Hi, > On Wed, 16 Jun 2021, Wang Yugui wrote: > > Hi, NeilBrown > > > > > On Sun, 13 Jun 2021, Wang Yugui wrote: > > > > Hi, > > > > > > > > Any idea about auto export multiple btrfs snapshots? > > > > > > > > One related patch is yet not merged to nfs-utils 2.5.3. > > > > From: "NeilBrown" <neilb@xxxxxxx> > > > > Subject: [PATCH/RFC v2 nfs-utils] Fix NFSv4 export of tmpfs filesystems. > > > > > > > > In this patch, an UUID is auto generated when a tmpfs have no UUID. > > > > > > > > for btrfs, multiple subvolume snapshot have the same filesystem UUID. > > > > Could we generate an UUID for btrfs subvol with 'filesystem UUID' + 'subvol ID'? > > > > > > You really need to ask this question of btrfs developers. 'mountd' > > > already has a special-case exception for btrfs, to prefer the uuid > > > provided by statfs64() rather than the uuid extracted from the block > > > device. It would be quite easy to add another exception. > > > But it would only be reasonable to do that if the btrfs team told us how > > > that wanted us to generate a UUID for a given mount point, and promised > > > that would always provide a unique stable result. > > > This is completely separate from the tmpfs patch you identified. > > > > Thanks a lot for the replay. > > > > Now btrfs statfs64() return 8 byte unique/stable result. > > > > It is based on two parts. > > 1) 16 byte blkid of file system. this is uniq/stable between btrfs filesystems. > > 2) 8 byte of btrfs sub volume objectid. this is uniq/stable inside a > > btrfs filesystem. > > > > the code of linux/fs/btrfs > > static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf) > > > > /* We treat it as constant endianness (it doesn't matter _which_) > > because we want the fsid to come out the same whether mounted > > on a big-endian or little-endian host */ > > buf->f_fsid.val[0] = be32_to_cpu(fsid[0]) ^ be32_to_cpu(fsid[2]); > > buf->f_fsid.val[1] = be32_to_cpu(fsid[1]) ^ be32_to_cpu(fsid[3]); > > /* Mask in the root object ID too, to disambiguate subvols */ > > buf->f_fsid.val[0] ^= > > BTRFS_I(d_inode(dentry))->root->root_key.objectid >> 32; > > buf->f_fsid.val[1] ^= > > BTRFS_I(d_inode(dentry))->root->root_key.objectid; > > > > > > for nfs, we need a 16 byte UUID now. > > > > The best way I though: > > 16 byte blkid , math add 8 byte btrfs sub volume objectid. > > but there is yet no a simple/easy way to get the raw value of 'btrfs sub > > volume objectid'. > > I'm a bit confused now. You started out talking about snapshots, but > now you are talking about sub volumes. Are they the same thing? > > NFS export of btrfs sub volumes has worked for the past 10 years I > believe. > > Can we go back to the beginning. What, exactly, is the problem you are > trying to solve? How can you demonstrate the problem? > > NeilBrown I nfs/exported a btrfs with 2 subvols and 2 snapshot(subvol). # btrfs subvolume list /mnt/test ID 256 gen 53 top level 5 path sub1 ID 260 gen 56 top level 5 path sub2 ID 261 gen 57 top level 5 path .snapshot/sub1-s1 ID 262 gen 57 top level 5 path .snapshot/sub2-s1 and then mount.nfs4 it to /nfs/test. # /bin/find /nfs/test/ /nfs/test/ find: File system loop detected; ‘/nfs/test/sub1’ is part of the same file system loop as ‘/nfs/test/’. /nfs/test/.snapshot find: File system loop detected; ‘/nfs/test/.snapshot/sub1-s1’ is part of the same file system loop as ‘/nfs/test/’. find: File system loop detected; ‘/nfs/test/.snapshot/sub2-s1’ is part of the same file system loop as ‘/nfs/test/’. /nfs/test/dir1 /nfs/test/dir1/a.txt find: File system loop detected; ‘/nfs/test/sub2’ is part of the same file system loop as ‘/nfs/test/’ /bin/find report 'File system loop detected'. so I though there is something wrong. but when I checked the file content through /mnt/test and /nfs/test, the file through /mnt/test/xxx and /nfs/test/xxx return the same result. I have used nfs/crossmnt, and then I thought that btrfs subvol/snapshot support is through 'nfs/crossmnt' feature. but in fact, it is not through nfs/crossmnt feature? /bin/find report 'File system loop detected', it means that vfs cache on nfs client side will have some problem? Best Regards Wang Yugui (wangyugui@xxxxxxxxxxxx) 2021/06/17
