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Documentation / md / raid5-cache.txt


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	RAID5 cache
2	
3	Raid 4/5/6 could include an extra disk for data cache besides normal RAID
4	disks. The role of RAID disks isn't changed with the cache disk. The cache disk
5	caches data to the RAID disks. The cache can be in write-through (supported
6	since 4.4) or write-back mode (supported since 4.10). mdadm (supported since
7	3.4) has a new option '--write-journal' to create array with cache. Please
8	refer to mdadm manual for details. By default (RAID array starts), the cache is
9	in write-through mode. A user can switch it to write-back mode by:
10	
11	echo "write-back" > /sys/block/md0/md/journal_mode
12	
13	And switch it back to write-through mode by:
14	
15	echo "write-through" > /sys/block/md0/md/journal_mode
16	
17	In both modes, all writes to the array will hit cache disk first. This means
18	the cache disk must be fast and sustainable.
19	
20	-------------------------------------
21	write-through mode:
22	
23	This mode mainly fixes the 'write hole' issue. For RAID 4/5/6 array, an unclean
24	shutdown can cause data in some stripes to not be in consistent state, eg, data
25	and parity don't match. The reason is that a stripe write involves several RAID
26	disks and it's possible the writes don't hit all RAID disks yet before the
27	unclean shutdown. We call an array degraded if it has inconsistent data. MD
28	tries to resync the array to bring it back to normal state. But before the
29	resync completes, any system crash will expose the chance of real data
30	corruption in the RAID array. This problem is called 'write hole'.
31	
32	The write-through cache will cache all data on cache disk first. After the data
33	is safe on the cache disk, the data will be flushed onto RAID disks. The
34	two-step write will guarantee MD can recover correct data after unclean
35	shutdown even the array is degraded. Thus the cache can close the 'write hole'.
36	
37	In write-through mode, MD reports IO completion to upper layer (usually
38	filesystems) after the data is safe on RAID disks, so cache disk failure
39	doesn't cause data loss. Of course cache disk failure means the array is
40	exposed to 'write hole' again.
41	
42	In write-through mode, the cache disk isn't required to be big. Several
43	hundreds megabytes are enough.
44	
45	--------------------------------------
46	write-back mode:
47	
48	write-back mode fixes the 'write hole' issue too, since all write data is
49	cached on cache disk. But the main goal of 'write-back' cache is to speed up
50	write. If a write crosses all RAID disks of a stripe, we call it full-stripe
51	write. For non-full-stripe writes, MD must read old data before the new parity
52	can be calculated. These synchronous reads hurt write throughput. Some writes
53	which are sequential but not dispatched in the same time will suffer from this
54	overhead too. Write-back cache will aggregate the data and flush the data to
55	RAID disks only after the data becomes a full stripe write. This will
56	completely avoid the overhead, so it's very helpful for some workloads. A
57	typical workload which does sequential write followed by fsync is an example.
58	
59	In write-back mode, MD reports IO completion to upper layer (usually
60	filesystems) right after the data hits cache disk. The data is flushed to raid
61	disks later after specific conditions met. So cache disk failure will cause
62	data loss.
63	
64	In write-back mode, MD also caches data in memory. The memory cache includes
65	the same data stored on cache disk, so a power loss doesn't cause data loss.
66	The memory cache size has performance impact for the array. It's recommended
67	the size is big. A user can configure the size by:
68	
69	echo "2048" > /sys/block/md0/md/stripe_cache_size
70	
71	Too small cache disk will make the write aggregation less efficient in this
72	mode depending on the workloads. It's recommended to use a cache disk with at
73	least several gigabytes size in write-back mode.
74	
75	--------------------------------------
76	The implementation:
77	
78	The write-through and write-back cache use the same disk format. The cache disk
79	is organized as a simple write log. The log consists of 'meta data' and 'data'
80	pairs. The meta data describes the data. It also includes checksum and sequence
81	ID for recovery identification. Data can be IO data and parity data. Data is
82	checksumed too. The checksum is stored in the meta data ahead of the data. The
83	checksum is an optimization because MD can write meta and data freely without
84	worry about the order. MD superblock has a field pointed to the valid meta data
85	of log head.
86	
87	The log implementation is pretty straightforward. The difficult part is the
88	order in which MD writes data to cache disk and RAID disks. Specifically, in
89	write-through mode, MD calculates parity for IO data, writes both IO data and
90	parity to the log, writes the data and parity to RAID disks after the data and
91	parity is settled down in log and finally the IO is finished. Read just reads
92	from raid disks as usual.
93	
94	In write-back mode, MD writes IO data to the log and reports IO completion. The
95	data is also fully cached in memory at that time, which means read must query
96	memory cache. If some conditions are met, MD will flush the data to RAID disks.
97	MD will calculate parity for the data and write parity into the log. After this
98	is finished, MD will write both data and parity into RAID disks, then MD can
99	release the memory cache. The flush conditions could be stripe becomes a full
100	stripe write, free cache disk space is low or free in-kernel memory cache space
101	is low.
102	
103	After an unclean shutdown, MD does recovery. MD reads all meta data and data
104	from the log. The sequence ID and checksum will help us detect corrupted meta
105	data and data. If MD finds a stripe with data and valid parities (1 parity for
106	raid4/5 and 2 for raid6), MD will write the data and parities to RAID disks. If
107	parities are incompleted, they are discarded. If part of data is corrupted,
108	they are discarded too. MD then loads valid data and writes them to RAID disks
109	in normal way.
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