Revision 474095e46cd14421821da3201a9fd6a4c070996b authored by Linus Torvalds on 24 April 2015, 16:28:01 UTC, committed by Linus Torvalds on 24 April 2015, 16:28:01 UTC
Pull md updates from Neil Brown: "More updates that usual this time. A few have performance impacts which hould mostly be positive, but RAID5 (in particular) can be very work-load ensitive... We'll have to wait and see. Highlights: - "experimental" code for managing md/raid1 across a cluster using DLM. Code is not ready for general use and triggers a WARNING if used. However it is looking good and mostly done and having in mainline will help co-ordinate development. - RAID5/6 can now batch multiple (4K wide) stripe_heads so as to handle a full (chunk wide) stripe as a single unit. - RAID6 can now perform read-modify-write cycles which should help performance on larger arrays: 6 or more devices. - RAID5/6 stripe cache now grows and shrinks dynamically. The value set is used as a minimum. - Resync is now allowed to go a little faster than the 'mininum' when there is competing IO. How much faster depends on the speed of the devices, so the effective minimum should scale with device speed to some extent" * tag 'md/4.1' of git://neil.brown.name/md: (58 commits) md/raid5: don't do chunk aligned read on degraded array. md/raid5: allow the stripe_cache to grow and shrink. md/raid5: change ->inactive_blocked to a bit-flag. md/raid5: move max_nr_stripes management into grow_one_stripe and drop_one_stripe md/raid5: pass gfp_t arg to grow_one_stripe() md/raid5: introduce configuration option rmw_level md/raid5: activate raid6 rmw feature md/raid6 algorithms: xor_syndrome() for SSE2 md/raid6 algorithms: xor_syndrome() for generic int md/raid6 algorithms: improve test program md/raid6 algorithms: delta syndrome functions raid5: handle expansion/resync case with stripe batching raid5: handle io error of batch list RAID5: batch adjacent full stripe write raid5: track overwrite disk count raid5: add a new flag to track if a stripe can be batched raid5: use flex_array for scribble data md raid0: access mddev->queue (request queue member) conditionally because it is not set when accessed from dm-raid md: allow resync to go faster when there is competing IO. md: remove 'go_faster' option from ->sync_request() ...
lzo.txt
LZO stream format as understood by Linux's LZO decompressor
===========================================================
Introduction
This is not a specification. No specification seems to be publicly available
for the LZO stream format. This document describes what input format the LZO
decompressor as implemented in the Linux kernel understands. The file subject
of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on
the compressor nor on any other implementations though it seems likely that
the format matches the standard one. The purpose of this document is to
better understand what the code does in order to propose more efficient fixes
for future bug reports.
Description
The stream is composed of a series of instructions, operands, and data. The
instructions consist in a few bits representing an opcode, and bits forming
the operands for the instruction, whose size and position depend on the
opcode and on the number of literals copied by previous instruction. The
operands are used to indicate :
- a distance when copying data from the dictionary (past output buffer)
- a length (number of bytes to copy from dictionary)
- the number of literals to copy, which is retained in variable "state"
as a piece of information for next instructions.
Optionally depending on the opcode and operands, extra data may follow. These
extra data can be a complement for the operand (eg: a length or a distance
encoded on larger values), or a literal to be copied to the output buffer.
The first byte of the block follows a different encoding from other bytes, it
seems to be optimized for literal use only, since there is no dictionary yet
prior to that byte.
Lengths are always encoded on a variable size starting with a small number
of bits in the operand. If the number of bits isn't enough to represent the
length, up to 255 may be added in increments by consuming more bytes with a
rate of at most 255 per extra byte (thus the compression ratio cannot exceed
around 255:1). The variable length encoding using #bits is always the same :
length = byte & ((1 << #bits) - 1)
if (!length) {
length = ((1 << #bits) - 1)
length += 255*(number of zero bytes)
length += first-non-zero-byte
}
length += constant (generally 2 or 3)
For references to the dictionary, distances are relative to the output
pointer. Distances are encoded using very few bits belonging to certain
ranges, resulting in multiple copy instructions using different encodings.
Certain encodings involve one extra byte, others involve two extra bytes
forming a little-endian 16-bit quantity (marked LE16 below).
After any instruction except the large literal copy, 0, 1, 2 or 3 literals
are copied before starting the next instruction. The number of literals that
were copied may change the meaning and behaviour of the next instruction. In
practice, only one instruction needs to know whether 0, less than 4, or more
literals were copied. This is the information stored in the <state> variable
in this implementation. This number of immediate literals to be copied is
generally encoded in the last two bits of the instruction but may also be
taken from the last two bits of an extra operand (eg: distance).
End of stream is declared when a block copy of distance 0 is seen. Only one
instruction may encode this distance (0001HLLL), it takes one LE16 operand
for the distance, thus requiring 3 bytes.
IMPORTANT NOTE : in the code some length checks are missing because certain
instructions are called under the assumption that a certain number of bytes
follow because it has already been garanteed before parsing the instructions.
They just have to "refill" this credit if they consume extra bytes. This is
an implementation design choice independant on the algorithm or encoding.
Byte sequences
First byte encoding :
0..17 : follow regular instruction encoding, see below. It is worth
noting that codes 16 and 17 will represent a block copy from
the dictionary which is empty, and that they will always be
invalid at this place.
18..21 : copy 0..3 literals
state = (byte - 17) = 0..3 [ copy <state> literals ]
skip byte
22..255 : copy literal string
length = (byte - 17) = 4..238
state = 4 [ don't copy extra literals ]
skip byte
Instruction encoding :
0 0 0 0 X X X X (0..15)
Depends on the number of literals copied by the last instruction.
If last instruction did not copy any literal (state == 0), this
encoding will be a copy of 4 or more literal, and must be interpreted
like this :
0 0 0 0 L L L L (0..15) : copy long literal string
length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte)
state = 4 (no extra literals are copied)
If last instruction used to copy between 1 to 3 literals (encoded in
the instruction's opcode or distance), the instruction is a copy of a
2-byte block from the dictionary within a 1kB distance. It is worth
noting that this instruction provides little savings since it uses 2
bytes to encode a copy of 2 other bytes but it encodes the number of
following literals for free. It must be interpreted like this :
0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance
length = 2
state = S (copy S literals after this block)
Always followed by exactly one byte : H H H H H H H H
distance = (H << 2) + D + 1
If last instruction used to copy 4 or more literals (as detected by
state == 4), the instruction becomes a copy of a 3-byte block from the
dictionary from a 2..3kB distance, and must be interpreted like this :
0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance
length = 3
state = S (copy S literals after this block)
Always followed by exactly one byte : H H H H H H H H
distance = (H << 2) + D + 2049
0 0 0 1 H L L L (16..31)
Copy of a block within 16..48kB distance (preferably less than 10B)
length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte)
Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
distance = 16384 + (H << 14) + D
state = S (copy S literals after this block)
End of stream is reached if distance == 16384
0 0 1 L L L L L (32..63)
Copy of small block within 16kB distance (preferably less than 34B)
length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
distance = D + 1
state = S (copy S literals after this block)
0 1 L D D D S S (64..127)
Copy 3-4 bytes from block within 2kB distance
state = S (copy S literals after this block)
length = 3 + L
Always followed by exactly one byte : H H H H H H H H
distance = (H << 3) + D + 1
1 L L D D D S S (128..255)
Copy 5-8 bytes from block within 2kB distance
state = S (copy S literals after this block)
length = 5 + L
Always followed by exactly one byte : H H H H H H H H
distance = (H << 3) + D + 1
Authors
This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
analysis of the decompression code available in Linux 3.16-rc5. The code is
tricky, it is possible that this document contains mistakes or that a few
corner cases were overlooked. In any case, please report any doubt, fix, or
proposed updates to the author(s) so that the document can be updated.
Computing file changes ...