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Date: Sat, 19 Jan 2013 14:39:28 -0800
Message-ID: <CAA3ZYrAF2wAseR0V+b2ko7ZaVKY0EyB4y34kZS=m-ZS4Eu5awg@mail.gmail.com>
Subject: Re: IBM blade server abysmal disk write performances
From: Dieter BSD <dieterbsd@gmail.com>
To: freebsd-hackers@freebsd.org
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Cc: gibbs@FreeBSD.org, scottl@FreeBSD.org, mjacob@FreeBSD.org
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Stefan writes:
> I seem to remember, that drives of that time required the write cache
> to be enabled to get any speed-up from tagged commands. This was no
> risk with SCSI drives, since the cache did not make the drives lye
> about command completion (i.e. the status for the write was only
> returned when the cached data had been written to disk, independently
> of the write cache enable).

Interesting.  Is there a way to tell, other than coming up with
some way to actually test it, whether a particular drive waits until
the data has been written to non-volatile memory (the platters in
conventional disks) before sending the command completion message?

I'm having thoughts of putting sensing resistors in the disk's
power cable, attaching an oscilloscope, and displaying the
timing of data on the data cable along with power usage from
seeking.