linux-kernel - Re: [PATCH v3] f2fs: introduce flush

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Message-ID: <aJr0Tk36ZNiCg8mQ@infradead.org>
Date: Tue, 12 Aug 2025 00:59:10 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Chao Yu <chao@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>, jaegeuk@...nel.org,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v3] f2fs: introduce flush_policy sysfs entry

On Tue, Aug 12, 2025 at 03:53:54PM +0800, Chao Yu wrote:
> > What did you use before?  At least for older qemu the default was
> > buffered I/O, which can lead to very expensive fua or flush calls.
> 
> Previously, I didn't use any cache= option, as manual described, it
> should equal to cache=wrteback.

Modern qemu actually split the cache option.  You absolute want
cache.direct=on.  If you don't do simulated power fail testing by
killing qemu (or run real workloads for the matter, but who does that
:)) it might make sense to just ignore the flushes with cache.no-flush=on
as well, which is what I do for my test VMs on the laptop.