Commit 5212e11f authored by Vishal Verma's avatar Vishal Verma Committed by Dan Williams
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nd_btt: atomic sector updates

BTT stands for Block Translation Table, and is a way to provide power
fail sector atomicity semantics for block devices that have the ability
to perform byte granularity IO. It relies on the capability of libnvdimm
namespace devices to do byte aligned IO.

The BTT works as a stacked blocked device, and reserves a chunk of space
from the backing device for its accounting metadata. It is a bio-based
driver because all IO is done synchronously, and there is no queuing or
asynchronous completions at either the device or the driver level.

The BTT uses 'lanes' to index into various 'on-disk' data structures,
and lanes also act as a synchronization mechanism in case there are more
CPUs than available lanes. We did a comparison between two lane lock
strategies - first where we kept an atomic counter around that tracked
which was the last lane that was used, and 'our' lane was determined by
atomically incrementing that. That way, for the nr_cpus > nr_lanes case,
theoretically, no CPU would be blocked waiting for a lane. The other
strategy was to use the cpu number we're scheduled on to and hash it to
a lane number. Theoretically, this could block an IO that could've
otherwise run using a different, free lane. But some fio workloads
showed that the direct cpu -> lane hash performed faster than tracking
'last lane' - my reasoning is the cache thrash caused by moving the
atomic variable made that approach slower than simply waiting out the
in-progress IO. This supports the conclusion that the driver can be a
very simple bio-based one that does synchronous IOs instead of queuing.

Cc: Andy Lutomirski <>
Cc: Boaz Harrosh <>
Cc: H. Peter Anvin <>
Cc: Jens Axboe <>
Cc: Ingo Molnar <>
Cc: Christoph Hellwig <>
Cc: Neil Brown <>
Cc: Jeff Moyer <>
Cc: Dave Chinner <>
Cc: Greg KH <>
[jmoyer: fix nmi watchdog timeout in btt_map_init]
[jmoyer: move btt initialization to module load path]
[jmoyer: fix memory leak in the btt initialization path]
[jmoyer: Don't overwrite corrupted arenas]
Signed-off-by: default avatarVishal Verma <>
Signed-off-by: default avatarDan Williams <>
parent 8c2f7e86
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
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
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
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 matches the map entry, then log_entry.old is free.
- If does not match the map entry, then is free.
(This case can only be caused by power-fails/unsafe shutdowns)
g. Summarizing - Read and Write flows
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)
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
- 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);
......@@ -902,6 +902,7 @@ static int acpi_nfit_init_mapping(struct acpi_nfit_desc *acpi_desc,
} 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;
......@@ -8,11 +8,11 @@ menuconfig LIBNVDIMM
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.
......@@ -20,6 +20,7 @@ config BLK_DEV_PMEM
tristate "PMEM: Persistent memory block device support"
depends on HAS_IOMEM
select ND_BTT if BTT
Memory ranges for PMEM are described by either an NFIT
(NVDIMM Firmware Interface Table, see CONFIG_NFIT_ACPI), a
......@@ -33,7 +34,22 @@ config BLK_DEV_PMEM
Say Y if you want to use an NVDIMM
config ND_BTT
config BTT
def_bool y
bool "BTT: Block Translation Table (atomic sector updates)"
default y if LIBNVDIMM
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,
Select Y if unsure
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
This diff is collapsed.
......@@ -19,6 +19,39 @@
#define BTT_SIG_LEN 16
#define MAP_ENT_SIZE 4
#define MAP_TRIM_SHIFT 31
#define MAP_ERR_SHIFT 30
#define MAP_LBA_MASK (~((1 << MAP_TRIM_SHIFT) | (1 << MAP_ERR_SHIFT)))
#define MAP_ENT_NORMAL 0xC0000000
#define LOG_ENT_SIZE sizeof(struct log_entry)
#define ARENA_MIN_SIZE (1UL << 24) /* 16 MB */
#define ARENA_MAX_SIZE (1ULL << 39) /* 512 GB */
#define RTT_VALID (1UL << 31)
#define RTT_INVALID 0
#define BTT_PG_SIZE 4096
#define LOG_SEQ_INIT 1
#define IB_FLAG_ERROR 0x00000001
#define IB_FLAG_ERROR_MASK 0x00000001
enum btt_init_state {
struct log_entry {
__le32 lba;
__le32 old_map;
__le32 new_map;
__le32 seq;
__le64 padding[2];
struct btt_sb {
u8 signature[BTT_SIG_LEN];
......@@ -42,4 +75,112 @@ struct btt_sb {
__le64 checksum;
struct free_entry {
u32 block;
u8 sub;
u8 seq;
struct aligned_lock {
union {
spinlock_t lock;
u8 cacheline_padding[L1_CACHE_BYTES];
* struct arena_info - handle for an arena
* @size: Size in bytes this arena occupies on the raw device.
* This includes arena metadata.
* @external_lba_start: The first external LBA in this arena.
* @internal_nlba: Number of internal blocks available in the arena
* including nfree reserved blocks
* @internal_lbasize: Internal and external lba sizes may be different as
* we can round up 'odd' external lbasizes such as 520B
* to be aligned.
* @external_nlba: Number of blocks contributed by the arena to the number
* reported to upper layers. (internal_nlba - nfree)
* @external_lbasize: LBA size as exposed to upper layers.
* @nfree: A reserve number of 'free' blocks that is used to
* handle incoming writes.
* @version_major: Metadata layout version major.
* @version_minor: Metadata layout version minor.
* @nextoff: Offset in bytes to the start of the next arena.
* @infooff: Offset in bytes to the info block of this arena.
* @dataoff: Offset in bytes to the data area of this arena.
* @mapoff: Offset in bytes to the map area of this arena.
* @logoff: Offset in bytes to the log area of this arena.
* @info2off: Offset in bytes to the backup info block of this arena.
* @freelist: Pointer to in-memory list of free blocks
* @rtt: Pointer to in-memory "Read Tracking Table"
* @map_locks: Spinlocks protecting concurrent map writes
* @nd_btt: Pointer to parent nd_btt structure.
* @list: List head for list of arenas
* @debugfs_dir: Debugfs dentry
* @flags: Arena flags - may signify error states.
* arena_info is a per-arena handle. Once an arena is narrowed down for an
* IO, this struct is passed around for the duration of the IO.
struct arena_info {
u64 size; /* Total bytes for this arena */
u64 external_lba_start;
u32 internal_nlba;
u32 internal_lbasize;
u32 external_nlba;
u32 external_lbasize;
u32 nfree;
u16 version_major;
u16 version_minor;
/* Byte offsets to the different on-media structures */
u64 nextoff;
u64 infooff;
u64 dataoff;
u64 mapoff;
u64 logoff;
u64 info2off;
/* Pointers to other in-memory structures for this arena */
struct free_entry *freelist;
u32 *rtt;
struct aligned_lock *map_locks;
struct nd_btt *nd_btt;
struct list_head list;
struct dentry *debugfs_dir;
/* Arena flags */
u32 flags;
* struct btt - handle for a BTT instance
* @btt_disk: Pointer to the gendisk for BTT device
* @btt_queue: Pointer to the request queue for the BTT device
* @arena_list: Head of the list of arenas
* @debugfs_dir: Debugfs dentry
* @nd_btt: Parent nd_btt struct
* @nlba: Number of logical blocks exposed to the upper layers
* after removing the amount of space needed by metadata
* @rawsize: Total size in bytes of the available backing device
* @lbasize: LBA size as requested and presented to upper layers.
* This is sector_size + size of any metadata.
* @sector_size: The Linux sector size - 512 or 4096
* @lanes: Per-lane spinlocks
* @init_lock: Mutex used for the BTT initialization
* @init_state: Flag describing the initialization state for the BTT
* @num_arenas: Number of arenas in the BTT instance
struct btt {
struct gendisk *btt_disk;
struct request_queue *btt_queue;
struct list_head arena_list;
struct dentry *debugfs_dir;
struct nd_btt *nd_btt;
u64 nlba;
unsigned long long rawsize;
u32 lbasize;
u32 sector_size;
struct nd_region *nd_region;
struct mutex init_lock;
int init_state;
int num_arenas;
......@@ -348,7 +348,8 @@ struct device *nd_btt_create(struct nd_region *nd_region)
u64 nd_btt_sb_checksum(struct btt_sb *btt_sb)
u64 sum, sum_save;
u64 sum;
__le64 sum_save;
sum_save = btt_sb->checksum;
btt_sb->checksum = 0;
......@@ -76,6 +76,30 @@ static bool is_namespace_io(struct device *dev)
return dev ? dev->type == &namespace_io_device_type : false;
const char *nvdimm_namespace_disk_name(struct nd_namespace_common *ndns,
char *name)
struct nd_region *nd_region = to_nd_region(ndns->dev.parent);
const char *suffix = "";
if (ndns->claim && is_nd_btt(ndns->claim))
suffix = "s";
if (is_namespace_pmem(&ndns->dev) || is_namespace_io(&ndns->dev))
sprintf(name, "pmem%d%s", nd_region->id, suffix);
else if (is_namespace_blk(&ndns->dev)) {
struct nd_namespace_blk *nsblk;
nsblk = to_nd_namespace_blk(&ndns->dev);
sprintf(name, "ndblk%d.%d%s", nd_region->id, nsblk->id, suffix);
} else {
return NULL;
return name;
static ssize_t nstype_show(struct device *dev,
struct device_attribute *attr, char *buf)
......@@ -20,6 +20,12 @@
#include "label.h"
enum {
* Limits the maximum number of block apertures a dimm can
* support and is an input to the geometry/on-disk-format of a
* BTT instance
......@@ -75,6 +81,11 @@ static inline struct nd_namespace_index *to_next_namespace_index(
for (res = (ndd)->dpa.child, next = res ? res->sibling : NULL; \
res; res = next, next = next ? next->sibling : NULL)
struct nd_percpu_lane {
int count;
spinlock_t lock;
struct nd_region {
struct device dev;
struct ida ns_ida;
......@@ -84,9 +95,10 @@ struct nd_region {
u16 ndr_mappings;
u64 ndr_size;
u64 ndr_start;
int id;
int id, num_lanes;
void *provider_data;
struct nd_interleave_set *nd_set;
struct nd_percpu_lane __percpu *lane;
struct nd_mapping mapping[0];
......@@ -100,9 +112,11 @@ static inline unsigned nd_inc_seq(unsigned seq)
return next[seq & 3];
struct btt;
struct nd_btt {
struct device dev;
struct nd_namespace_common *ndns;
struct btt *btt;
unsigned long lbasize;
u8 *uuid;
int id;
......@@ -157,6 +171,8 @@ static inline struct device *nd_btt_create(struct nd_region *nd_region)
struct nd_region *to_nd_region(struct device *dev);
unsigned int nd_region_acquire_lane(struct nd_region *nd_region);
void nd_region_release_lane(struct nd_region *nd_region, unsigned int lane);
int nd_region_to_nstype(struct nd_region *nd_region);
int nd_region_register_namespaces(struct nd_region *nd_region, int *err);
u64 nd_region_interleave_set_cookie(struct nd_region *nd_region);
......@@ -172,4 +188,8 @@ struct resource *nvdimm_allocate_dpa(struct nvdimm_drvdata *ndd,
resource_size_t n);
resource_size_t nvdimm_namespace_capacity(struct nd_namespace_common *ndns);
struct nd_namespace_common *nvdimm_namespace_common_probe(struct device *dev);
int nvdimm_namespace_attach_btt(struct nd_namespace_common *ndns);
int nvdimm_namespace_detach_btt(struct nd_namespace_common *ndns);
const char *nvdimm_namespace_disk_name(struct nd_namespace_common *ndns,
char *name);
#endif /* __ND_H__ */
......@@ -160,7 +160,6 @@ static void pmem_detach_disk(struct pmem_device *pmem)
static int pmem_attach_disk(struct nd_namespace_common *ndns,
struct pmem_device *pmem)
struct nd_region *nd_region = to_nd_region(ndns->dev.parent);
struct gendisk *disk;
pmem->pmem_queue = blk_alloc_queue(GFP_KERNEL);
......@@ -183,7 +182,7 @@ static int pmem_attach_disk(struct nd_namespace_common *ndns,
disk->private_data = pmem;
disk->queue = pmem->pmem_queue;
disk->flags = GENHD_FL_EXT_DEVT;
sprintf(disk->disk_name, "pmem%d", nd_region->id);
nvdimm_namespace_disk_name(ndns, disk->disk_name);
disk->driverfs_dev = &ndns->dev;
set_capacity(disk, pmem->size >> 9);
pmem->pmem_disk = disk;
......@@ -211,17 +210,6 @@ static int pmem_rw_bytes(struct nd_namespace_common *ndns,
return 0;
static int nvdimm_namespace_attach_btt(struct nd_namespace_common *ndns)
/* TODO */
return -ENXIO;
static void nvdimm_namespace_detach_btt(struct nd_namespace_common *ndns)
/* TODO */