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Documentation / nvdimm / btt.txt


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

1	BTT - Block Translation Table
2	=============================
3	
4	
5	1. Introduction
6	---------------
7	
8	Persistent memory based storage is able to perform IO at byte (or more
9	accurately, cache line) granularity. However, we often want to expose such
10	storage as traditional block devices. The block drivers for persistent memory
11	will do exactly this. However, they do not provide any atomicity guarantees.
12	Traditional SSDs typically provide protection against torn sectors in hardware,
13	using stored energy in capacitors to complete in-flight block writes, or perhaps
14	in firmware. We don't have this luxury with persistent memory - if a write is in
15	progress, and we experience a power failure, the block will contain a mix of old
16	and new data. Applications may not be prepared to handle such a scenario.
17	
18	The Block Translation Table (BTT) provides atomic sector update semantics for
19	persistent memory devices, so that applications that rely on sector writes not
20	being torn can continue to do so. The BTT manifests itself as a stacked block
21	device, and reserves a portion of the underlying storage for its metadata. At
22	the heart of it, is an indirection table that re-maps all the blocks on the
23	volume. It can be thought of as an extremely simple file system that only
24	provides atomic sector updates.
25	
26	
27	2. Static Layout
28	----------------
29	
30	The underlying storage on which a BTT can be laid out is not limited in any way.
31	The BTT, however, splits the available space into chunks of up to 512 GiB,
32	called "Arenas".
33	
34	Each arena follows the same layout for its metadata, and all references in an
35	arena are internal to it (with the exception of one field that points to the
36	next arena). The following depicts the "On-disk" metadata layout:
37	
38	
39	  Backing Store     +------->  Arena
40	+---------------+   |   +------------------+
41	|               |   |   | Arena info block |
42	|    Arena 0    +---+   |       4K         |
43	|     512G      |       +------------------+
44	|               |       |                  |
45	+---------------+       |                  |
46	|               |       |                  |
47	|    Arena 1    |       |   Data Blocks    |
48	|     512G      |       |                  |
49	|               |       |                  |
50	+---------------+       |                  |
51	|       .       |       |                  |
52	|       .       |       |                  |
53	|       .       |       |                  |
54	|               |       |                  |
55	|               |       |                  |
56	+---------------+       +------------------+
57	                        |                  |
58	                        |     BTT Map      |
59	                        |                  |
60	                        |                  |
61	                        +------------------+
62	                        |                  |
63	                        |     BTT Flog     |
64	                        |                  |
65	                        +------------------+
66	                        | Info block copy  |
67	                        |       4K         |
68	                        +------------------+
69	
70	
71	3. Theory of Operation
72	----------------------
73	
74	
75	a. The BTT Map
76	--------------
77	
78	The map is a simple lookup/indirection table that maps an LBA to an internal
79	block. Each map entry is 32 bits. The two most significant bits are special
80	flags, and the remaining form the internal block number.
81	
82	Bit      Description
83	31 - 30	: Error and Zero flags - Used in the following way:
84		 Bit		      Description
85		31 30
86		-----------------------------------------------------------------------
87		 00	Initial state. Reads return zeroes; Premap = Postmap
88		 01	Zero state: Reads return zeroes
89		 10	Error state: Reads fail; Writes clear 'E' bit
90		 11	Normal Block – has valid postmap
91	
92	
93	29 - 0	: Mappings to internal 'postmap' blocks
94	
95	
96	Some of the terminology that will be subsequently used:
97	
98	External LBA  : LBA as made visible to upper layers.
99	ABA           : Arena Block Address - Block offset/number within an arena
100	Premap ABA    : The block offset into an arena, which was decided upon by range
101			checking the External LBA
102	Postmap ABA   : The block number in the "Data Blocks" area obtained after
103			indirection from the map
104	nfree	      : The number of free blocks that are maintained at any given time.
105			This is the number of concurrent writes that can happen to the
106			arena.
107	
108	
109	For example, after adding a BTT, we surface a disk of 1024G. We get a read for
110	the external LBA at 768G. This falls into the second arena, and of the 512G
111	worth of blocks that this arena contributes, this block is at 256G. Thus, the
112	premap ABA is 256G. We now refer to the map, and find out the mapping for block
113	'X' (256G) points to block 'Y', say '64'. Thus the postmap ABA is 64.
114	
115	
116	b. The BTT Flog
117	---------------
118	
119	The BTT provides sector atomicity by making every write an "allocating write",
120	i.e. Every write goes to a "free" block. A running list of free blocks is
121	maintained in the form of the BTT flog. 'Flog' is a combination of the words
122	"free list" and "log". The flog contains 'nfree' entries, and an entry contains:
123	
124	lba     : The premap ABA that is being written to
125	old_map : The old postmap ABA - after 'this' write completes, this will be a
126		  free block.
127	new_map : The new postmap ABA. The map will up updated to reflect this
128		  lba->postmap_aba mapping, but we log it here in case we have to
129		  recover.
130	seq	: Sequence number to mark which of the 2 sections of this flog entry is
131		  valid/newest. It cycles between 01->10->11->01 (binary) under normal
132		  operation, with 00 indicating an uninitialized state.
133	lba'	: alternate lba entry
134	old_map': alternate old postmap entry
135	new_map': alternate new postmap entry
136	seq'	: alternate sequence number.
137	
138	Each of the above fields is 32-bit, making one entry 32 bytes. Entries are also
139	padded to 64 bytes to avoid cache line sharing or aliasing. Flog updates are
140	done such that for any entry being written, it:
141	a. overwrites the 'old' section in the entry based on sequence numbers
142	b. writes the 'new' section such that the sequence number is written last.
143	
144	
145	c. The concept of lanes
146	-----------------------
147	
148	While 'nfree' describes the number of concurrent IOs an arena can process
149	concurrently, 'nlanes' is the number of IOs the BTT device as a whole can
150	process.
151	 nlanes = min(nfree, num_cpus)
152	A lane number is obtained at the start of any IO, and is used for indexing into
153	all the on-disk and in-memory data structures for the duration of the IO. If
154	there are more CPUs than the max number of available lanes, than lanes are
155	protected by spinlocks.
156	
157	
158	d. In-memory data structure: Read Tracking Table (RTT)
159	------------------------------------------------------
160	
161	Consider a case where we have two threads, one doing reads and the other,
162	writes. We can hit a condition where the writer thread grabs a free block to do
163	a new IO, but the (slow) reader thread is still reading from it. In other words,
164	the reader consulted a map entry, and started reading the corresponding block. A
165	writer started writing to the same external LBA, and finished the write updating
166	the map for that external LBA to point to its new postmap ABA. At this point the
167	internal, postmap block that the reader is (still) reading has been inserted
168	into the list of free blocks. If another write comes in for the same LBA, it can
169	grab this free block, and start writing to it, causing the reader to read
170	incorrect data. To prevent this, we introduce the RTT.
171	
172	The RTT is a simple, per arena table with 'nfree' entries. Every reader inserts
173	into rtt[lane_number], the postmap ABA it is reading, and clears it after the
174	read is complete. Every writer thread, after grabbing a free block, checks the
175	RTT for its presence. If the postmap free block is in the RTT, it waits till the
176	reader clears the RTT entry, and only then starts writing to it.
177	
178	
179	e. In-memory data structure: map locks
180	--------------------------------------
181	
182	Consider a case where two writer threads are writing to the same LBA. There can
183	be a race in the following sequence of steps:
184	
185	free[lane] = map[premap_aba]
186	map[premap_aba] = postmap_aba
187	
188	Both threads can update their respective free[lane] with the same old, freed
189	postmap_aba. This has made the layout inconsistent by losing a free entry, and
190	at the same time, duplicating another free entry for two lanes.
191	
192	To solve this, we could have a single map lock (per arena) that has to be taken
193	before performing the above sequence, but we feel that could be too contentious.
194	Instead we use an array of (nfree) map_locks that is indexed by
195	(premap_aba modulo nfree).
196	
197	
198	f. Reconstruction from the Flog
199	-------------------------------
200	
201	On startup, we analyze the BTT flog to create our list of free blocks. We walk
202	through all the entries, and for each lane, of the set of two possible
203	'sections', we always look at the most recent one only (based on the sequence
204	number). The reconstruction rules/steps are simple:
205	- Read map[log_entry.lba].
206	- If log_entry.new matches the map entry, then log_entry.old is free.
207	- If log_entry.new does not match the map entry, then log_entry.new is free.
208	  (This case can only be caused by power-fails/unsafe shutdowns)
209	
210	
211	g. Summarizing - Read and Write flows
212	-------------------------------------
213	
214	Read:
215	
216	1.  Convert external LBA to arena number + pre-map ABA
217	2.  Get a lane (and take lane_lock)
218	3.  Read map to get the entry for this pre-map ABA
219	4.  Enter post-map ABA into RTT[lane]
220	5.  If TRIM flag set in map, return zeroes, and end IO (go to step 8)
221	6.  If ERROR flag set in map, end IO with EIO (go to step 8)
222	7.  Read data from this block
223	8.  Remove post-map ABA entry from RTT[lane]
224	9.  Release lane (and lane_lock)
225	
226	Write:
227	
228	1.  Convert external LBA to Arena number + pre-map ABA
229	2.  Get a lane (and take lane_lock)
230	3.  Use lane to index into in-memory free list and obtain a new block, next flog
231	        index, next sequence number
232	4.  Scan the RTT to check if free block is present, and spin/wait if it is.
233	5.  Write data to this free block
234	6.  Read map to get the existing post-map ABA entry for this pre-map ABA
235	7.  Write flog entry: [premap_aba / old postmap_aba / new postmap_aba / seq_num]
236	8.  Write new post-map ABA into map.
237	9.  Write old post-map entry into the free list
238	10. Calculate next sequence number and write into the free list entry
239	11. Release lane (and lane_lock)
240	
241	
242	4. Error Handling
243	=================
244	
245	An arena would be in an error state if any of the metadata is corrupted
246	irrecoverably, either due to a bug or a media error. The following conditions
247	indicate an error:
248	- Info block checksum does not match (and recovering from the copy also fails)
249	- All internal available blocks are not uniquely and entirely addressed by the
250	  sum of mapped blocks and free blocks (from the BTT flog).
251	- Rebuilding free list from the flog reveals missing/duplicate/impossible
252	  entries
253	- A map entry is out of bounds
254	
255	If any of these error conditions are encountered, the arena is put into a read
256	only state using a flag in the info block.
257	
258	
259	5. Usage
260	========
261	
262	The BTT can be set up on any disk (namespace) exposed by the libnvdimm subsystem
263	(pmem, or blk mode). The easiest way to set up such a namespace is using the
264	'ndctl' utility [1]:
265	
266	For example, the ndctl command line to setup a btt with a 4k sector size is:
267	
268	    ndctl create-namespace -f -e namespace0.0 -m sector -l 4k
269	
270	See ndctl create-namespace --help for more options.
271	
272	[1]: https://github.com/pmem/ndctl
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