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

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Message-ID: <aJruCTOjcj1nEk-S@infradead.org>
Date: Tue, 12 Aug 2025 00:32:25 -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 02:28:46PM +0800, Chao Yu wrote:
> BTW, I suffered extremely long latency of checkpoint which may block every
> update operations when testing generic/299 w/ mode=lfs mount option in qemu,
> then I propose to use PREFLUSH instead of FUA to resolve this issue.
> 
> "F2FS-fs (vdc): checkpoint was blocked for 24495 ms"
> 
> I just realize that using cache=directsync option in qemu can avoid FUA hang
> issue, anyway, let me test more w/ this option.

Well, for decent qemu performance you always want to use DIRECT I/O.
directsync is generally not a very good idea as it forces every write
to be synchronous and will give you very bad performance.

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.