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From: Warner Losh <imp@bsdimp.com>
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Date: Thu, 7 Apr 2011 15:23:31 -0600
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To: Garrett Cooper <yanegomi@gmail.com>
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Cc: svn-src-head@freebsd.org, Alexander Best <arundel@freebsd.org>,
	Alexander Motin <mav@freebsd.org>, src-committers@freebsd.org,
	svn-src-all@freebsd.org
Subject: Re: svn commit: r220412 - in head: share/man/man4 sys/cam/ata
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On Apr 7, 2011, at 3:00 PM, Garrett Cooper wrote:

> On Thu, Apr 7, 2011 at 12:03 PM, Alexander Best <arundel@freebsd.org> =
wrote:
>> On Thu Apr  7 11, Alexander Motin wrote:
>>> Author: mav
>>> Date: Thu Apr  7 08:17:53 2011
>>> New Revision: 220412
>>> URL: http://svn.freebsd.org/changeset/base/220412
>>>=20
>>> Log:
>>>   Make ada(4) driver to control device write cache, same as ata(4) =
does.
>>>   Add kern.cam.ada.write_cache sysctl/tunable to control it alike =
hw.ata.wc.
>>=20
>> how hard would it be to support per device sysctls/tunables? i'd =
really like to
>> do:
>>=20
>> kern.cam.ada.0.write_cache=3D0 (root fs)
>> kern.cam.ada.1.write_cache=3D1 (/usr, /var, etc.)
>=20
> Does it really make sense to turn on write caching for one drive and
> not the other(s)?

Think about /usr/obj or /tmp and ask that question again.  Or any =
filesystem that's mounted that you don't really care about the contents =
of across a power cycle.  If you have to recreate it, that's OK.   In =
those cases, you may want the speed increase over safety that =
write_cache gives you.  Or maybe you have a drive that's doing write =
caching correctly and one that doesn't.

Warner