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'. > > A simple but good enough way: > 1) first 8 byte copy from blkid > 2) second 8 byte copy from btrfs_statfs() > the uniq/stable of multiple subvolume inside a btrfs filesystem is kept. By the way, the random 16 byte UUID still have very little chance to conflict. Could we keep the first 4 byte of the UUID of nfs/tmpfs alwasy ZERO? , the first 4 byte zero will limit the conflict inside nfs/tmpfs, and it is easy to diag. Here we use the first same 8 byte for UUID of btrfs and nfs/btrfs, so it is easy to diag too. Best Regards Wang Yugui (wangyugui@xxxxxxxxxxxx) 2021/06/15
