perftune.py: tuning 'write_cache' is bogus and dangerous

[vladzcloudius]

HEAD: 0d250d15ace37973a1fbf449cb06adbcc5f8a9da

Description Changing a value of /sys/block/.../queue/write_cache is bogus and dangerous because it doesn't change the behavior of the underlying HW and only changes the way kernel "thinks" the HW behaves.

Read more about this here: https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt

[soyacz]

A reminder for this bug - as we still set this value for GCE and Azure preinstalled images. If this setting is bogus, we should stop doing that. cc @slivne

[vladzcloudius]

cc @avikivity @xemul

While there was "some" reason to use it on GCE in the past (which is no longer the case) there is no reason whatsoever to use it on Azure AFAIK.

@slivne could you, please, clarify why have you decided to use it on Azure?

[fgelcer]

@yaronkaikov FYI