On Thu, Apr 7, 2011 at 2:23 PM, Warner Losh <[email protected]> wrote:
>
> On Apr 7, 2011, at 3:00 PM, Garrett Cooper wrote:
>
>> On Thu, Apr 7, 2011 at 12:03 PM, Alexander Best <[email protected]> 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
>>>>
>>>> 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.
>>>
>>> how hard would it be to support per device sysctls/tunables? i'd really
>>> like to
>>> do:
>>>
>>> kern.cam.ada.0.write_cache=0 (root fs)
>>> kern.cam.ada.1.write_cache=1 (/usr, /var, etc.)
>>
>> 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.
Alex, Warner: thanks for the practical example; I hadn't really
considered that :).
Cheers!
-Garrett
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