Cache size not correct

[francrouge]

Hi all 

 

I got a question for you.

 

I have a cache pool option in my setup.

 

I have 3 ssd

 

1 x240gb

1x240gb

1x256gb

 

My probleme is that the total is 368gb in unraid.

 

How could this be right ?

 

unraid-diagnostics-20181127-0456.zip

[John_M]

That is exactly correct. See here:

[John_M]

I'm not sure what capacity you were expecting to see but BTRFS RAID 1 is a little different from traditional RAID 1. With three devices you don't end up with a triple mirror. Instead you get exactly two copies of any block. I believe the blocks are allocated to the three devices in a round robin fashion. The fact that the three disks aren't of equal capacity makes it complicated to visualise but think of it like this: half of the 256 GB device is mirrored on each of the two 240 GB devices, leaving 240 - 128 = 112 GB that's mirrored between the two 240 GB devices. 256 + 112 = 368 GB.

[francrouge]

O okay thx for the info

I'm not sure what capacity you were expecting to see but BTRFS RAID 1 is a little different from traditional RAID 1. With three devices you don't end up with a triple mirror. Instead you get exactly two copies of any block. I believe the blocks are allocated to the three devices in a round robin fashion. The fact that the three disks aren't of equal capacity makes it complicated to visualise but think of it like this: half of the 256 GB device is mirrored on each of the two 240 GB devices, leaving 240 - 128 = 112 GB that's mirrored between the two 240 GB devices. 256 + 112 = 368 GB.

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