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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 : TRIM flag - marks if the block was trimmed or discarded
+30 : ERROR flag - marks an error block. Cleared on write.
+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 16 bytes. 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 entry 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. It is
+protected by a spinlock.
+
+
+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. In-kernel usage
+==================
+
+Any block driver that supports byte granularity IO to the storage may register
+with the BTT. It will have to provide the rw_bytes interface in its
+block_device_operations struct:
+
+ int (*rw_bytes)(struct gendisk *, void *, size_t, off_t, int rw);
+
+It may register with the BTT after it adds its own gendisk, using btt_init:
+
+ struct btt *btt_init(struct gendisk *disk, unsigned long long rawsize,
+ u32 lbasize, u8 uuid[], int maxlane);
+
+note that maxlane is the maximum amount of concurrency the driver wishes to
+allow the BTT to use.
+
+The BTT 'disk' appears as a stacked block device that grabs the underlying block
+device in the O_EXCL mode.
+
+When the driver wishes to remove the backing disk, it should similarly call
+btt_fini using the same struct btt* handle that was provided to it by btt_init.
+
+ void btt_fini(struct btt *btt);
+