Re: WAPBL and write cacheing (was: SATA write performance problems)

[christos%astron.com@localhost (Christos Zoulas)]

In article 
<CAJcb3foogeoiw_XgnRKoVyKigAB+vf8jSiFFiZ8ZgpyjRijmZQ%mail.gmail.com@localhost>,
Andy Ruhl  <acruhl%gmail.com@localhost> wrote:
>On Thu, Jan 3, 2013 at 4:54 AM, Lars Heidieker
><lars.heidieker%googlemail.com@localhost> wrote:
>> On Thu, Jan 3, 2013 at 12:49 PM, Edgar Fuß <ef%math.uni-bonn.de@localhost> 
>> wrote:
>>>> Doesn't this depend on filesystem journaling?
>>> Can someone please enlighten me?
>>> Is it safe to use write cacheing on a SATA drive with FFS/WAPBL on it?
>>
>> AFAIK it depends on the drive, if it doesn't lie about the command to
>> flush the cache it's safe.
>> WAPBL sends such a command on commit.
>
>So I think what you are saying is that WAPBL asks the drive to flush
>it's volatile cache before the journal update is done?
>
>There was talk a while back on some list (I don't remember if it was a
>NetBSD list) that certain OS behavior (maybe not NetBSD) was flushing
>cache so often that drive cache performance benefits were essentially
>negated. So the drive would ignore some of the cache requests which
>leaves systems using journaling vulnerable. The fail-safe was to just
>turn off cache completely.

Actually it was the addition of:

    sysctl -w vfs.wapbl.flush_disk_cache=0

and the discussion on the actual behavior of various cache flushing
commands on different types of buses and drives.

christos