Add basic RAIDZ intro

Signed-off-by: George Melikov <mail@gmelikov.ru>

docs/Basic Concepts/RAIDZ.rst

@@ -0,0 +1,89 @@
+RAIDZ
+=====
+
+tl;dr: RAIDZ is effective for large block sizes and sequential workloads.
+
+Introduction
+~~~~~~~~~~~~
+
+RAIDZ is a variation on RAID-5 that allows for better distribution of parity
+and eliminates the RAID-5 “write hole” (in which data and parity become
+inconsistent after a power loss).
+Data and parity is striped across all disks within a raidz group.
+
+A raidz group can have single, double, or triple parity, meaning that the raidz
+group can sustain one, two, or three failures, respectively, without losing any
+data. The ``raidz1`` vdev type specifies a single-parity raidz group; the ``raidz2``
+vdev type specifies a double-parity raidz group; and the ``raidz3`` vdev type
+specifies a triple-parity raidz group. The ``raidz`` vdev type is an alias for
+raidz1.
+
+A raidz group with N disks of size X with P parity disks can hold
+approximately (N-P)*X bytes and can withstand P devices failing without
+losing data. The minimum number of devices in a raidz group is one more
+than the number of parity disks. The recommended number is between 3 and 9
+to help increase performance.
+
+
+Space efficiency
+~~~~~~~~~~~~~~~~
+
+Actual used space for a block in RAIDZ is based on several points:
+
+- minimal write size is disk sector size (can be set via `ashift` vdev parameter)
+
+- stripe width in RAIDZ is dynamic, and starts with at least one data block part, or up to
+  ``disks count`` minus ``parity number`` parts of data block
+
+- one block of data with size of ``recordsize`` is
+  splitted equally via ``sector size`` parts
+  and written on each stripe on RAIDZ vdev
+- each stripe of data will have a part of block
+
+- in addition to data one, two or three blocks of parity should be written,
+  one per disk; so, for raidz2 of 5 disks there will be 3 blocks of data and
+  2 blocks of parity
+
+Due to these inputs, if ``recordsize`` is less or equal to sector size,
+then RAIDZ's parity size will be effictively equal to mirror with same redundancy.
+For example, for raidz1 of 3 disks with ``ashift=12`` and ``recordsize=4K``
+we will allocate on disk:
+
+- one 4K block of data
+
+- one 4K padding block
+
+, and usable space ratio will be 50%, same as with double mirror.
+
+
+Another example for ``ashift=12`` and ``recordsize=128K`` for raidz1 of 3 disks:
+
+- total stripe width is 3
+
+- one stripe can have up to 2 data parts of 4K size because of 1 parity blocks
+
+- we will have 128K/2 = 64 stripes with 8K of data and 4K of parity each
+
+, so usable space ratio in this case will be 66%.
+
+
+If RAIDZ will have more disks, it's stripe width will be larger, and space
+efficiency better too.
+
+You can find actual parity cost per RAIDZ size here:
+
+.. raw:: html
+
+    <iframe src="https://docs.google.com/spreadsheets/d/1tf4qx1aMJp8Lo_R6gpT689wTjHv6CGVElrPqTA0w_ZY/pub?embed=true" height="1000px" width="100%"></iframe>
+
+(`source <https://docs.google.com/spreadsheets/d/1tf4qx1aMJp8Lo_R6gpT689wTjHv6CGVElrPqTA0w_ZY/edit>`__)
+
+
+Performance considerations
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Write
+^^^^^
+
+Because of full stripe width, one block write will write stripe part on each disk.
+One RAIDZ vdev has a write IOPS of one slowest disk because of that in worst case.