+++ /dev/null
-======================
-Capabilities in CephFS
-======================
-When a client wants to operate on an inode, it will query the MDS in various
-ways, which will then grant the client a set of **capabilities**. These
-grant the client permissions to operate on the inode in various ways. One
-of the major differences from other network filesystems (e.g NFS or SMB) is
-that the capabilities granted are quite granular, and it's possible that
-multiple clients can hold different capabilities on the same inodes.
-
-Types of Capabilities
----------------------
-There are several "generic" capability bits. These denote what sort of ability
-the capability grants.
-
-::
-
- /* generic cap bits */
- #define CEPH_CAP_GSHARED 1 /* client can reads (s) */
- #define CEPH_CAP_GEXCL 2 /* client can read and update (x) */
- #define CEPH_CAP_GCACHE 4 /* (file) client can cache reads (c) */
- #define CEPH_CAP_GRD 8 /* (file) client can read (r) */
- #define CEPH_CAP_GWR 16 /* (file) client can write (w) */
- #define CEPH_CAP_GBUFFER 32 /* (file) client can buffer writes (b) */
- #define CEPH_CAP_GWREXTEND 64 /* (file) client can extend EOF (a) */
- #define CEPH_CAP_GLAZYIO 128 /* (file) client can perform lazy io (l) */
-
-These are then shifted by a particular number of bits. These denote a part of
-the inode's data or metadata on which the capability is being granted:
-
-::
-
- /* per-lock shift */
- #define CEPH_CAP_SAUTH 2 /* A */
- #define CEPH_CAP_SLINK 4 /* L */
- #define CEPH_CAP_SXATTR 6 /* X */
- #define CEPH_CAP_SFILE 8 /* F */
-
-Only certain generic cap types are ever granted for some of those "shifts",
-however. In particular, only the FILE shift ever has more than the first two
-bits.
-
-::
-
- | AUTH | LINK | XATTR | FILE
- 2 4 6 8
-
-From the above, we get a number of constants, that are generated by taking
-each bit value and shifting to the correct bit in the word:
-
-::
-
- #define CEPH_CAP_AUTH_SHARED (CEPH_CAP_GSHARED << CEPH_CAP_SAUTH)
-
-These bits can then be or'ed together to make a bitmask denoting a set of
-capabilities.
-
-There is one exception:
-
-::
-
- #define CEPH_CAP_PIN 1 /* no specific capabilities beyond the pin */
-
-The "pin" just pins the inode into memory, without granting any other caps.
-
-Graphically:
-
-::
-
- +---+---+---+---+---+---+---+---+
- | p | _ |As x |Ls x |Xs x |
- +---+---+---+---+---+---+---+---+
- |Fs x c r w b a l |
- +---+---+---+---+---+---+---+---+
-
-The second bit is currently unused.
-
-Abilities granted by each cap:
-------------------------------
-While that is how capabilities are granted (and communicated), the important
-bit is what they actually allow the client to do:
-
-* PIN: this just pins the inode into memory. This is sufficient to allow the
- client to get to the inode number, as well as other immutable things like
- major or minor numbers in a device inode, or symlink contents.
-
-* AUTH: this grants the ability to get to the authentication-related metadata.
- In particular, the owner, group and mode. Note that doing a full permission
- check may require getting at ACLs as well, which are stored in xattrs.
-
-* LINK: the link count of the inode
-
-* XATTR: ability to access or manipulate xattrs. Note that since ACLs are
- stored in xattrs, it's also sometimes necessary to access them when checking
- permissions.
-
-* FILE: this is the big one. These allow the client to access and manipulate
- file data. It also covers certain metadata relating to file data -- the
- size, mtime, atime and ctime, in particular.
-
-Shorthand:
-----------
-Note that the client logging can also present a compact representation of the
-capabilities. For example:
-
-::
- pAsLsXsFs
-
-The 'p' represents the pin. Each capital letter corresponds to the shift
-values, and the lowercase letters after each shift are for the actual
-capabilities granted in each shift.
Code Review / stor4nfv.git / blobdiff