Re: cache+barriers vs cache+nobarriers vs disabled cache+barriers vs disabled cache+nobarriers

[Martin Steigerwald <Martin@lichtvoll.de>]

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Am Donnerstag 15 März 2007 schrieb Leon Kolchinsky:
> Hello All,
>
>
> After reading http://oss.sgi.com/projects/xfs/faq.html#wcache
> and some posts on the list I've got the following question:
>
> If I have disabled write cache on the disk (hdparm -W0 /dev/hda) and by
> default FS is mounted with "barrier" enabled, Is there any taste in
> enabling "barrier"(by default) because write cache is disabled anyway
> or may be it's a good idea to mount with "nobarriers" in this case?

Hello Leon!

It is not needed to enable barriers when write cache is disabled. Enabling 
barriers in this case shouldn't have any visible effect I think.

> I thought that "barrier" is on by default to somewhat minimize
> potential dangers of enabled write cache? But if write cache is
> disabled, would "barrier" option just slow down the FS performance
> (which is already slowed down by "hdparm -W0 /dev/had" anyway)?

I think it wouldn't slow down any more, except maybe a minimal slow down 
due to a little bit more of code executed inside XFS.

But why do you want to disable write cache in the first case? As long as 
you are using 2.6.17.7 or later you can safely enable barriers and and 
write cache. At least that is my experience upto 2.6.20.1 with old IDE 
drivers and now since some hours on my ThinkPad T42 2.6.20.3 with libata 
drivers.

With write barriers XFS and enabled write cache, XFS will not be as fast 
as without write barriers and with enabled write cache - which is the 
unsafe combination -, but it will still be faster than with disabled 
write cache.

Regards,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7

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