X-Git-Url: https://gerrit.opnfv.org/gerrit/gitweb?a=blobdiff_plain;f=src%2Fceph%2Fdoc%2Fcephfs%2Fmultimds.rst;fp=src%2Fceph%2Fdoc%2Fcephfs%2Fmultimds.rst;h=11814c14e2dd02fbeb2f98c116a8bfe22cefb64e;hb=812ff6ca9fcd3e629e49d4328905f33eee8ca3f5;hp=0000000000000000000000000000000000000000;hpb=15280273faafb77777eab341909a3f495cf248d9;p=stor4nfv.git diff --git a/src/ceph/doc/cephfs/multimds.rst b/src/ceph/doc/cephfs/multimds.rst new file mode 100644 index 0000000..11814c1 --- /dev/null +++ b/src/ceph/doc/cephfs/multimds.rst @@ -0,0 +1,147 @@ + +Configuring multiple active MDS daemons +--------------------------------------- + +*Also known as: multi-mds, active-active MDS* + +Each CephFS filesystem is configured for a single active MDS daemon +by default. To scale metadata performance for large scale systems, you +may enable multiple active MDS daemons, which will share the metadata +workload with one another. + +When should I use multiple active MDS daemons? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You should configure multiple active MDS daemons when your metadata performance +is bottlenecked on the single MDS that runs by default. + +Adding more daemons may not increase performance on all workloads. Typically, +a single application running on a single client will not benefit from an +increased number of MDS daemons unless the application is doing a lot of +metadata operations in parallel. + +Workloads that typically benefit from a larger number of active MDS daemons +are those with many clients, perhaps working on many separate directories. + + +Increasing the MDS active cluster size +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Each CephFS filesystem has a *max_mds* setting, which controls +how many ranks will be created. The actual number of ranks +in the filesystem will only be increased if a spare daemon is +available to take on the new rank. For example, if there is only one MDS daemon running, and max_mds is set to two, no second rank will be created. + +Set ``max_mds`` to the desired number of ranks. In the following examples +the "fsmap" line of "ceph status" is shown to illustrate the expected +result of commands. + +:: + + # fsmap e5: 1/1/1 up {0=a=up:active}, 2 up:standby + + ceph fs set max_mds 2 + + # fsmap e8: 2/2/2 up {0=a=up:active,1=c=up:creating}, 1 up:standby + # fsmap e9: 2/2/2 up {0=a=up:active,1=c=up:active}, 1 up:standby + +The newly created rank (1) will pass through the 'creating' state +and then enter this 'active state'. + +Standby daemons +~~~~~~~~~~~~~~~ + +Even with multiple active MDS daemons, a highly available system **still +requires standby daemons** to take over if any of the servers running +an active daemon fail. + +Consequently, the practical maximum of ``max_mds`` for highly available systems +is one less than the total number of MDS servers in your system. + +To remain available in the event of multiple server failures, increase the +number of standby daemons in the system to match the number of server failures +you wish to withstand. + +Decreasing the number of ranks +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +All ranks, including the rank(s) to be removed must first be active. This +means that you must have at least max_mds MDS daemons available. + +First, set max_mds to a lower number, for example we might go back to +having just a single active MDS: + +:: + + # fsmap e9: 2/2/2 up {0=a=up:active,1=c=up:active}, 1 up:standby + ceph fs set max_mds 1 + # fsmap e10: 2/2/1 up {0=a=up:active,1=c=up:active}, 1 up:standby + +Note that we still have two active MDSs: the ranks still exist even though +we have decreased max_mds, because max_mds only restricts creation +of new ranks. + +Next, use the ``ceph mds deactivate `` command to remove the +unneeded rank: + +:: + + ceph mds deactivate cephfs_a:1 + telling mds.1:1 172.21.9.34:6806/837679928 to deactivate + + # fsmap e11: 2/2/1 up {0=a=up:active,1=c=up:stopping}, 1 up:standby + # fsmap e12: 1/1/1 up {0=a=up:active}, 1 up:standby + # fsmap e13: 1/1/1 up {0=a=up:active}, 2 up:standby + +See :doc:`/cephfs/administration` for more details which forms ```` can +take. + +The deactivated rank will first enter the stopping state for a period +of time while it hands off its share of the metadata to the remaining +active daemons. This phase can take from seconds to minutes. If the +MDS appears to be stuck in the stopping state then that should be investigated +as a possible bug. + +If an MDS daemon crashes or is killed while in the 'stopping' state, a +standby will take over and the rank will go back to 'active'. You can +try to deactivate it again once it has come back up. + +When a daemon finishes stopping, it will respawn itself and go +back to being a standby. + + +Manually pinning directory trees to a particular rank +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +In multiple active metadata server configurations, a balancer runs which works +to spread metadata load evenly across the cluster. This usually works well +enough for most users but sometimes it is desirable to override the dynamic +balancer with explicit mappings of metadata to particular ranks. This can allow +the administrator or users to evenly spread application load or limit impact of +users' metadata requests on the entire cluster. + +The mechanism provided for this purpose is called an ``export pin``, an +extended attribute of directories. The name of this extended attribute is +``ceph.dir.pin``. Users can set this attribute using standard commands: + +:: + + setfattr -n ceph.dir.pin -v 2 path/to/dir + +The value of the extended attribute is the rank to assign the directory subtree +to. A default value of ``-1`` indicates the directory is not pinned. + +A directory's export pin is inherited from its closest parent with a set export +pin. In this way, setting the export pin on a directory affects all of its +children. However, the parents pin can be overriden by setting the child +directory's export pin. For example: + +:: + + mkdir -p a/b + # "a" and "a/b" both start without an export pin set + setfattr -n ceph.dir.pin -v 1 a/ + # a and b are now pinned to rank 1 + setfattr -n ceph.dir.pin -v 0 a/b + # a/b is now pinned to rank 0 and a/ and the rest of its children are still pinned to rank 1 +