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BTT - Block Translation Table
=============================


1. Introduction
---------------

Persistent memory based storage is able to perform IO at byte (or more
accurately, cache line) granularity. However, we often want to expose such
storage as traditional block devices. The block drivers for persistent memory
will do exactly this. However, they do not provide any atomicity guarantees.
Traditional SSDs typically provide protection against torn sectors in hardware,
using stored energy in capacitors to complete in-flight block writes, or perhaps
in firmware. We don't have this luxury with persistent memory - if a write is in
progress, and we experience a power failure, the block will contain a mix of old
and new data. Applications may not be prepared to handle such a scenario.

The Block Translation Table (BTT) provides atomic sector update semantics for
persistent memory devices, so that applications that rely on sector writes not
being torn can continue to do so. The BTT manifests itself as a stacked block
device, and reserves a portion of the underlying storage for its metadata. At
the heart of it, is an indirection table that re-maps all the blocks on the
volume. It can be thought of as an extremely simple file system that only
provides atomic sector updates.


2. Static Layout
----------------

The underlying storage on which a BTT can be laid out is not limited in any way.
The BTT, however, splits the available space into chunks of up to 512 GiB,
called "Arenas".

Each arena follows the same layout for its metadata, and all references in an
arena are internal to it (with the exception of one field that points to the
next arena). The following depicts the "On-disk" metadata layout:


  Backing Store     +------->  Arena
+---------------+   |   +------------------+
|               |   |   | Arena info block |
|    Arena 0    +---+   |       4K         |
|     512G      |       +------------------+
|               |       |                  |
+---------------+       |                  |
|               |       |                  |
|    Arena 1    |       |   Data Blocks    |
|     512G      |       |                  |
|               |       |                  |
+---------------+       |                  |
|       .       |       |                  |
|       .       |       |                  |
|       .       |       |                  |
|               |       |                  |
|               |       |                  |
+---------------+       +------------------+
                        |                  |
                        |     BTT Map      |
                        |                  |
                        |                  |
                        +------------------+
                        |                  |
                        |     BTT Flog     |
                        |                  |
                        +------------------+
                        | Info block copy  |
                        |       4K         |
                        +------------------+


3. Theory of Operation
----------------------


a. The BTT Map
--------------

The map is a simple lookup/indirection table that maps an LBA to an internal
block. Each map entry is 32 bits. The two most significant bits are special
flags, and the remaining form the internal block number.

Bit      Description
31 - 30	: Error and Zero flags - Used in the following way:
	 Bit		      Description
	31 30
	-----------------------------------------------------------------------
	 00	Initial state. Reads return zeroes; Premap = Postmap
	 01	Zero state: Reads return zeroes
	 10	Error state: Reads fail; Writes clear 'E' bit
	 11	Normal Block – has valid postmap


29 - 0	: Mappings to internal 'postmap' blocks


Some of the terminology that will be subsequently used:

External LBA  : LBA as made visible to upper layers.
ABA           : Arena Block Address - Block offset/number within an arena
Premap ABA    : The block offset into an arena, which was decided upon by range
		checking the External LBA
Postmap ABA   : The block number in the "Data Blocks" area obtained after
		indirection from the map
nfree	      : The number of free blocks that are maintained at any given time.
		This is the number of concurrent writes that can happen to the
		arena.


For example, after adding a BTT, we surface a disk of 1024G. We get a read for
the external LBA at 768G. This falls into the second arena, and of the 512G
worth of blocks that this arena contributes, this block is at 256G. Thus, the
premap ABA is 256G. We now refer to the map, and find out the mapping for block
'X' (256G) points to block 'Y', say '64'. Thus the postmap ABA is 64.


b. The BTT Flog
---------------

The BTT provides sector atomicity by making every write an "allocating write",
i.e. Every write goes to a "free" block. A running list of free blocks is
maintained in the form of the BTT flog. 'Flog' is a combination of the words
"free list" and "log". The flog contains 'nfree' entries, and an entry contains:

lba     : The premap ABA that is being written to
old_map : The old postmap ABA - after 'this' write completes, this will be a
	  free block.
new_map : The new postmap ABA. The map will up updated to reflect this
	  lba->postmap_aba mapping, but we log it here in case we have to
	  recover.
seq	: Sequence number to mark which of the 2 sections of this flog entry is
	  valid/newest. It cycles between 01->10->11->01 (binary) under normal
	  operation, with 00 indicating an uninitialized state.
lba'	: alternate lba entry
old_map': alternate old postmap entry
new_map': alternate new postmap entry
seq'	: alternate sequence number.

Each of the above fields is 32-bit, making one entry 32 bytes. Entries are also
padded to 64 bytes to avoid cache line sharing or aliasing. Flog updates are
done such that for any entry being written, it:
a. overwrites the 'old' section in the entry based on sequence numbers
b. writes the 'new' section such that the sequence number is written last.


c. The concept of lanes
-----------------------

While 'nfree' describes the number of concurrent IOs an arena can process
concurrently, 'nlanes' is the number of IOs the BTT device as a whole can
process.
 nlanes = min(nfree, num_cpus)
A lane number is obtained at the start of any IO, and is used for indexing into
all the on-disk and in-memory data structures for the duration of the IO. If
there are more CPUs than the max number of available lanes, than lanes are
protected by spinlocks.


d. In-memory data structure: Read Tracking Table (RTT)
------------------------------------------------------

Consider a case where we have two threads, one doing reads and the other,
writes. We can hit a condition where the writer thread grabs a free block to do
a new IO, but the (slow) reader thread is still reading from it. In other words,
the reader consulted a map entry, and started reading the corresponding block. A
writer started writing to the same external LBA, and finished the write updating
the map for that external LBA to point to its new postmap ABA. At this point the
internal, postmap block that the reader is (still) reading has been inserted
into the list of free blocks. If another write comes in for the same LBA, it can
grab this free block, and start writing to it, causing the reader to read
incorrect data. To prevent this, we introduce the RTT.

The RTT is a simple, per arena table with 'nfree' entries. Every reader inserts
into rtt[lane_number], the postmap ABA it is reading, and clears it after the
read is complete. Every writer thread, after grabbing a free block, checks the
RTT for its presence. If the postmap free block is in the RTT, it waits till the
reader clears the RTT entry, and only then starts writing to it.


e. In-memory data structure: map locks
--------------------------------------

Consider a case where two writer threads are writing to the same LBA. There can
be a race in the following sequence of steps:

free[lane] = map[premap_aba]
map[premap_aba] = postmap_aba

Both threads can update their respective free[lane] with the same old, freed
postmap_aba. This has made the layout inconsistent by losing a free entry, and
at the same time, duplicating another free entry for two lanes.

To solve this, we could have a single map lock (per arena) that has to be taken
before performing the above sequence, but we feel that could be too contentious.
Instead we use an array of (nfree) map_locks that is indexed by
(premap_aba modulo nfree).


f. Reconstruction from the Flog
-------------------------------

On startup, we analyze the BTT flog to create our list of free blocks. We walk
through all the entries, and for each lane, of the set of two possible
'sections', we always look at the most recent one only (based on the sequence
number). The reconstruction rules/steps are simple:
- Read map[log_entry.lba].
- If log_entry.new matches the map entry, then log_entry.old is free.
- If log_entry.new does not match the map entry, then log_entry.new is free.
  (This case can only be caused by power-fails/unsafe shutdowns)


g. Summarizing - Read and Write flows
-------------------------------------

Read:

1.  Convert external LBA to arena number + pre-map ABA
2.  Get a lane (and take lane_lock)
3.  Read map to get the entry for this pre-map ABA
4.  Enter post-map ABA into RTT[lane]
5.  If TRIM flag set in map, return zeroes, and end IO (go to step 8)
6.  If ERROR flag set in map, end IO with EIO (go to step 8)
7.  Read data from this block
8.  Remove post-map ABA entry from RTT[lane]
9.  Release lane (and lane_lock)

Write:

1.  Convert external LBA to Arena number + pre-map ABA
2.  Get a lane (and take lane_lock)
3.  Use lane to index into in-memory free list and obtain a new block, next flog
        index, next sequence number
4.  Scan the RTT to check if free block is present, and spin/wait if it is.
5.  Write data to this free block
6.  Read map to get the existing post-map ABA entry for this pre-map ABA
7.  Write flog entry: [premap_aba / old postmap_aba / new postmap_aba / seq_num]
8.  Write new post-map ABA into map.
9.  Write old post-map entry into the free list
10. Calculate next sequence number and write into the free list entry
11. Release lane (and lane_lock)


4. Error Handling
=================

An arena would be in an error state if any of the metadata is corrupted
irrecoverably, either due to a bug or a media error. The following conditions
indicate an error:
- Info block checksum does not match (and recovering from the copy also fails)
- All internal available blocks are not uniquely and entirely addressed by the
  sum of mapped blocks and free blocks (from the BTT flog).
- Rebuilding free list from the flog reveals missing/duplicate/impossible
  entries
- A map entry is out of bounds

If any of these error conditions are encountered, the arena is put into a read
only state using a flag in the info block.


5. Usage
========

The BTT can be set up on any disk (namespace) exposed by the libnvdimm subsystem
(pmem, or blk mode). The easiest way to set up such a namespace is using the
'ndctl' utility [1]:

For example, the ndctl command line to setup a btt with a 4k sector size is:

    ndctl create-namespace -f -e namespace0.0 -m sector -l 4k

See ndctl create-namespace --help for more options.

[1]: https://github.com/pmem/ndctl