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FAQ: how come write is faster than read?

Posted: Wed Oct 23, 2019 11:40 pm
by katsura
i'm guessing the write cache on the storage device is contributing to this behavior. to be confirmed...

fio exhibits the same issue.

APPLE SSD SM0512F:
  • Read: 26.8MB/s
  • Write: 102MB/s

Code: Select all

$ fio --name=test --ioengine=posixaio --direct=1 --gtod_reduce=1 --bs=4k --iodepth=1 --size=500M --readwrite=randread
...
Run status group 0 (all jobs):
   READ: bw=25.6MiB/s (26.8MB/s), 25.6MiB/s-25.6MiB/s (26.8MB/s-26.8MB/s), io=500MiB (524MB), run=19542-19542msec

Code: Select all

$ fio --name=test --ioengine=posixaio --direct=1 --gtod_reduce=1 --bs=4k --iodepth=1 --size=500M --readwrite=randwrite
...
Run status group 0 (all jobs):
  WRITE: bw=97.5MiB/s (102MB/s), 97.5MiB/s-97.5MiB/s (102MB/s-102MB/s), io=500MiB (524MB), run=5130-5130msec