Re: [PATCH 5/7] vfs: Add wbcflush sysfs knob to disable storagedevice writeback cache flushes

[Jeff Garzik]

Jens Axboe wrote:
> On Mon, Mar 30 2009, Jeff Garzik wrote:
> > Jens Axboe wrote:
> > > On Mon, Mar 30 2009, Bartlomiej Zolnierkiewicz wrote:
> > > > On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
> > > > > Add a sysfs knob to disable storage device writeback cache flushes.
> > > > The horde of casual desktop users (with me included) would probably prefer
> > > > having two settings -- one for filesystem barriers and one for fsync().
> > > >
> > > > IOW I prefer higher performance at the cost of risking losing few last
> > > > seconds/minutes of work in case of crash / powerfailure but I would still
> > > > like to have the filesystem in the consistent state after such accident.
> > > The knob is meant to control whether we really need to send a flush to
> > > the device or not, so it's an orthogonal issue to what you are talking
> > > about. For battery backed caches, we never need to flush. This knob is
> > > useful IFF we have devices with write back caches that STILL do a cache
> > > flush.
> > How do installers and/or kernels detect a battery-backed cache that does
> > not need flush?
>
> They obviously can't, otherwise it would not be an issue at all. And
> whether it's an issue is up for debate, until someone can point at such
> a device. You could add a white/blacklist.

Sorry, I guess I misinterpreted your dual "IFF" statement :)

I completely agree that the suggested knob, for disabling cache flush 
for these battery-backed devices, is at the present time addressing an 
entirely theoretical argument AFAICS.

	Jeff


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