nd_btt: atomic sector updates

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);

		} else {

			nd_mapping->size = nfit_mem->bdw->capacity;

			nd_mapping->start = nfit_mem->bdw->start_address;

			ndr_desc->num_lanes = nfit_mem->bdw->windows;

			blk_valid = 1;

		}

	  NFIT, or otherwise can discover NVDIMM resources, a libnvdimm

	  bus is registered to advertise PMEM (persistent memory)

	  namespaces (/dev/pmemX) and BLK (sliding mmio window(s))

	  namespaces (/dev/ndX). A PMEM namespace refers to a memory

	  resource that may span multiple DIMMs and support DAX (see

	  CONFIG_DAX).  A BLK namespace refers to an NVDIMM control

	  region which exposes an mmio register set for windowed

	  access mode to non-volatile memory.

	  namespaces (/dev/ndblkX.Y). A PMEM namespace refers to a

	  memory resource that may span multiple DIMMs and support DAX

	  (see CONFIG_DAX).  A BLK namespace refers to an NVDIMM control

	  region which exposes an mmio register set for windowed access

	  mode to non-volatile memory.

if LIBNVDIMM

	tristate "PMEM: Persistent memory block device support"

	default LIBNVDIMM

	depends on HAS_IOMEM

	select ND_BTT if BTT

	help

	  Memory ranges for PMEM are described by either an NFIT

	  (NVDIMM Firmware Interface Table, see CONFIG_NFIT_ACPI), a

	  Say Y if you want to use an NVDIMM

config ND_BTT

	tristate

config BTT

	def_bool y

	bool "BTT: Block Translation Table (atomic sector updates)"

	default y if LIBNVDIMM

	help

	  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 (a

	  guarantee that typical disks provide) can continue to do so.

	  The BTT manifests itself as an alternate personality for an

	  NVDIMM namespace, i.e. a namespace can be in raw mode (pmemX,

	  ndblkX.Y, etc...), or 'sectored' mode, (pmemXs, ndblkX.Ys,

	  etc...).

	  Select Y if unsure

endif

obj-$(CONFIG_LIBNVDIMM) += libnvdimm.o

obj-$(CONFIG_BLK_DEV_PMEM) += nd_pmem.o

obj-$(CONFIG_ND_BTT) += nd_btt.o

nd_pmem-y := pmem.o

nd_btt-y := btt.o

libnvdimm-y := core.o

libnvdimm-y += bus.o

libnvdimm-y += dimm_devs.o