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Re: WAPBL and write cacheing (was: SATA write performance problems)
Andy Ruhl <acruhl%gmail.com@localhost> wrote:
>On Thu, Jan 3, 2013 at 4:54 AM, Lars Heidieker
>> On Thu, Jan 3, 2013 at 12:49 PM, Edgar Fuß <ef%math.uni-bonn.de@localhost>
>>>> 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.
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