Re: [PATCH v4 0/8] fuse,virtiofs: support per-file DAX
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From: Vivek Goyal <vgoyal@redhat.com>
Date: 2021-08-17 14:57:53
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linux-fsdevel
On Tue, Aug 17, 2021 at 09:22:53PM +0800, JeffleXu wrote:
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.
Downloading 2MB chunk should not be a big issue (IMHO). And if this turns out to be real concern, we could experiment with a smaller mapping granularity.
quoted
Other reason mentioned I think was that only certain files benefit from DAX. But not much details are there after that. It will be nice to hear a more concrete use case and more details about this usage.Apart from our internal requirement, more fine grained control for DAX shall be general and more flexible. Glad to hear more discussion from community.
Sure it will be more general and flexible. But there needs to be 1-2 good concrete use cases to justify additional complexity. And I don't think that so far a good use case has come forward. Thanks Vivek _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization