CentOS slows down when copying large files

[Abdelrahman Said]

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# don't buffer too many writes
echo $((16*1024*1024)) > /proc/sys/vm/dirty_background_bytes
echo $((48*1024*1024)) > /proc/sys/vm/dirty_bytes

# enable completely fair queuing
echo cfq > /sys/block/sda/queue/scheduler

Adding CFQ on top seems to solve both problems. I no longer have to wait until dd is finished, oowriter starts in parallel. The remedy in later kernels is called writeback throttling (4.10+) but RH did not backport this. However, CFQ does something similar: https://patchwork.kernel.org/patch/9661671/

Thank you very much, chemal. I confirm your test results. With CFQ, oowriter now doesn't wait until 'dd' is finished. Of course, it is slower to start than normal, but it starts. And the copying doesn't completely hog the system now.

Did RH backport that kyber scheduler or is there any other way to try it with the current distro kernel?

[chemal]

Kyber is for mq-blk only. You either need an NVME SSD or you can try the scsi_mod.use_blk_mq=y technology preview option.

https://access.redhat.com/documentation ... es/storage