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

Jeff Garzik
2009-03-30T17:56:32Z

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?

Jeff