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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id p14sm2022341wro.3.2021.08.17.03.37.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Aug 2021 03:37:22 -0700 (PDT) Date: Tue, 17 Aug 2021 11:37:20 +0100 From: "Dr. David Alan Gilbert" To: Miklos Szeredi Cc: Jeffle Xu , virtualization@lists.linux-foundation.org, virtio-fs-list , Joseph Qi , linux-fsdevel@vger.kernel.org, Vivek Goyal Subject: Re: [Virtio-fs] [PATCH v4 0/8] fuse,virtiofs: support per-file DAX Message-ID: References: <20210817022220.17574-1-jefflexu@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org * Miklos Szeredi (miklos@szeredi.hu) wrote: > On Tue, 17 Aug 2021 at 11:32, Dr. David Alan Gilbert > wrote: > > > > * Miklos Szeredi (miklos@szeredi.hu) wrote: > > > On Tue, 17 Aug 2021 at 04:22, Jeffle Xu wrote: > > > > > > > > This patchset adds support of per-file DAX for virtiofs, which is > > > > inspired by Ira Weiny's work on ext4[1] and xfs[2]. > > > > > > Can you please explain the background of this change in detail? > > > > > > Why would an admin want to enable DAX for a particular virtiofs file > > > and not for others? > > > > Where we're contending on virtiofs dax cache size it makes a lot of > > sense; it's quite expensive for us to map something into the cache > > (especially if we push something else out), so selectively DAXing files > > that are expected to be hot could help reduce cache churn. > > If this is a performance issue, it should be fixed in a way that > doesn't require hand tuning like you suggest, I think. I'd agree that would be nice; however: a) It looks like other filesystems already gave something admin selectable b) Trying to write clever heuristics is only going to work in some cases; being able to say 'DAX this directory' might work better in practice. > I'm not sure what the ext4/xfs case for per-file DAX is. Maybe that > can help understand the virtiofs case as well. Yep, I don't understand the case with real nvdimm hardware. Dave > Thanks, > Miklos > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK