HDD performance decreasing a lot

[Massimo-B]

Hi, I'm running several single device btrfs from 1TB up to 4TB, most are SSD, NVMe, some are 1TB HDDs. I use btrbk a lot to store snapshots to other btrfs. I'm running bees on all btrfs, especially on the bigger btrfs for deduping snapshots from different machines.

I plan to get one single 20TB HDD for NAS data, server data and also all the snapshots from other machines centralized on this big btrfs.

But as all non-rotating SSDs are fine, the rotating HDDs performance are seriously decreasing. Most probably due to fragmentation of extents? Usually I can measure fragmentation only per file and I did not find any serious high fragmentation. How could I analyze the overall fragmentation of the whole btrfs?

For instance: My 4TB NVMe receiving snapshots currently currently has 441 subvolumes. btrfs filesystem usage -g says 1759GiB free, min 960GiB. Runs fine. The 1TB HDD has 74 subvolumes, 200GiB free. But running btrbk, transferring ~100MB takes 10 to 20 minutes and doing a noisy lot of IO. bees is almost never finishing even after days.

The planned 20TB HDD would be connected to a 24/7 thin-client with small CPU and only 8 GB of RAM. Due to the size there could be around 800 subvolumes over time... I doubt that will work if I'm already struggling with the small 1TB disk. I'm quite sure that bees is responsible for the degraded performance. But without bees I would never be able to dedupe snapshots received from different machines.