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My BTRFS file system has plenty of space free, but the "SINGLE" section seems to be almost full. Please see the output of btrfs fi usage here:

btrfs fi usage /
Overall:
    Device size:          37.95TiB
    Device allocated:         15.85TiB
    Device unallocated:       22.11TiB
    Device missing:          0.00B
    Used:             15.48TiB
    Free (estimated):         22.46TiB  (min: 11.41TiB)
    Data ratio:               1.00
    Metadata ratio:           2.00
    Global reserve:      512.00MiB  (used: 0.00B)

Data,single: Size:15.79TiB, Used:15.43TiB
   /dev/sda3      15.79TiB

Metadata,DUP: Size:28.00GiB, Used:21.67GiB
   /dev/sda3      56.00GiB

System,DUP: Size:8.00MiB, Used:1.95MiB
   /dev/sda3      16.00MiB

Unallocated:
   /dev/sda3      22.11TiB

Is this high "Data, single" usage fine or should something be reconfigured / cleaned up to keep the file system operating smoothly? Currently, the file system has a very slow write performance.

Andi
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  • 40TB device? What device is this? Maybe this device is slow and not btrfs – ppuschmann Jan 04 '21 at 10:45
  • @ppuschmann this is a Hardware Raid (LSI MegaRaid), Raid Level 10 with 2 spans and 7 disks per span. I did a write test using dd on the freshly initialized device and write speed was pretty decent (i don't remember the exact number though). However back then, I wrote one big file with dd. Now, it seems slow, and we (probably) have mainly "small" disk access, running several mysql servers, writing small files etc. – Andi Jan 05 '21 at 11:16

1 Answers1

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If this is apart of an SSD or similar (NVMes for instance) then I would be worried about this yes.

Even on spinners those shouldn't be loaded than 85% at the very most. In order to prevent the heads from potentially corrupting the data on the spinning platters.

Assure that all flash storage devices are lesser of that of 75% in order to make sure the controller(s) are able to "breathe" and therefore "move unused" data blocks around.

An SSD (or any other flash storage device like a NVMe drive or even your thumb drive) operates ideally under such conditions. Cause think about it, if you took a box full (or even too full) of marbles, they aren't to move around or even much right? Same idea here.

Rickuku
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    The BTRFS filesystem is running on a hardware RAID10, built on classic (spinning) HDDs. What I do not understand is: What is the data single section? Why is it almost full even though the entire volume is less than half filled? – Andi Dec 29 '20 at 21:04
  • I understand, yea this is strange at barely half full. If you believe this not an error in the partitioning it's possible that the file system itself was not sized appropriately. I am not quite sure on how BTRFS works in this regards so I couldn't tell ya on the top of my head. :( – Rickuku Dec 29 '20 at 21:27