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

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Message-ID: <dec5b832-53f1-4274-902c-418f01df9458@kernel.org>
Date: Tue, 12 Aug 2025 15:53:54 +0800
From: Chao Yu <chao@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: chao@...nel.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 8/12/25 15:32, Christoph Hellwig wrote:
> 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.

Yeah, I think that may hurt the performance too, at least, I don't see
any obvious change for time cost of generic/299 testcases, but still I
need to run all my testcase to see what will happen. :)

generic/299 115s ...  113s

> 
> 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.

Thanks,