Unraid 3diskraid 5 cache pool

[deeks]

Hi guys,

 

I would like to create a cache pool of 3ssd's of 480gb each in raid5 config. I have applied the -dconvert=raid5 -mconvert=raid1 command and expected about 960gb of free space using the carvox.org raid calulator. Unraid shows met 1,4Tb of free space howver. How do i know my raid setup is correct?

Edited January 24, 2020 by deeks

[JorgeB]

10 hours ago, deeks said:

How do i know my raid setup is correct?

You can check or post the output of

btrfs fi usage -T /mnt/cache

Note that for now when using raid5/6 with Unraid free space will be incorrectly reported, i.e., it doesn't account for parity, used space is correct, so say you have a raid5 pool using three 1TB devices with 500GB used, Unraid will report 500GB used and 2.5TB free when in reality you need to subtract space used by parity, so you have 1.5TB free.

[deeks]

Hi Johnnie.black - thanks for your response. This is the output I get.  :

 

 

Linux 4.19.88-Unraid.
Last login: Sat Jan 25 10:22:33 +0100 2020 on /dev/pts/0.
root@unRaid:~# btrfs fi usage -T /mnt/cache
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
Overall:
    Device size:                   1.31TiB
    Device allocated:                0.00B
    Device unallocated:            1.31TiB
    Device missing:                  0.00B
    Used:                            0.00B
    Free (estimated):                0.00B      (min: 8.00EiB)
    Data ratio:                       0.00
    Metadata ratio:                   0.00
    Global reserve:               16.00MiB      (used: 0.00B)

             Data      Metadata  System              
Id Path      RAID5     RAID5     RAID5    Unallocated
-- --------- --------- --------- -------- -----------
 1 /dev/sdc1   1.00GiB 512.00MiB 32.00MiB   445.60GiB
 2 /dev/sde1   1.00GiB 512.00MiB 32.00MiB   445.60GiB
 3 /dev/sdb1   1.00GiB 512.00MiB 32.00MiB   445.60GiB
-- --------- --------- --------- -------- -----------
   Total       2.00GiB   1.00GiB 64.00MiB     1.30TiB
   Used      768.00KiB 112.00KiB 16.00KiB     

 

I wiped the 3 cache disks so yes, used space is almost nil. But not getting a correct free space indication is a bit of a bummer as one might run out of free space without prior warning I suppose.

[deeks]

I found link  https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-space-emergency-response/ I gather that its a btrfs thing one has to live with

[deeks]

In terms of raid setup, what are considerations for setting raid 5 for btrfs metadata and system volumes as well?

[JorgeB]

13 hours ago, deeks said:

-mconvert=raid1

You metadata is raid5, not raid1 like you said you did, I recommend using raid1 for metadata, it's much safer.

[deeks]

yeah I tinkered around a bit with the mconvert last night...will revert to raid 1 😀

[deeks]

So with the raid5 setup I do need to check usage in stead of free space more than anything for my cache disks. Thanks for helping out!

 

What I do still wonder is why my previous cache disk setup (2 ssd in raid1) did not start moving data to disk as it ran out of disk space. My cache folders were set to "prefer" in the share settings. 

 

I would have thought that Unraid woud not further fill the cache pool a a certain point. It didn't....

[JorgeB]

2 minutes ago, deeks said:

My cache folders were set to "prefer" in the share settings.

Prefer moves from array to cache, you want cache="yes".

[deeks]

got it - thanks for that!