linux-kernel - Re: [PATCH] f2fs: add sysfs entry to avoid FUA

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Date:   Fri, 27 May 2022 17:26:32 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Eric Biggers <ebiggers@...nel.org>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-block@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH] f2fs: add sysfs entry to avoid FUA

On 05/28, Dave Chinner wrote:
> On Fri, May 27, 2022 at 09:33:55PM +0000, Eric Biggers wrote:
> > [+Cc linux-block for FUA, and linux-xfs for iomap]
> 
> linux-fsdevel should really be used for iomap stuff...
> 
> > 
> > On Fri, May 27, 2022 at 01:59:55PM -0700, Jaegeuk Kim wrote:
> > > Some UFS storage gives slower performance on FUA than write+cache_flush.
> > > Let's give a way to manage it.
> > > 
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > 
> > Should the driver even be saying that it has FUA support in this case?  If the
> > driver didn't claim FUA support, that would also solve this problem.
> 
> Agreed, this is a hardware problem that need to addressed with a
> driver quirk to stop it advertising FUA support. The high level
> fs/iomap code should always issue FUA writes where possible and
> the lower layers tell the block layer whether to issue the FUA as
> a FUA or write+cache flush pair.

I was thinking to turn off FUA in driver side quickly tho, one concern
was the bandwidth vs. latency. What if the device can support FUA having
short latency while giving low bandwidth? In that case, we still have
a room to utilize FUA for small-sized  writes such as filesystem metadata
writes, but avoid DIO w/ FUA for sequential write stream. Is this just
HW problem? Or, does SW need to use FUA more efficiently?

> 
> And, quite frankly, exposing this sort of "hardware needs help" knob
> as a sysfs variable is exactly the sort of thing we should never do.
> 
> Users have no idea how to tune stuff like this correctly (even if
> they knew it existed!), yet we know exactly what hardware has this
> problem and the kernel already has mechanisms that would allow it to
> just Do The Right Thing. IOWs, we can fix this without the user even
> having to know that they have garbage hardware that needs special
> help....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com