[dm-devel] BIO_RW_SYNCIO
Mikulas Patocka
mpatocka at redhat.com
Mon Jul 26 21:53:40 UTC 2010
Hi Jens
I found out that when I remove BIO_RW_SYNCIO from dm-kcopyd.c, performance
when writing to the snapshot origin is increased twice. In this workload,
snapshot driver reads a lot of 8k chunks from one place on the disk and
writes them to the other place on the same disk. Without BIO_RW_SYNCIO, it
is twice faster with CFQ scheduler.
What does BIO_RW_SYNCIO exactly do? Does it attempt to decrease latency
and decrease throughput? (so that disk head seeks more between the source
and destination and it causes performance degradation) Is there some
general guidance, when use BIO_RW_SYNCIO and when not?
Mikulas
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