Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

[Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>]

2017-04-02 17:28 GMT+02:00 Thor Lancelot Simon <tls%panix.com@localhost>:
> However -- I believe for the 20-30% of SAS drives you mention as shipping
> with WCE set, it should be possible to obtain nearly identical performance
> and more safety by setting the Queue Algorithm Modifier bit in the control
> mode page to 1.  This allows the drive to arbitrarily reorder SIMPLE
> writes so long as the precedence rules with HEAD and ORDERED commands
> are respected.

Is there any reason we wouldn't want to set QAM=1 by default for
sd(4)? Seems like pretty obvious performance improvement tweak.

MODE SENSE has a flag to tell it this field is settable, so should be
pretty safe to set relying on that.

I can make this change (maybe with sysctl to control it?),
unfortunately I don't have any hw to test if it actually makes
measurable difference. Any volunteers to test a patch?

Jaromir