docs: collocated and non-collocated scenario docs

Signed-off-by: Andrew Schoen <aschoen@redhat.com>

docs/source/osds/scenarios.rst

@@ -1,6 +1,148 @@
 OSD Scenarios
 =============
 
+The following are all of the available options for the ``osd_scenario`` config
+setting. Defining an ``osd_scenario`` is mandatory for using ``ceph-ansible``.
+
+collocated
+----------
+This OSD scenario uses ``ceph-disk`` to create OSDs with collocated journals
+from raw devices.
+
+Use ``osd_scenario: collocated`` to enable this scenario. This scenario also
+has the following required configuration options:
+
+- ``devices``
+
+This scenario has the following optional configuration options:
+
+- ``osd_objectstore``: defaults to ``filestore`` if not set. Available options are ``filestore`` or ``bluestore``.
+  You can only select ``bluestore`` with the ceph release is Luminous or greater.
+
+- ``dmcrypt``: defaults to ``false`` if not set.
+
+This scenario supports encrypting your OSDs by setting ``dmcrypt: True``.
+
+If ``osd_objectstore: filestore`` is enabled both 'ceph data' and 'ceph journal' partitions
+will be stored on the same device.
+
+If ``osd_objectstore: bluestore`` is enabled 'ceph data', 'ceph block', 'ceph block.db', 'ceph block.wal' will be stored
+on the same device. The device will get 2 partitions:
+
+- One for 'data', called 'ceph data'
+
+- One for 'ceph block', 'ceph block.db', 'ceph block.wal' called 'ceph block'
+
+Example of what you will get::
+
+    [root@ceph-osd0 ~]# blkid /dev/sda*
+    /dev/sda: PTTYPE="gpt"
+    /dev/sda1: UUID="9c43e346-dd6e-431f-92d8-cbed4ccb25f6" TYPE="xfs" PARTLABEL="ceph data" PARTUUID="749c71c9-ed8f-4930-82a7-a48a3bcdb1c7"
+    /dev/sda2: PARTLABEL="ceph block" PARTUUID="e6ca3e1d-4702-4569-abfa-e285de328e9d"
+
+An example of using the ``collocated`` OSD scenario with encryption would look like::
+
+    osd_scenario: collocated
+    dmcrypt: true
+
+non-collocated
+--------------
+
+This OSD scenario uses ``ceph-disk`` to create OSDs from raw devices with journals that
+exit on a dedicated device.
+
+Use ``osd_scenario: non-collocated`` to enable this scenario. This scenario also
+has the following required configuration options:
+
+- ``devices``
+
+This scenario has the following optional configuration options:
+
+- ``dedicated_devices``: defaults to ``devices`` if not set
+
+- ``osd_objectstore``: defaults to ``filestore`` if not set. Available options are ``filestore`` or ``bluestore``.
+  You can only select ``bluestore`` with the ceph release is Luminous or greater.
+
+- ``dmcrypt``: defaults to ``false`` if not set.
+
+This scenario supports encrypting your OSDs by setting ``dmcrypt: True``.
+
+If ``osd_objectstore: filestore`` is enabled 'ceph data' and 'ceph journal' partitions
+will be stored on different devices:
+- 'ceph data' will be stored on the device listed in ``devices``
+- 'ceph journal' will be stored on the device listed in ``dedicated_devices``
+
+Let's take an example, imagine ``devices`` was declared like this::
+
+    devices:
+      - /dev/sda
+      - /dev/sdb
+      - /dev/sdc
+      - /dev/sdd
+
+And ``dedicated_devices`` was declared like this::
+
+    dedicated_devices:
+      - /dev/sdf
+      - /dev/sdf
+      - /dev/sdg
+      - /dev/sdg
+
+This will result in the following mapping:
+
+- /dev/sda will have /dev/sdf1 as journal
+
+- /dev/sdb will have /dev/sdf2 as a journal
+
+- /dev/sdc will have /dev/sdg1 as a journal
+
+- /dev/sdd will have /dev/sdg2 as a journal
+
+
+.. note::
+    On a containerized scenario we only support A SINGLE journal
+    for all the OSDs on a given machine. If you don't, bad things will happen
+    This is a limitation we plan to fix at some point.
+
+
+If ``osd_objectstore: bluestore`` is enabled, both 'ceph block.db' and 'ceph block.wal' partitions will be stored
+on a dedicated device.
+
+So the following will happen:
+
+- The devices listed in ``devices`` will get 2 partitions, one for 'block' and one for 'data'. 'data' is only 100MB big and do not store any of your data, it's just a bunch of Ceph metadata. 'block' will store all your actual data.
+
+- The devices in ``dedicated_devices`` will get 1 partition for RocksDB DB, called 'block.db' and one for RocksDB WAL, called 'block.wal'
+
+By default ``dedicated_devices`` will represent block.db
+
+Example of what you will get::
+
+    [root@ceph-osd0 ~]# blkid /dev/sd*
+    /dev/sda: PTTYPE="gpt"
+    /dev/sda1: UUID="c6821801-2f21-4980-add0-b7fc8bd424d5" TYPE="xfs" PARTLABEL="ceph data" PARTUUID="f2cc6fa8-5b41-4428-8d3f-6187453464d0"
+    /dev/sda2: PARTLABEL="ceph block" PARTUUID="ea454807-983a-4cf2-899e-b2680643bc1c"
+    /dev/sdb: PTTYPE="gpt"
+    /dev/sdb1: PARTLABEL="ceph block.db" PARTUUID="af5b2d74-4c08-42cf-be57-7248c739e217"
+    /dev/sdb2: PARTLABEL="ceph block.wal" PARTUUID="af3f8327-9aa9-4c2b-a497-cf0fe96d126a"
+
+There is more device granularity for Bluestore ONLY if ``osd_objectstore: bluestore`` is enabled by setting the
+``bluestore_wal_devices`` config option.
+
+By default, if ``bluestore_wal_devices`` is empty, it will get the content of ``dedicated_devices``.
+If set, then you will have a dedicated partition on a specific device for block.wal.
+
+Example of what you will get::
+
+    [root@ceph-osd0 ~]# blkid /dev/sd*
+    /dev/sda: PTTYPE="gpt"
+    /dev/sda1: UUID="39241ae9-d119-4335-96b3-0898da8f45ce" TYPE="xfs" PARTLABEL="ceph data" PARTUUID="961e7313-bdb7-49e7-9ae7-077d65c4c669"
+    /dev/sda2: PARTLABEL="ceph block" PARTUUID="bff8e54e-b780-4ece-aa16-3b2f2b8eb699"
+    /dev/sdb: PTTYPE="gpt"
+    /dev/sdb1: PARTLABEL="ceph block.db" PARTUUID="0734f6b6-cc94-49e9-93de-ba7e1d5b79e3"
+    /dev/sdc: PTTYPE="gpt"
+    /dev/sdc1: PARTLABEL="ceph block.wal" PARTUUID="824b84ba-6777-4272-bbbd-bfe2a25cecf3"
+
 lvm
 ---
 This OSD scenario uses ``ceph-volume`` to create OSDs from logical volumes and