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Date: Tue, 28 May 2019 09:13:56 -0400 From: "Theodore Ts'o" <tytso@....edu> To: Sahitya Tummala <stummala@...eaurora.org> Cc: Andreas Dilger <adilger.kernel@...ger.ca>, linux-ext4@...r.kernel.org Subject: Re: fsync_mode mount option for ext4 On Tue, May 28, 2019 at 09:18:30AM +0530, Sahitya Tummala wrote: > > Yes, but fsync_mode=nobarrier is little different than > a general nobarrier option. The fsync_mode=nobarrier is > only controlling the flush policy for fsync() path, unlike > the nobarrier mount option which is applicable at all > places in the filesystem. What are you really trying to accomplish with fsync_mode=nobarrier? And when does that distinction have a difference? What sort of guarantees are you trying to offer, given a particular hardware and software design? I gather that fsync_mode=nobarrier means one of two things: * "screw you, application writer; your data consistency means nothing to me", OR * "we have sufficient guarantees --- e.g., UPS/battery protection to guarantee that even if we lose AC mains, or the user press and holds the power button for eight seconds, we will give storage devices a sufficient grace period to write everything to persistent storage. We also have the appropriate hardware to warn of an impending low-battery shutdown and software to perform a graceful shutdown in that eventuality." If it's the latter, then nobarrier works just as well --- even better. If it's the former, *why* is it considered a good thing to ignore the requests of userspace? And without any hardware assurances to provide a backstop against power drop, do you care or not care about file system consistency? Why do you want the distinction between fsymc_mode=nobarrier and nobarrier? When would this distinction be considered a good thing? - Ted
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