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

Jeff Garzik
2009-03-30T19:10:43Z

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