Re: [PATCH v4 0/8] fuse,virtiofs: support per-file DAX
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From: JeffleXu <jefflexu@linux.alibaba.com>
Date: 2021-09-03 05:31:02
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linux-fsdevel
On 8/17/21 10:08 PM, Miklos Szeredi wrote:
On Tue, 17 Aug 2021 at 15:22, JeffleXu [off-list ref] wrote:quoted
On 8/17/21 8:39 PM, Vivek Goyal wrote:quoted
On Tue, Aug 17, 2021 at 10:06:53AM +0200, Miklos Szeredi wrote:quoted
On Tue, 17 Aug 2021 at 04:22, Jeffle Xu [off-list ref] wrote:quoted
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?Initially I thought that they needed it because they are downloading files on the fly from server. So they don't want to enable dax on the file till file is completely downloaded.Right, it's our initial requirement.quoted
But later I realized that they should be able to block in FUSE_SETUPMAPPING call and make sure associated file section has been downloaded before returning and solve the problem. So that can't be the primary reason.Saying we want to access 4KB of one file inside guest, if it goes through FUSE request routine, then the fuse daemon only need to download this 4KB from remote server. But if it goes through DAX, then the fuse daemon need to download the whole DAX window (e.g., 2MB) from remote server, so called amplification. Maybe we could decrease the DAX window size, but it's a trade off.That could be achieved with a plain fuse filesystem on the host (which will get 4k READ requests for accesses to mapped area inside guest). Since this can be done selectively for files which are not yet downloaded, the extra layer wouldn't be a performance problem. Is there a reason why that wouldn't work?
I didn't realize this mechanism (working around from user space) before sending this patch set. After learning the virtualization and KVM stuffs, I find that, as Vivek Goyal replied in [1], virtiofsd/qemu need to somehow hook the user page fault and then download the remained part. IMHO, this mechanism (as you proposed by implementing a plain fuse filesystem on the host) seems a little bit sophisticated so far. [1] https://lore.kernel.org/linux-fsdevel/YR08KnP8cO8LjKY7@redhat.com/ (local) -- Thanks, Jeffle _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization