io - What are front merges in I/O scheduling and how can I tune this parameter? -


my linux uses deadline algorithm i/o scheduling. 1 of parameters front_merges parameter under /sys/block/sda/queue/iosched/front_merges. default set 1, means front merges occur. 1 can set 0 gain performance boost if don't expect front merges occur.

  1. what front merges exactly? can depict please?
  2. how know or test whether front merges occur on system or not?

when example reading in file, start end. it's beneficial merge requests same or adjacent blocks. called merging, because newer request added @ of existing one. enabled deadline scheduler.

a lot less common read blocks in reverse order , have newer request added in front of existing one.

both incur overhead check whether merge appropriate. merging, pays off, front merging depends more on workload.

i not aware of way profile this, apart modifying , recompiling kernel. interestingly, till at least rhel 5.2, front_merges turned off default. in more recent rhel versions it's on default. leave way.


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