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-rw-r--r--Documentation/nvdimm/btt.txt24
1 files changed, 17 insertions, 7 deletions
diff --git a/Documentation/nvdimm/btt.txt b/Documentation/nvdimm/btt.txt
index 95134d5ec4a0..b91443f577dc 100644
--- a/Documentation/nvdimm/btt.txt
+++ b/Documentation/nvdimm/btt.txt
@@ -80,9 +80,17 @@ 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
+31 - 30 : Error and Zero flags - Used in the following way:
+ Bit Description
+ 31 30
+ -----------------------------------------------------------------------
+ 00 Initial state. Reads return zeroes; Premap = Postmap
+ 01 Zero state: Reads return zeroes
+ 10 Error state: Reads fail; Writes clear 'E' bit
+ 11 Normal Block – has valid postmap
+
+
+29 - 0 : Mappings to internal 'postmap' blocks
Some of the terminology that will be subsequently used:
@@ -127,10 +135,11 @@ 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
+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 entry such that the sequence number is written last.
+b. writes the 'new' section such that the sequence number is written last.
c. The concept of lanes
@@ -141,8 +150,9 @@ 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.
+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)