On 2019-12-20 11:53 a.m., Marc Lehmann wrote:

> 
>    Filesystem               Size  Used Avail Use% Mounted on
>    /dev/mapper/xmnt-cold15   27T   23T     0 100% /cold1
> 
>    Overall:
>        Device size:                       24216.49GiB
>        Device allocated:                  20894.89GiB
>        Device unallocated:                 3321.60GiB
>        Device missing:                        0.00GiB
>        Used:                              20893.68GiB
>        Free (estimated):                   3322.73GiB      (min: 1661.93GiB)
>        Data ratio:                               1.00
>        Metadata ratio:                           2.00
>        Global reserve:                        0.50GiB      (used: 0.00GiB)
> 
>    Data,single: Size:20839.01GiB, Used:20837.88GiB (99.99%)
>       /dev/mapper/xmnt-cold15      9288.01GiB
>       /dev/mapper/xmnt-cold12      7427.00GiB
>       /dev/mapper/xmnt-cold13      4124.00GiB
> 
>    Metadata,RAID1: Size:27.91GiB, Used:27.90GiB (99.97%)
>       /dev/mapper/xmnt-cold15        25.44GiB
>       /dev/mapper/xmnt-cold12        24.46GiB
>       /dev/mapper/xmnt-cold13         5.91GiB
> 
>    System,RAID1: Size:0.03GiB, Used:0.00GiB (6.69%)
>       /dev/mapper/xmnt-cold15         0.03GiB
>       /dev/mapper/xmnt-cold12         0.03GiB
> 
>    Unallocated:
>       /dev/mapper/xmnt-cold15         0.01GiB
>       /dev/mapper/xmnt-cold12         0.00GiB
>       /dev/mapper/xmnt-cold13      3321.59GiB
> 

You don't need hints, the problem is right here.

Your Metadata is Raid 1, (which requires minimum of 2 devices,) Your
allocated metadata is full (27.90GB / 27.91 GB) and you only have 1
device left with unallocated space, so no new metadata space can be
allocated until you fix that.