1================
2RAID 4/5/6 cache
3================
4
5Raid 4/5/6 could include an extra disk for data cache besides normal RAID
6disks. The role of RAID disks isn't changed with the cache disk. The cache disk
7caches data to the RAID disks. The cache can be in write-through (supported
8since 4.4) or write-back mode (supported since 4.10). mdadm (supported since
93.4) has a new option '--write-journal' to create array with cache. Please
10refer to mdadm manual for details. By default (RAID array starts), the cache is
11in write-through mode. A user can switch it to write-back mode by::
12
13	echo "write-back" > /sys/block/md0/md/journal_mode
14
15And switch it back to write-through mode by::
16
17	echo "write-through" > /sys/block/md0/md/journal_mode
18
19In both modes, all writes to the array will hit cache disk first. This means
20the cache disk must be fast and sustainable.
21
22write-through mode
23==================
24
25This mode mainly fixes the 'write hole' issue. For RAID 4/5/6 array, an unclean
26shutdown can cause data in some stripes to not be in consistent state, eg, data
27and parity don't match. The reason is that a stripe write involves several RAID
28disks and it's possible the writes don't hit all RAID disks yet before the
29unclean shutdown. We call an array degraded if it has inconsistent data. MD
30tries to resync the array to bring it back to normal state. But before the
31resync completes, any system crash will expose the chance of real data
32corruption in the RAID array. This problem is called 'write hole'.
33
34The write-through cache will cache all data on cache disk first. After the data
35is safe on the cache disk, the data will be flushed onto RAID disks. The
36two-step write will guarantee MD can recover correct data after unclean
37shutdown even the array is degraded. Thus the cache can close the 'write hole'.
38
39In write-through mode, MD reports IO completion to upper layer (usually
40filesystems) after the data is safe on RAID disks, so cache disk failure
41doesn't cause data loss. Of course cache disk failure means the array is
42exposed to 'write hole' again.
43
44In write-through mode, the cache disk isn't required to be big. Several
45hundreds megabytes are enough.
46
47write-back mode
48===============
49
50write-back mode fixes the 'write hole' issue too, since all write data is
51cached on cache disk. But the main goal of 'write-back' cache is to speed up
52write. If a write crosses all RAID disks of a stripe, we call it full-stripe
53write. For non-full-stripe writes, MD must read old data before the new parity
54can be calculated. These synchronous reads hurt write throughput. Some writes
55which are sequential but not dispatched in the same time will suffer from this
56overhead too. Write-back cache will aggregate the data and flush the data to
57RAID disks only after the data becomes a full stripe write. This will
58completely avoid the overhead, so it's very helpful for some workloads. A
59typical workload which does sequential write followed by fsync is an example.
60
61In write-back mode, MD reports IO completion to upper layer (usually
62filesystems) right after the data hits cache disk. The data is flushed to raid
63disks later after specific conditions met. So cache disk failure will cause
64data loss.
65
66In write-back mode, MD also caches data in memory. The memory cache includes
67the same data stored on cache disk, so a power loss doesn't cause data loss.
68The memory cache size has performance impact for the array. It's recommended
69the size is big. A user can configure the size by::
70
71	echo "2048" > /sys/block/md0/md/stripe_cache_size
72
73Too small cache disk will make the write aggregation less efficient in this
74mode depending on the workloads. It's recommended to use a cache disk with at
75least several gigabytes size in write-back mode.
76
77The implementation
78==================
79
80The write-through and write-back cache use the same disk format. The cache disk
81is organized as a simple write log. The log consists of 'meta data' and 'data'
82pairs. The meta data describes the data. It also includes checksum and sequence
83ID for recovery identification. Data can be IO data and parity data. Data is
84checksumed too. The checksum is stored in the meta data ahead of the data. The
85checksum is an optimization because MD can write meta and data freely without
86worry about the order. MD superblock has a field pointed to the valid meta data
87of log head.
88
89The log implementation is pretty straightforward. The difficult part is the
90order in which MD writes data to cache disk and RAID disks. Specifically, in
91write-through mode, MD calculates parity for IO data, writes both IO data and
92parity to the log, writes the data and parity to RAID disks after the data and
93parity is settled down in log and finally the IO is finished. Read just reads
94from raid disks as usual.
95
96In write-back mode, MD writes IO data to the log and reports IO completion. The
97data is also fully cached in memory at that time, which means read must query
98memory cache. If some conditions are met, MD will flush the data to RAID disks.
99MD will calculate parity for the data and write parity into the log. After this
100is finished, MD will write both data and parity into RAID disks, then MD can
101release the memory cache. The flush conditions could be stripe becomes a full
102stripe write, free cache disk space is low or free in-kernel memory cache space
103is low.
104
105After an unclean shutdown, MD does recovery. MD reads all meta data and data
106from the log. The sequence ID and checksum will help us detect corrupted meta
107data and data. If MD finds a stripe with data and valid parities (1 parity for
108raid4/5 and 2 for raid6), MD will write the data and parities to RAID disks. If
109parities are incompleted, they are discarded. If part of data is corrupted,
110they are discarded too. MD then loads valid data and writes them to RAID disks
111in normal way.
112