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Thu, 24 Jun 2021 21:58:59 +0000 Subject: Re: any idea about auto export multiple btrfs snapshots? To: NeilBrown , "J. Bruce Fields" Cc: Wang Yugui , linux-nfs@vger.kernel.org References: <162432531379.17441.15110145423567943074@noble.neil.brown.name> <20210622112253.DAEE.409509F4@e16-tech.com> <20210622151407.C002.409509F4@e16-tech.com> <162440994038.28671.7338874000115610814@noble.neil.brown.name> <20210623153548.GF20232@fieldses.org> <162448589701.28671.8402117125966499268@noble.neil.brown.name> From: Patrick Goetz Message-ID: <3ac9c4f3-6bc2-9753-6f0f-937aa4f13efa@math.utexas.edu> Date: Thu, 24 Jun 2021 16:58:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: <162448589701.28671.8402117125966499268@noble.neil.brown.name> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [67.198.113.142] X-ClientProxiedBy: SN4PR0501CA0104.namprd05.prod.outlook.com (2603:10b6:803:42::21) To BYAPR06MB3848.namprd06.prod.outlook.com (2603:10b6:a02:8c::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.4] (67.198.113.142) by SN4PR0501CA0104.namprd05.prod.outlook.com (2603:10b6:803:42::21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4287.8 via Frontend Transport; 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Bruce Fields wrote: >> Is there any hope of solving this problem within btrfs? >> >> It doesn't seem like it should have been that difficult for it to give >> subvolumes separate superblocks and vfsmounts. >> >> But this has come up before, and I think the answer may have been that >> it's just too late to fix. > > It is never too late to do the right thing! > > Probably the best approach to fixing this completely on the btrfs side > would be to copy the auto-mount approach used in NFS. NFS sees multiple > different volumes on the server and transparently creates new vfsmounts, > using the automount infrastructure to mount and unmount them. I'm very confused about what you're talking about. Is this documented somewhere? I mean, I do use autofs, but see that as a separate software system working with NFS. > BTRFS > effective sees multiple volumes on the block device and could do the > same thing. > > I can only think of one change to the user-space API (other than > /proc/mounts contents) that this would cause and I suspect it could be > resolved if needed. > > Currently when you 'stat' the mountpoint of a btrfs subvol you see the > root of that subvol. However when you 'stat' the mountpoint of an NFS > sub-filesystem (before any access below there) you see the mountpoint > (s_dev matches the parent). This is how automounts are expected to work > and if btrfs were switched to use automounts for subvols, stating the > mountpoint would initially show the mountpoint, not the subvol root. > > If this were seen to be a problem I doubt it would be hard to add > optional functionality to automount so that 'stat' triggers the mount. > > All we really need is: > 1/ someone to write the code > 2/ someone to review the code > 3/ someone to accept the code > > How hard can it be :-) > > NeilBrown >