[PATCH v2 25/29] docs: md: convert to ReST

Mauro Carvalho Chehab
2019-06-18T20:55:24Z

Rename the md documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
---
Documentation/md/index.rst | 12 ++
.../md/{md-cluster.txt => md-cluster.rst} | 188 ++++++++++++------
.../md/{raid5-cache.txt => raid5-cache.rst} | 28 +--
.../md/{raid5-ppl.txt => raid5-ppl.rst} | 2 +
4 files changed, 153 insertions(+), 77 deletions(-)
create mode 100644 Documentation/md/index.rst
rename Documentation/md/{md-cluster.txt => md-cluster.rst} (68%)
rename Documentation/md/{raid5-cache.txt => raid5-cache.rst} (92%)
rename Documentation/md/{raid5-ppl.txt => raid5-ppl.rst} (98%)

diff --git a/Documentation/md/index.rst b/Documentation/md/index.rst
new file mode 100644
index 000000000000..c4db34ed327d
--- /dev/null
+++ b/Documentation/md/index.rst
@@ -0,0 +1,12 @@
+:orphan:
+
+====
+RAID
+====
+
+.. toctree::
+ :maxdepth: 1
+
+ md-cluster
+ raid5-cache
+ raid5-ppl
diff --git a/Documentation/md/md-cluster.txt b/Documentation/md/md-cluster.rst
similarity index 68%
rename from Documentation/md/md-cluster.txt
rename to Documentation/md/md-cluster.rst
index e1055f105cf5..96eb52cec7eb 100644
--- a/Documentation/md/md-cluster.txt
+++ b/Documentation/md/md-cluster.rst
@@ -1,19 +1,24 @@
+==========
+MD Cluster
+==========
+
The cluster MD is a shared-device RAID for a cluster, it supports
two levels: raid1 and raid10 (limited support).

1. On-disk format
+=================

Separate write-intent-bitmaps are used for each cluster node.
The bitmaps record all writes that may have been started on that node,
-and may not yet have finished. The on-disk layout is:
+and may not yet have finished. The on-disk layout is::

-0 4k 8k 12k
--------------------------------------------------------------------
-| idle | md super | bm super [0] + bits |
-| bm bits[0, contd] | bm super[1] + bits | bm bits[1, contd] |
-| bm super[2] + bits | bm bits [2, contd] | bm super[3] + bits |
-| bm bits [3, contd] | | |
+ 0 4k 8k 12k
+ -------------------------------------------------------------------
+ | idle | md super | bm super [0] + bits |
+ | bm bits[0, contd] | bm super[1] + bits | bm bits[1, contd] |
+ | bm super[2] + bits | bm bits [2, contd] | bm super[3] + bits |
+ | bm bits [3, contd] | | |

During "normal" functioning we assume the filesystem ensures that only
one node writes to any given block at a time, so a write request will
@@ -28,10 +33,12 @@ node) is writing.

2. DLM Locks for management
+===========================

There are three groups of locks for managing the device:

2.1 Bitmap lock resource (bm_lockres)
+-------------------------------------

The bm_lockres protects individual node bitmaps. They are named in
the form bitmap000 for node 1, bitmap001 for node 2 and so on. When a
@@ -48,6 +55,7 @@ There are three groups of locks for managing the device:
joins the cluster.

2.2 Message passing locks
+-------------------------

Each node has to communicate with other nodes when starting or ending
resync, and for metadata superblock updates. This communication is
@@ -55,116 +63,155 @@ There are three groups of locks for managing the device:
with the Lock Value Block (LVB) of one of the "message" lock.

2.3 new-device management
+-------------------------

A single lock: "no-new-dev" is used to co-ordinate the addition of
new devices - this must be synchronized across the array.
Normally all nodes hold a concurrent-read lock on this device.

3. Communication
+================

Messages can be broadcast to all nodes, and the sender waits for all
other nodes to acknowledge the message before proceeding. Only one
message can be processed at a time.

3.1 Message Types
+-----------------

There are six types of messages which are passed:

- 3.1.1 METADATA_UPDATED: informs other nodes that the metadata has
+3.1.1 METADATA_UPDATED
+^^^^^^^^^^^^^^^^^^^^^^
+
+ informs other nodes that the metadata has
been updated, and the node must re-read the md superblock. This is
performed synchronously. It is primarily used to signal device
failure.

- 3.1.2 RESYNCING: informs other nodes that a resync is initiated or
+3.1.2 RESYNCING
+^^^^^^^^^^^^^^^
+ informs other nodes that a resync is initiated or
ended so that each node may suspend or resume the region. Each
RESYNCING message identifies a range of the devices that the
sending node is about to resync. This overrides any previous
notification from that node: only one ranged can be resynced at a
time per-node.

- 3.1.3 NEWDISK: informs other nodes that a device is being added to
+3.1.3 NEWDISK
+^^^^^^^^^^^^^
+
+ informs other nodes that a device is being added to
the array. Message contains an identifier for that device. See
below for further details.

- 3.1.4 REMOVE: A failed or spare device is being removed from the
+3.1.4 REMOVE
+^^^^^^^^^^^^
+
+ A failed or spare device is being removed from the
array. The slot-number of the device is included in the message.

- 3.1.5 RE_ADD: A failed device is being re-activated - the assumption
+ 3.1.5 RE_ADD:
+
+ A failed device is being re-activated - the assumption
is that it has been determined to be working again.

- 3.1.6 BITMAP_NEEDS_SYNC: if a node is stopped locally but the bitmap
+ 3.1.6 BITMAP_NEEDS_SYNC:
+
+ If a node is stopped locally but the bitmap
isn't clean, then another node is informed to take the ownership of
resync.

3.2 Communication mechanism
+---------------------------

The DLM LVB is used to communicate within nodes of the cluster. There
are three resources used for the purpose:

- 3.2.1 token: The resource which protects the entire communication
+3.2.1 token
+^^^^^^^^^^^
+ The resource which protects the entire communication
system. The node having the token resource is allowed to
communicate.

- 3.2.2 message: The lock resource which carries the data to
- communicate.
+3.2.2 message
+^^^^^^^^^^^^^
+ The lock resource which carries the data to communicate.

- 3.2.3 ack: The resource, acquiring which means the message has been
+3.2.3 ack
+^^^^^^^^^
+
+ The resource, acquiring which means the message has been
acknowledged by all nodes in the cluster. The BAST of the resource
is used to inform the receiving node that a node wants to
communicate.

The algorithm is:

- 1. receive status - all nodes have concurrent-reader lock on "ack".
+ 1. receive status - all nodes have concurrent-reader lock on "ack"::

- sender receiver receiver
- "ack":CR "ack":CR "ack":CR
+ sender receiver receiver
+ "ack":CR "ack":CR "ack":CR

- 2. sender get EX on "token"
- sender get EX on "message"
- sender receiver receiver
- "token":EX "ack":CR "ack":CR
- "message":EX
- "ack":CR
+ 2. sender get EX on "token",
+ sender get EX on "message"::
+
+ sender receiver receiver
+ "token":EX "ack":CR "ack":CR
+ "message":EX
+ "ack":CR

Sender checks that it still needs to send a message. Messages
received or other events that happened while waiting for the
"token" may have made this message inappropriate or redundant.

- 3. sender writes LVB.
+ 3. sender writes LVB
+
sender down-convert "message" from EX to CW
+
sender try to get EX of "ack"
- [ wait until all receivers have *processed* the "message" ]
-
- [ triggered by bast of "ack" ]
- receiver get CR on "message"
- receiver read LVB
- receiver processes the message
- [ wait finish ]
- receiver releases "ack"
- receiver tries to get PR on "message"
-
- sender receiver receiver
- "token":EX "message":CR "message":CR
- "message":CW
- "ack":EX
+
+ ::
+
+ [ wait until all receivers have *processed* the "message" ]
+
+ [ triggered by bast of "ack" ]
+ receiver get CR on "message"
+ receiver read LVB
+ receiver processes the message
+ [ wait finish ]
+ receiver releases "ack"
+ receiver tries to get PR on "message"
+
+ sender receiver receiver
+ "token":EX "message":CR "message":CR
+ "message":CW
+ "ack":EX

4. triggered by grant of EX on "ack" (indicating all receivers
have processed message)
+
sender down-converts "ack" from EX to CR
+
sender releases "message"
+
sender releases "token"
- receiver upconvert to PR on "message"
- receiver get CR of "ack"
- receiver release "message"

- sender receiver receiver
- "ack":CR "ack":CR "ack":CR
+ ::
+
+ receiver upconvert to PR on "message"
+ receiver get CR of "ack"
+ receiver release "message"
+
+ sender receiver receiver
+ "ack":CR "ack":CR "ack":CR

4. Handling Failures
+====================

4.1 Node Failure
+----------------

When a node fails, the DLM informs the cluster with the slot
number. The node starts a cluster recovery thread. The cluster
@@ -177,11 +224,11 @@ The algorithm is:
- cleans the bitmap of the failed node
- releases bitmap<number> lock of the failed node
- initiates resync of the bitmap on the current node
- md_check_recovery is invoked within recover_bitmaps,
- then md_check_recovery -> metadata_update_start/finish,
- it will lock the communication by lock_comm.
- Which means when one node is resyncing it blocks all
- other nodes from writing anywhere on the array.
+ md_check_recovery is invoked within recover_bitmaps,
+ then md_check_recovery -> metadata_update_start/finish,
+ it will lock the communication by lock_comm.
+ Which means when one node is resyncing it blocks all
+ other nodes from writing anywhere on the array.

The resync process is the regular md resync. However, in a clustered
environment when a resync is performed, it needs to tell other nodes
@@ -198,6 +245,7 @@ The algorithm is:
particular I/O range should be suspended or not.

4.2 Device Failure
+==================

Device failures are handled and communicated with the metadata update
routine. When a node detects a device failure it does not allow
@@ -205,38 +253,41 @@ The algorithm is:
acknowledged by all other nodes.

5. Adding a new Device
+----------------------

For adding a new device, it is necessary that all nodes "see" the new
device to be added. For this, the following algorithm is used:

- 1. Node 1 issues mdadm --manage /dev/mdX --add /dev/sdYY which issues
+ 1. Node 1 issues mdadm --manage /dev/mdX --add /dev/sdYY which issues
ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CLUSTER_ADD)
- 2. Node 1 sends a NEWDISK message with uuid and slot number
- 3. Other nodes issue kobject_uevent_env with uuid and slot number
+ 2. Node 1 sends a NEWDISK message with uuid and slot number
+ 3. Other nodes issue kobject_uevent_env with uuid and slot number
(Steps 4,5 could be a udev rule)
- 4. In userspace, the node searches for the disk, perhaps
+ 4. In userspace, the node searches for the disk, perhaps
using blkid -t SUB_UUID=""
- 5. Other nodes issue either of the following depending on whether
+ 5. Other nodes issue either of the following depending on whether
the disk was found:
ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CANDIDATE and
- disc.number set to slot number)
+ disc.number set to slot number)
ioctl(CLUSTERED_DISK_NACK)
- 6. Other nodes drop lock on "no-new-devs" (CR) if device is found
- 7. Node 1 attempts EX lock on "no-new-dev"
- 8. If node 1 gets the lock, it sends METADATA_UPDATED after
+ 6. Other nodes drop lock on "no-new-devs" (CR) if device is found
+ 7. Node 1 attempts EX lock on "no-new-dev"
+ 8. If node 1 gets the lock, it sends METADATA_UPDATED after
unmarking the disk as SpareLocal
- 9. If not (get "no-new-dev" lock), it fails the operation and sends
+ 9. If not (get "no-new-dev" lock), it fails the operation and sends
METADATA_UPDATED.
10. Other nodes get the information whether a disk is added or not
by the following METADATA_UPDATED.

-6. Module interface.
+6. Module interface
+===================

There are 17 call-backs which the md core can make to the cluster
module. Understanding these can give a good overview of the whole
process.

6.1 join(nodes) and leave()
+---------------------------

These are called when an array is started with a clustered bitmap,
and when the array is stopped. join() ensures the cluster is
@@ -244,11 +295,13 @@ The algorithm is:
Only the first 'nodes' nodes in the cluster can use the array.

6.2 slot_number()
+-----------------

Reports the slot number advised by the cluster infrastructure.
Range is from 0 to nodes-1.

6.3 resync_info_update()
+------------------------

This updates the resync range that is stored in the bitmap lock.
The starting point is updated as the resync progresses. The
@@ -256,6 +309,7 @@ The algorithm is:
It does *not* send a RESYNCING message.

6.4 resync_start(), resync_finish()
+-----------------------------------

These are called when resync/recovery/reshape starts or stops.
They update the resyncing range in the bitmap lock and also
@@ -265,8 +319,8 @@ The algorithm is:
resync_finish() also sends a BITMAP_NEEDS_SYNC message which
allows some other node to take over.

-6.5 metadata_update_start(), metadata_update_finish(),
- metadata_update_cancel().
+6.5 metadata_update_start(), metadata_update_finish(), metadata_update_cancel()
+-------------------------------------------------------------------------------

metadata_update_start is used to get exclusive access to
the metadata. If a change is still needed once that access is
@@ -275,6 +329,7 @@ The algorithm is:
can be used to release the lock.

6.6 area_resyncing()
+--------------------

This combines two elements of functionality.

@@ -289,6 +344,7 @@ The algorithm is:
a node failure.

6.7 add_new_disk_start(), add_new_disk_finish(), new_disk_ack()
+---------------------------------------------------------------

These are used to manage the new-disk protocol described above.
When a new device is added, add_new_disk_start() is called before
@@ -300,17 +356,20 @@ The algorithm is:
new_disk_ack() is called.

6.8 remove_disk()
+-----------------

This is called when a spare or failed device is removed from
the array. It causes a REMOVE message to be send to other nodes.

6.9 gather_bitmaps()
+--------------------

This sends a RE_ADD message to all other nodes and then
gathers bitmap information from all bitmaps. This combined
bitmap is then used to recovery the re-added device.

6.10 lock_all_bitmaps() and unlock_all_bitmaps()
+------------------------------------------------

These are called when change bitmap to none. If a node plans
to clear the cluster raid's bitmap, it need to make sure no other
@@ -319,6 +378,7 @@ The algorithm is:
accordingly.

7. Unsupported features
+=======================

There are somethings which are not supported by cluster MD yet.

diff --git a/Documentation/md/raid5-cache.txt b/Documentation/md/raid5-cache.rst
similarity index 92%
rename from Documentation/md/raid5-cache.txt
rename to Documentation/md/raid5-cache.rst
index 2b210f295786..d7a15f44a7c3 100644
--- a/Documentation/md/raid5-cache.txt
+++ b/Documentation/md/raid5-cache.rst
@@ -1,4 +1,6 @@
-RAID5 cache
+================
+RAID 4/5/6 cache
+================

Raid 4/5/6 could include an extra disk for data cache besides normal RAID
disks. The role of RAID disks isn't changed with the cache disk. The cache disk
@@ -6,19 +8,19 @@ caches data to the RAID disks. The cache can be in write-through (supported
since 4.4) or write-back mode (supported since 4.10). mdadm (supported since
3.4) has a new option '--write-journal' to create array with cache. Please
refer to mdadm manual for details. By default (RAID array starts), the cache is
-in write-through mode. A user can switch it to write-back mode by:
+in write-through mode. A user can switch it to write-back mode by::

-echo "write-back" > /sys/block/md0/md/journal_mode
+ echo "write-back" > /sys/block/md0/md/journal_mode

-And switch it back to write-through mode by:
+And switch it back to write-through mode by::

-echo "write-through" > /sys/block/md0/md/journal_mode
+ echo "write-through" > /sys/block/md0/md/journal_mode

In both modes, all writes to the array will hit cache disk first. This means
the cache disk must be fast and sustainable.

--------------------------------------
-write-through mode:
+write-through mode
+==================

This mode mainly fixes the 'write hole' issue. For RAID 4/5/6 array, an unclean
shutdown can cause data in some stripes to not be in consistent state, eg, data
@@ -42,8 +44,8 @@ exposed to 'write hole' again.
In write-through mode, the cache disk isn't required to be big. Several
hundreds megabytes are enough.

---------------------------------------
-write-back mode:
+write-back mode
+===============

write-back mode fixes the 'write hole' issue too, since all write data is
cached on cache disk. But the main goal of 'write-back' cache is to speed up
@@ -64,16 +66,16 @@ data loss.
In write-back mode, MD also caches data in memory. The memory cache includes
the same data stored on cache disk, so a power loss doesn't cause data loss.
The memory cache size has performance impact for the array. It's recommended
-the size is big. A user can configure the size by:
+the size is big. A user can configure the size by::

-echo "2048" > /sys/block/md0/md/stripe_cache_size
+ echo "2048" > /sys/block/md0/md/stripe_cache_size

Too small cache disk will make the write aggregation less efficient in this
mode depending on the workloads. It's recommended to use a cache disk with at
least several gigabytes size in write-back mode.

---------------------------------------
-The implementation:
+The implementation
+==================

The write-through and write-back cache use the same disk format. The cache disk
is organized as a simple write log. The log consists of 'meta data' and 'data'
diff --git a/Documentation/md/raid5-ppl.txt b/Documentation/md/raid5-ppl.rst
similarity index 98%
rename from Documentation/md/raid5-ppl.txt
rename to Documentation/md/raid5-ppl.rst
index bfa092589e00..357e5515bc55 100644
--- a/Documentation/md/raid5-ppl.txt
+++ b/Documentation/md/raid5-ppl.rst
@@ -1,4 +1,6 @@
+==================
Partial Parity Log
+==================

Partial Parity Log (PPL) is a feature available for RAID5 arrays. The issue
addressed by PPL is that after a dirty shutdown, parity of a particular stripe
--
2.21.0