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diff --git a/Documentation/driver-api/nvdimm/btt.rst b/Documentation/driver-api/nvdimm/btt.rst new file mode 100644 index 000000000..107395c04 --- /dev/null +++ b/Documentation/driver-api/nvdimm/btt.rst @@ -0,0 +1,285 @@ +============================= +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:: + + == == ==================================================== + 31 30 Description + == == ==================================================== + 0 0 Initial state. Reads return zeroes; Premap = Postmap + 0 1 Zero state: Reads return zeroes + 1 0 Error state: Reads fail; Writes clear 'E' bit + 1 1 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 |