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Why is BTRFS scrub speed limited to 800mb/s? Where is the bottleneck?

TexasUnraid
in General Support

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So had a BTRFS corruption error on a drive and decided to run a scrub of all my drives to check them out but noticed something odd.

 

The total scrub speed for the system seems to be bottlenecked to around ~800mb/s?

 

AKA, if I start 4x drives scrubbing I get 200mb/s per drive but if I start a 5th drive they all drop to ~160mb/s for the same total system read speed.

 

If I do a parity check on the other hand I can get the full speed of all drives and over 2GB/s easily.

 

The system is a 2x 10c/20t 2680v2 and CPU usage is only around ~30%.

 

I would think adding more scrub jobs would be more threads and thus easily spread over the 40 threads I have. The SAS card can sustain far more then 800mb/s as shown by the parity check.

 

I can't figure out where the bottleneck could be, anyone have any ideas?

Edited by TexasUnraid

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