ceph/doc/rbd/rbd-persistent-cache.rst

   1 =======================
   2  RBD Persistent Cache
   3 =======================
   4
   5 .. index:: Ceph Block Device; Persistent Cache
   6
   7 Shared, Read-only Parent Image Cache
   8 ====================================
   9
  10 `Cloned RBD images`_ from a parent usually only modify a small portion of
  11 the image. For example, in a VDI workload, the VMs are cloned from the same
  12 base image and initially only differ by hostname and IP address. During the
  13 booting stage, all of these VMs would re-read portions of duplicate parent
  14 image data from the RADOS cluster. If we have a local cache of the parent
  15 image, this will help to speed up the read process on one host, as well as
  16 to save the client to cluster network traffic.
  17 RBD shared read-only parent image cache requires explicitly enabling in
  18 ``ceph.conf``. The ``ceph-immutable-object-cache`` daemon is responsible for
  19 caching the parent content on the local disk, and future reads on that data
  20 will be serviced from the local cache.
  21
  22 .. note:: RBD shared read-only parent image cache requires the Ceph Nautilus release or later.
  23
  24 .. ditaa::  +--------------------------------------------------------+
  25             |                         QEMU                           |
  26             +--------------------------------------------------------+
  27             |                librbd (cloned images)                  |
  28             +-------------------+-+----------------------------------+
  29             |      librados     | |  ceph--immutable--object--cache  |
  30             +-------------------+ +----------------------------------+
  31             |      OSDs/Mons    | |     local cached parent image    |
  32             +-------------------+ +----------------------------------+
  33
  34
  35 Enable RBD Shared Read-only Parent Image Cache
  36 ----------------------------------------------
  37
  38 To enable RBD shared read-only parent image cache, the following Ceph settings
  39 need to added in the ``[client]`` `section`_ of your ``ceph.conf`` file.
  40
  41 ``rbd parent cache enabled = true``
  42
  43
  44 Immutable Object Cache Daemon
  45 =============================
  46
  47 The ``ceph-immutable-object-cache`` daemon is responsible for caching parent
  48 image content within its local caching directory. For better performance it's
  49 recommended to use SSDs as the underlying storage.
  50
  51 The key components of the daemon are:
  52
  53 #. **Domain socket based IPC:** The daemon will listen on a local domain
  54    socket on start up and wait for connections from librbd clients.
  55
  56 #. **LRU based promotion/demotion policy:** The daemon will maintain
  57    in-memory statistics of cache-hits on each cache file. It will demote the
  58    cold cache if capacity reaches to the configured threshold.
  59
  60 #. **File-based caching store:** The daemon will maintain a simple file
  61    based cache store. On promotion the RADOS objects will be fetched from
  62    RADOS cluster and stored in the local caching directory.
  63
  64 On opening each cloned rbd image, ``librbd`` will try to connect to the
  65 cache daemon over its domain socket. If it's successfully connected,
  66 ``librbd`` will automatically check with the daemon on the subsequent reads.
  67 If there's a read that's not cached, the daemon will promote the RADOS object
  68 to local caching directory, so the next read on that object will be serviced
  69 from local file. The daemon also maintains simple LRU statistics so if there's
  70 not enough capacity it will delete some cold cache files.
  71
  72 Here are some important cache options correspond to the following settings:
  73
  74 - ``immutable_object_cache_path`` The immutable object cache data directory.
  75
  76 - ``immutable_object_cache_max_size`` The max size for immutable cache.
  77
  78 - ``immutable_object_cache_watermark`` The watermark for the cache. If the
  79   capacity reaches to this watermark, the daemon will delete cold cache based
  80   on the LRU statistics.
  81
  82 The ``ceph-immutable-object-cache`` daemon is available within the optional
  83 ``ceph-immutable-object-cache`` distribution package.
  84
  85 .. important:: ``ceph-immutable-object-cache`` daemon requires the ability to
  86    connect RADOS clusters.
  87
  88 ``ceph-immutable-object-cache`` daemon should use a unique Ceph user ID.
  89 To `create a Ceph user`_, with ``ceph`` specify the ``auth get-or-create``
  90 command, user name, monitor caps, and OSD caps::
  91
  92   ceph auth get-or-create client.ceph-immutable-object-cache.{unique id} mon 'allow r' osd 'profile rbd-read-only'
  93
  94 The ``ceph-immutable-object-cache`` daemon can be managed by ``systemd`` by specifying the user
  95 ID as the daemon instance::
  96
  97   systemctl enable ceph-immutable-object-cache@immutable-object-cache.{unique id}
  98
  99 The ``ceph-immutable-object-cache`` can also be run in foreground by ``ceph-immutable-object-cache`` command::
 100
 101   ceph-immutable-object-cache -f --log-file={log_path}
 102
 103 .. _Cloned RBD Images: ../rbd-snapshot/#layering
 104 .. _section: ../../rados/configuration/ceph-conf/#configuration-sections
 105 .. _create a Ceph user: ../../rados/operations/user-management#add-a-user
 106