Re: How to debug intermittent increasing md/inflight but no disk activity?

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On 11/07/2024 01:12, Dave Chinner wrote:
Probably not a lot you can do short of reconfiguring your RAID6
storage devices to handle small IOs better. However, in general,
RAID6 /always sucks/ for small IOs, and the only way to fix this
problem is to use high performance SSDs to give you a massive excess
of write bandwidth to burn on write amplification....
RAID5/6 has the same issues with NVME drives.
Major issue is the bitmap.

5 disk NVMe RAID5, 64K chunk

Test                   BW         IOPS
bitmap internal 64M    700KiB/s   174
bitmap internal 128M   702KiB/s   175
bitmap internal 512M   1142KiB/s  285
bitmap internal 1024M  40.4MiB/s  10.3k
bitmap internal 2G     66.5MiB/s  17.0k
bitmap external 64M    67.8MiB/s  17.3k
bitmap external 1024M  76.5MiB/s  19.6k
bitmap none            80.6MiB/s  20.6k
Single disk 1K         54.1MiB/s  55.4k
Single disk 4K         269MiB/s   68.8k

Tested with fio --filename=/dev/md/raid5 --direct=1 --rw=randwrite --bs=4k --ioengine=libaio --iodepth=1 --runtime=60 --numjobs=1 --group_reporting --time_based --name=Raid5




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