svn commit: r220412 - in head: share/man/man4 sys/cam/ata

[Warner Losh]

On Apr 7, 2011, at 3:00 PM, Garrett Cooper wrote:

> On Thu, Apr 7, 2011 at 12:03 PM, Alexander Best <arundel at 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
>>> 
>>> 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.

Warner