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authorNiels de Vos <ndevos@redhat.com>2016-05-05 17:30:26 +0200
committerNiels de Vos <ndevos@redhat.com>2016-05-07 02:38:44 -0700
commit23b48ac03b60417fbc2d93399a91a57b850f8fbd (patch)
tree47b76e14d705a2f3634e288775df9214a5a6a264 /done/GlusterFS 3.8/Mandatory Locks.md
parent4acbcae57ae28c67be2bf802eff8c6991eb22596 (diff)
Move "Mandatory Locks" for 3.8 to "done"
Change-Id: Ifd0695dd146f0ceb99e4978945b3156d6f13d17a Signed-off-by: Niels de Vos <ndevos@redhat.com> Reviewed-on: http://review.gluster.org/14227 Reviewed-by: Anoop C S <anoopcs@redhat.com> Reviewed-by: Poornima G <pgurusid@redhat.com>
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+Feature
+-------
+Provide mandatory lock support for Multiprotocol environment.
+
+Summary
+-------
+POSIX.1 does not specify any scheme for mandatory locking. Whereas Linux kernel
+provide support for mandatory locks based on file mode bits which is explained
+at <https://www.kernel.org/doc/Documentation/filesystems/mandatory-locking.txt>.
+But the proposed feature does not adhere completely to the semantics described
+by linux kernel. Instead we enforce core mandatory lock semantics at its byte
+range granularity level as detailed below without the help of file mode bits.
+
+Owners
+------
+Anoop C S <anoopcs@redhat.com>
+
+Poornima G <pgurusid@redhat.com>
+
+Raghavendra Talur <rtalur@redhat.com>
+
+Rajesh Joseph <rjoseph@redhat.com>
+
+Current status
+--------------
+As of now we have POSIX locks translator loaded at the server side to handle
+advisory type in-memory locks and we have the support for mandatory locks
+according to linux kernel semantics. But as we move forward with respect to
+integration of GlusterFS with other protocols like NFS or SMB, apart from
+default POSIX style advisory locks(or even mandatory locks by linux kernel)
+we will have to add support for applications to make use of mandatory locks
+independent of file mode bits defined by linux kernel. This is a mandatory
+requirement for GlusterFS to cope with Multiprotocol environment.
+
+Related Feature Requests and Bugs
+---------------------------------
+* BZ 762184 - Support mandatory locking in glusterfs
+
+ <https://bugzilla.redhat.com/show_bug.cgi?id=762184>
+
+* BZ 1194546 - Write behind returns success for a write irrespective of a
+ conflicting lock held by another application.
+
+ <https://bugzilla.redhat.com/show_bug.cgi?id=1194546>
+
+* BZ 1287099 - Race between mandatory lock request and ongoing read/write
+
+ <https://bugzilla.redhat.com/show_bug.cgi?id=1287099>
+
+Detailed Description
+--------------------
+* By default, mandatory locking will be disabled for a volume. With all
+ the patches listed towards the end, 4 modes{'off', 'file', 'forced' and
+ 'optimal'} are available for the volume set option 'mandatory-locking'
+ which will contain the value 'off' at start.
+
+* Following the POSIX standard, lock requests from all glusterfs native
+ clients will be considered as advisory locks. If the cluster is being
+ accessed by only native clients, then multiple access to a single file
+ can be controlled by setting the group-id bit in its file mode but
+ removing the group-execute bit (as per mandatory locks documentation for
+ linux kernel). This behaviour is achieved by setting the option
+ for mandatory locking to 'file'(implementation not yet done). As a result
+ normal fcntl locks on those files whose required bits are set/unset are
+ considered to be mandatory in nature and thereby performs byte-range
+ conflict check during every data modifying fops.
+
+* With other protocol clients accessing the cluster, to provide advanced
+ protection to files residing in a volume we can use 'forced' mode to ensure
+ volume-wide mandatory lock behaviour which will perform a conflicting
+ advisory/mandatory byte-range lock check before every read/write/truncate/
+ zerofill fop.
+
+* In a similar environment with different protocol clients accessing the
+ cluster, an 'optimal' mode is more suitable based on the following
+ semantics:
+
+ * Each read/write/truncate will check for conflicting mandatory locks
+ and correspondingly blocks/allows the fop.
+
+ * Locks from POSIX clients will be always advisory and any other POSIX
+ client can do read/write/truncate without conflict/overlap byte
+ range check.
+
+ * No clients are allowed to read/write into regions where a conflicting/
+ overlapping mandatory byte-range lock is being held by another client.
+
+ * Any gfapi client who require a mandatory lock on a particular
+ byte-range will have to use the glfs_common_lock() API to do so.
+
+* Mandatory locks can be of two types namely shared and exclusive with
+ semantics similar to the current fcntl locks. The following table explains
+ the extra checks made during various fops acting on overlapping byte range
+ for a particular file:
+
+
+ +----------------------------+-------------+----------------+
+ | Incoming FOP/Existing LOCK | SHARED_LOCK | EXCLUSIVE_LOCK |
+ +----------------------------+-------------+----------------+
+ | READ | Success | Wait/Fail |
+ | WRITE/FTRUNCATE/ZEROFILL | Wait/Fail | Wait/Fail |
+ | SHARED_LOCK | Granted | Wait/Fail |
+ | EXCLUSIVE_LOCK | Wait/Fail | Wait/Fail |
+ | OPEN (with O_TRUNC) | Fail | Fail |
+ +----------------------------+-------------+----------------+
+ * Wait = make the fop wait till the conflicting lock get released. This is
+ the default.
+ * Fail = return EAGAIN if fd flags contain O_NONBLOCK except in the case of
+ OPEN call with O_TRUNC flag where we return EAGAIN without checking
+ O_NONBLOCK.
+
+* Other than normal client initiated file ops, internal fops associated with
+ GlusterFS such as self-heal, rebalance etc will have to bypass byte-range
+ conflict check for mandatory locks and proceed as normal under optimal and
+ forced mandatory locking mode for a volume (See dependencies for more details
+ on problems associated with lock-healing and lock-migration w.r.t self-heal
+ and rebalance).
+
+Dependencies
+------------
+1. During disconnect between client and server, locks get cleaned up on server
+ side. When it comes back online we does not heal fcntl locks and therefore
+ no way to recover locks for that particular brick. Related problems and
+ proposed solutions can be reviewed at the following link:
+
+ <https://docs.google.com/document/d/1py5uDvvbbL3piEnuCa_vo37Kq_vZzHhzKgnc2OiGzg8/edit?pli=1#heading=h.2vntu84m9e2j>
+
+2. Race window where a mandatory lock request from a new client on a particular
+ byte range overlapping with a write from older client is ongoing in the
+ backend, we end up in granting the lock request which will break the
+ assumption given to latter. In this scenario we will have to check for
+ conflicting inodelk in that particular range for which the write is being
+ done by older client. But this check will not satisfy ongoing read use case
+ since reads are not associated with inodelk. Instead we can internally take
+ byte range locks for fops like read, write etc to ensure mandatory lock
+ semantics in a much better way. Following BZ has been created to track the
+ issue in future:
+
+ <https://bugzilla.redhat.com/show_bug.cgi?id=1287099>
+
+3. Considering rebalance of files we have to make sure that locks associated
+ with a file are also migrated to its new destination. Failure to migrate
+ locks may result in undesired access to files under new destination. Proposed
+ design can be tracked at the following link:
+
+ <https://github.com/gluster/glusterfs-specs/blob/master/accepted/Lock-Migration.md>
+
+Relevant patches
+----------------
+* <http://review.gluster.org/#/c/9768/>
+* <http://review.gluster.org/#/c/11177/>
+
+Benefit to GlusterFS
+--------------------
+This feature will allow other protocol clients like SMB or NFS accessing
+gluster volumes via libgfapi to make use of mandatory locking semantics
+described above.
+
+Scope
+-----
+
+#### Nature of proposed change
+For implementing this feature we make use of current locks translator to
+accomodate necessary changes without the introduction of a new translator. It is
+important to note that mandatory lock requests received when it is disabled for
+a volume will be stored as it is inside locks translator. But enforcement of
+advisory and mandatory lock requests will be done based on the current
+mandatory-locking mode.
+
+#### Implications on manageability
+New volume set option 'mandatory-locking' will be available accepting the
+following values:
+* off
+* file
+* optimal
+* forced
+
+Note:- These volume set options are taken into effect only after a subsequent
+start/restart of the volume.
+
+#### Implications on presentation layer
+New locking API namely glfs_lock() will be exposed to allow applications to
+apply for mandatory locks.
+
+#### Implications on persistence layer
+None
+#### Implications on 'GlusterFS' backend
+None
+#### Modification to GlusterFS metadata
+None
+#### Implications on 'glusterd'
+None
+
+How To Test
+-----------
+
+User Experience
+---------------
+
+Dependencies
+------------
+
+Documentation
+-------------
+TBD
+
+Status
+------
+In development
+
+Comments and Discussion
+-----------------------