a 250Gb SSD. The 250Gb is currently being use as a cache drive and it's almost full due to the VMs that's I'm running on it.

 

You can add multiple cache drives (two or more), but it will create a RAID-1 type setup. I don't think you'd get more space unless you added a larger drive than what you have.

 

If not, any chance there is a way to mount the 240Gb without being part of the array?

 

This is easy to do.

[itimpi]

Does all the cache drive have to be the same size? I have a 140Gb, 240Gb, and a 250Gb SSD. The 250Gb is currently being use as a cache drive and it's almost full due to the VMs that's I'm running on it. If not, any chance there is a way to mount the 240Gb without being part of the array?

The drives do not have to be the same size.  The cache uses a variant of RAID-1 when there is multiple drives so with multiple drives the available space is determined by the smallest amount that means data can be held on at least 2 drives.

 

You can also mount drives outside the array.  The Unassigned Devices plugin can help with managing this.

 

Note that if you have any license other than Pro all devices attached to the PC count against the license limits regardless of whether they are being used by unRAID.

[ziggie216]

Thanks! I this case, i'll use the Unassigned Devices instead of messing around with the cache drive.

[trurl]

If you used all 3 of those, 250, 240, 140, you would get 315. The extra space may not be worth using 2 extra ports, but you also get mirroring.

 

btrfs disk usage calculator

 

[afsilver]

On 8/24/2015 at 5:50 AM, trurl said:

If you used all 3 of those, 250, 240, 140, you would get 315. The extra space may not be worth using 2 extra ports, but you also get mirroring.

 

btrfs disk usage calculator

 

I really do not understand how the space is calculated.

I have 2 drives in cache pool

1 250GB

1 500GB

Unraid reports total capacity of 375GB. Which i get if i split the diff and add to the smallest drive.

 

I tired the btrfs calculator, under no circumstances do i get 375.

 

But does this mean i get "proper" RAID1 up to 250GB, then the remaining is duplicated on the 500GB drive only?

 

I am also planning to add another 750GB drive to the cache pool, which in my head would give me proper redundancy with 750GB usable space?

[Squid]

30 minutes ago, afsilver said:

Unraid reports total capacity of 375GB.

Known limitation of btrfs where it reports the wrong size

 

 

[trurl]

34 minutes ago, afsilver said:

I really do not understand how the space is calculated.

I have 2 drives in cache pool

1 250GB

1 500GB

Unraid reports total capacity of 375GB. Which i get if i split the diff and add to the smallest drive.

 

I tired the btrfs calculator, under no circumstances do i get 375.

 

But does this mean i get "proper" RAID1 up to 250GB, then the remaining is duplicated on the 500GB drive only?

 

I am also planning to add another 750GB drive to the cache pool, which in my head would give me proper redundancy with 750GB usable space?

with 2 disks, 250+500 raid1, you only get a mirror equal to the smaller disk. Nothing else. Mirror requires the copy of any data to be on a separate device. It wouldn't be redundant if it weren't. So, you only have total capacity of 250.

 

With 3 disks, 250+500+750 raid1, calculator gives mirror with total capacity of 750. The 750 is mirrored by the 250+500. So the copy of any data can exist on a separate device.

[afsilver]

1 hour ago, Squid said:

Known limitation of btrfs where it reports the wrong size

 

 

How does unraid handle it when i exceed 250GB on the cache then?

1 hour ago, trurl said:

with 2 disks, 250+500 raid1, you only get a mirror equal to the smaller disk. Nothing else. Mirror requires the copy of any data to be on a separate device. It wouldn't be redundant if it weren't. So, you only have total capacity of 250.

 

With 3 disks, 250+500+750 raid1, calculator gives mirror with total capacity of 750. The 750 is mirrored by the 250+500. So the copy of any data can exist on a separate device.

Thank you, this means I do understand it after all (hopefully) 😂

[JonathanM]

19 hours ago, afsilver said:

How does unraid handle it when i exceed 250GB on the cache then?

Poorly. There will still be free space showing when you get out of space errors.

It's a BTRFS thing, not really under unraid's control.

[Raidiculous]

From what I understood it was supposed to gracefully switch over to writing to the array when free space on the cache is unavailable. Is this able to happen if BTRFS isn't reporting the proper free space?

[JorgeB]

Currently btrfs can correctly report the free space when using different size devices, it's just the GUI that isn't doing it, but it should be fixed soon on an upcoming release.

[Ver7o]

I don't want to open a new thread. I have a 250GB cache drive which I'm playing to replace with 2x2TB drives. 

 

The way I want to go about it is just add one 2TB SSD, wait for rebalance to do its thing, then replace the 250GB drive with the second 2TB in its place. Would that be the correct way to go about it, without going the longer route of using mover and changing share options?

[JorgeB]

1 hour ago, Ver7o said:

Would that be the correct way to go about it

That's fine, you could also replace the existing one first (see FAQ notes about single devive replacement) then add the other one.