274 lines
11 KiB
Plaintext
274 lines
11 KiB
Plaintext
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BTT - Block Translation Table
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=============================
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1. Introduction
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---------------
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Persistent memory based storage is able to perform IO at byte (or more
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accurately, cache line) granularity. However, we often want to expose such
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storage as traditional block devices. The block drivers for persistent memory
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will do exactly this. However, they do not provide any atomicity guarantees.
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Traditional SSDs typically provide protection against torn sectors in hardware,
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using stored energy in capacitors to complete in-flight block writes, or perhaps
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in firmware. We don't have this luxury with persistent memory - if a write is in
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progress, and we experience a power failure, the block will contain a mix of old
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and new data. Applications may not be prepared to handle such a scenario.
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The Block Translation Table (BTT) provides atomic sector update semantics for
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persistent memory devices, so that applications that rely on sector writes not
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being torn can continue to do so. The BTT manifests itself as a stacked block
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device, and reserves a portion of the underlying storage for its metadata. At
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the heart of it, is an indirection table that re-maps all the blocks on the
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volume. It can be thought of as an extremely simple file system that only
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provides atomic sector updates.
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2. Static Layout
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----------------
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The underlying storage on which a BTT can be laid out is not limited in any way.
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The BTT, however, splits the available space into chunks of up to 512 GiB,
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called "Arenas".
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Each arena follows the same layout for its metadata, and all references in an
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arena are internal to it (with the exception of one field that points to the
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next arena). The following depicts the "On-disk" metadata layout:
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Backing Store +-------> Arena
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+---------------+ | +------------------+
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| | | | Arena info block |
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| Arena 0 +---+ | 4K |
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| 512G | +------------------+
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+---------------+ | |
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| Arena 1 | | Data Blocks |
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| 512G | | |
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+---------------+ | |
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| . | | |
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| . | | |
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+---------------+ +------------------+
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| BTT Map |
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+------------------+
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| BTT Flog |
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+------------------+
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| Info block copy |
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| 4K |
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+------------------+
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3. Theory of Operation
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----------------------
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a. The BTT Map
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--------------
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The map is a simple lookup/indirection table that maps an LBA to an internal
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block. Each map entry is 32 bits. The two most significant bits are special
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flags, and the remaining form the internal block number.
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Bit Description
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31 - 30 : Error and Zero flags - Used in the following way:
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Bit Description
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31 30
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-----------------------------------------------------------------------
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00 Initial state. Reads return zeroes; Premap = Postmap
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01 Zero state: Reads return zeroes
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10 Error state: Reads fail; Writes clear 'E' bit
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11 Normal Block – has valid postmap
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29 - 0 : Mappings to internal 'postmap' blocks
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Some of the terminology that will be subsequently used:
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External LBA : LBA as made visible to upper layers.
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ABA : Arena Block Address - Block offset/number within an arena
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Premap ABA : The block offset into an arena, which was decided upon by range
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checking the External LBA
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Postmap ABA : The block number in the "Data Blocks" area obtained after
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indirection from the map
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nfree : The number of free blocks that are maintained at any given time.
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This is the number of concurrent writes that can happen to the
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arena.
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For example, after adding a BTT, we surface a disk of 1024G. We get a read for
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the external LBA at 768G. This falls into the second arena, and of the 512G
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worth of blocks that this arena contributes, this block is at 256G. Thus, the
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premap ABA is 256G. We now refer to the map, and find out the mapping for block
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'X' (256G) points to block 'Y', say '64'. Thus the postmap ABA is 64.
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b. The BTT Flog
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---------------
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The BTT provides sector atomicity by making every write an "allocating write",
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i.e. Every write goes to a "free" block. A running list of free blocks is
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maintained in the form of the BTT flog. 'Flog' is a combination of the words
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"free list" and "log". The flog contains 'nfree' entries, and an entry contains:
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lba : The premap ABA that is being written to
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old_map : The old postmap ABA - after 'this' write completes, this will be a
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free block.
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new_map : The new postmap ABA. The map will up updated to reflect this
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lba->postmap_aba mapping, but we log it here in case we have to
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recover.
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seq : Sequence number to mark which of the 2 sections of this flog entry is
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valid/newest. It cycles between 01->10->11->01 (binary) under normal
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operation, with 00 indicating an uninitialized state.
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lba' : alternate lba entry
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old_map': alternate old postmap entry
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new_map': alternate new postmap entry
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seq' : alternate sequence number.
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Each of the above fields is 32-bit, making one entry 32 bytes. Entries are also
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padded to 64 bytes to avoid cache line sharing or aliasing. Flog updates are
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done such that for any entry being written, it:
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a. overwrites the 'old' section in the entry based on sequence numbers
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b. writes the 'new' section such that the sequence number is written last.
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c. The concept of lanes
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-----------------------
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While 'nfree' describes the number of concurrent IOs an arena can process
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concurrently, 'nlanes' is the number of IOs the BTT device as a whole can
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process.
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nlanes = min(nfree, num_cpus)
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A lane number is obtained at the start of any IO, and is used for indexing into
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all the on-disk and in-memory data structures for the duration of the IO. If
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there are more CPUs than the max number of available lanes, than lanes are
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protected by spinlocks.
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d. In-memory data structure: Read Tracking Table (RTT)
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------------------------------------------------------
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Consider a case where we have two threads, one doing reads and the other,
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writes. We can hit a condition where the writer thread grabs a free block to do
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a new IO, but the (slow) reader thread is still reading from it. In other words,
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the reader consulted a map entry, and started reading the corresponding block. A
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writer started writing to the same external LBA, and finished the write updating
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the map for that external LBA to point to its new postmap ABA. At this point the
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internal, postmap block that the reader is (still) reading has been inserted
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into the list of free blocks. If another write comes in for the same LBA, it can
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grab this free block, and start writing to it, causing the reader to read
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incorrect data. To prevent this, we introduce the RTT.
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The RTT is a simple, per arena table with 'nfree' entries. Every reader inserts
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into rtt[lane_number], the postmap ABA it is reading, and clears it after the
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read is complete. Every writer thread, after grabbing a free block, checks the
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RTT for its presence. If the postmap free block is in the RTT, it waits till the
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reader clears the RTT entry, and only then starts writing to it.
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e. In-memory data structure: map locks
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--------------------------------------
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Consider a case where two writer threads are writing to the same LBA. There can
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be a race in the following sequence of steps:
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free[lane] = map[premap_aba]
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map[premap_aba] = postmap_aba
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Both threads can update their respective free[lane] with the same old, freed
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postmap_aba. This has made the layout inconsistent by losing a free entry, and
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at the same time, duplicating another free entry for two lanes.
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To solve this, we could have a single map lock (per arena) that has to be taken
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before performing the above sequence, but we feel that could be too contentious.
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Instead we use an array of (nfree) map_locks that is indexed by
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(premap_aba modulo nfree).
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f. Reconstruction from the Flog
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-------------------------------
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On startup, we analyze the BTT flog to create our list of free blocks. We walk
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through all the entries, and for each lane, of the set of two possible
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'sections', we always look at the most recent one only (based on the sequence
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number). The reconstruction rules/steps are simple:
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- Read map[log_entry.lba].
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- If log_entry.new matches the map entry, then log_entry.old is free.
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- If log_entry.new does not match the map entry, then log_entry.new is free.
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(This case can only be caused by power-fails/unsafe shutdowns)
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g. Summarizing - Read and Write flows
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-------------------------------------
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Read:
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1. Convert external LBA to arena number + pre-map ABA
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2. Get a lane (and take lane_lock)
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3. Read map to get the entry for this pre-map ABA
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4. Enter post-map ABA into RTT[lane]
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5. If TRIM flag set in map, return zeroes, and end IO (go to step 8)
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6. If ERROR flag set in map, end IO with EIO (go to step 8)
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7. Read data from this block
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8. Remove post-map ABA entry from RTT[lane]
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9. Release lane (and lane_lock)
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Write:
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1. Convert external LBA to Arena number + pre-map ABA
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2. Get a lane (and take lane_lock)
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3. Use lane to index into in-memory free list and obtain a new block, next flog
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index, next sequence number
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4. Scan the RTT to check if free block is present, and spin/wait if it is.
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5. Write data to this free block
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6. Read map to get the existing post-map ABA entry for this pre-map ABA
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7. Write flog entry: [premap_aba / old postmap_aba / new postmap_aba / seq_num]
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8. Write new post-map ABA into map.
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9. Write old post-map entry into the free list
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10. Calculate next sequence number and write into the free list entry
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11. Release lane (and lane_lock)
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4. Error Handling
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=================
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An arena would be in an error state if any of the metadata is corrupted
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irrecoverably, either due to a bug or a media error. The following conditions
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indicate an error:
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- Info block checksum does not match (and recovering from the copy also fails)
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- All internal available blocks are not uniquely and entirely addressed by the
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sum of mapped blocks and free blocks (from the BTT flog).
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- Rebuilding free list from the flog reveals missing/duplicate/impossible
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entries
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- A map entry is out of bounds
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If any of these error conditions are encountered, the arena is put into a read
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only state using a flag in the info block.
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5. Usage
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========
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The BTT can be set up on any disk (namespace) exposed by the libnvdimm subsystem
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(pmem, or blk mode). The easiest way to set up such a namespace is using the
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'ndctl' utility [1]:
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For example, the ndctl command line to setup a btt with a 4k sector size is:
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ndctl create-namespace -f -e namespace0.0 -m sector -l 4k
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See ndctl create-namespace --help for more options.
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[1]: https://github.com/pmem/ndctl
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