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Merge pull request #21 from aldot/doc-tweaks

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documentation touch up, take 2
This commit is contained in:
Christopher Haster
2018-02-01 15:06:24 -06:00
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parent 6d8e0e21d0 436707c8d0
commit d82e34c3ee
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DESIGN.md
+45 -43
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@@ -27,16 +27,17 @@ cheap, and can be very granular. For NOR flash specifically, byte-level
programs are quite common. Erasing, however, requires an expensive operation
that forces the state of large blocks of memory to reset in a destructive
reaction that gives flash its name. The [Wikipedia entry](https://en.wikipedia.org/wiki/Flash_memory)
has more information if you are interesting in how this works.
has more information if you are interested in how this works.
This leaves us with an interesting set of limitations that can be simplified
to three strong requirements:
1. **Power-loss resilient** - This is the main goal of the littlefs and the
focus of this project. Embedded systems are usually designed without a
shutdown routine and a notable lack of user interface for recovery, so
filesystems targeting embedded systems must be prepared to lose power an
any given time.
focus of this project.
Embedded systems are usually designed without a shutdown routine and a
notable lack of user interface for recovery, so filesystems targeting
embedded systems must be prepared to lose power at any given time.
Despite this state of things, there are very few embedded filesystems that
handle power loss in a reasonable manner, and most can become corrupted if
@@ -52,7 +53,8 @@ to three strong requirements:
which stores a file allocation table (FAT) at a specific offset from the
beginning of disk. Every block allocation will update this table, and after
100,000 updates, the block will likely go bad, rendering the filesystem
unusable even if there are many more erase cycles available on the storage.
unusable even if there are many more erase cycles available on the storage
as a whole.
3. **Bounded RAM/ROM** - Even with the design difficulties presented by the
previous two limitations, we have already seen several flash filesystems
@@ -72,7 +74,7 @@ to three strong requirements:
## Existing designs?
There are of course, many different existing filesystem. Heres a very rough
There are of course, many different existing filesystem. Here is a very rough
summary of the general ideas behind some of them.
Most of the existing filesystems fall into the one big category of filesystem
@@ -80,21 +82,21 @@ designed in the early days of spinny magnet disks. While there is a vast amount
of interesting technology and ideas in this area, the nature of spinny magnet
disks encourage properties, such as grouping writes near each other, that don't
make as much sense on recent storage types. For instance, on flash, write
locality is not important and can actually increase wear destructively.
locality is not important and can actually increase wear.
One of the most popular designs for flash filesystems is called the
[logging filesystem](https://en.wikipedia.org/wiki/Log-structured_file_system).
The flash filesystems [jffs](https://en.wikipedia.org/wiki/JFFS)
and [yaffs](https://en.wikipedia.org/wiki/YAFFS) are good examples. In
logging filesystem, data is not store in a data structure on disk, but instead
and [yaffs](https://en.wikipedia.org/wiki/YAFFS) are good examples. In a
logging filesystem, data is not stored in a data structure on disk, but instead
the changes to the files are stored on disk. This has several neat advantages,
such as the fact that the data is written in a cyclic log format naturally
such as the fact that the data is written in a cyclic log format and naturally
wear levels as a side effect. And, with a bit of error detection, the entire
filesystem can easily be designed to be resilient to power loss. The
journalling component of most modern day filesystems is actually a reduced
journaling component of most modern day filesystems is actually a reduced
form of a logging filesystem. However, logging filesystems have a difficulty
scaling as the size of storage increases. And most filesystems compensate by
caching large parts of the filesystem in RAM, a strategy that is unavailable
caching large parts of the filesystem in RAM, a strategy that is inappropriate
for embedded systems.
Another interesting filesystem design technique is that of [copy-on-write (COW)](https://en.wikipedia.org/wiki/Copy-on-write).
@@ -107,14 +109,14 @@ where the COW data structures are synchronized.
## Metadata pairs
The core piece of technology that provides the backbone for the littlefs is
the concept of metadata pairs. The key idea here, is that any metadata that
the concept of metadata pairs. The key idea here is that any metadata that
needs to be updated atomically is stored on a pair of blocks tagged with
a revision count and checksum. Every update alternates between these two
pairs, so that at any time there is always a backup containing the previous
state of the metadata.
Consider a small example where each metadata pair has a revision count,
a number as data, and the xor of the block as a quick checksum. If
a number as data, and the XOR of the block as a quick checksum. If
we update the data to a value of 9, and then to a value of 5, here is
what the pair of blocks may look like after each update:
```
@@ -130,7 +132,7 @@ what the pair of blocks may look like after each update:
After each update, we can find the most up to date value of data by looking
at the revision count.
Now consider what the blocks may look like if we suddenly loss power while
Now consider what the blocks may look like if we suddenly lose power while
changing the value of data to 5:
```
block 1 block 2 block 1 block 2 block 1 block 2
@@ -149,7 +151,7 @@ check our checksum we notice that block 1 was corrupted. So we fall back to
block 2 and use the value 9.
Using this concept, the littlefs is able to update metadata blocks atomically.
There are a few other tweaks, such as using a 32 bit crc and using sequence
There are a few other tweaks, such as using a 32 bit CRC and using sequence
arithmetic to handle revision count overflow, but the basic concept
is the same. These metadata pairs define the backbone of the littlefs, and the
rest of the filesystem is built on top of these atomic updates.
@@ -161,7 +163,7 @@ requires two blocks for each block of data. I'm sure users would be very
unhappy if their storage was suddenly cut in half! Instead of storing
everything in these metadata blocks, the littlefs uses a COW data structure
for files which is in turn pointed to by a metadata block. When
we update a file, we create a copies of any blocks that are modified until
we update a file, we create copies of any blocks that are modified until
the metadata blocks are updated with the new copy. Once the metadata block
points to the new copy, we deallocate the old blocks that are no longer in use.
@@ -184,7 +186,7 @@ Here is what updating a one-block file may look like:
update data in file update metadata pair
```
It doesn't matter if we lose power while writing block 5 with the new data,
It doesn't matter if we lose power while writing new data to block 5,
since the old data remains unmodified in block 4. This example also
highlights how the atomic updates of the metadata blocks provide a
synchronization barrier for the rest of the littlefs.
@@ -206,7 +208,7 @@ files in filesystems. Of these, the littlefs uses a rather unique [COW](https://
data structure that allows the filesystem to reuse unmodified parts of the
file without additional metadata pairs.
First lets consider storing files in a simple linked-list. What happens when
First lets consider storing files in a simple linked-list. What happens when we
append a block? We have to change the last block in the linked-list to point
to this new block, which means we have to copy out the last block, and change
the second-to-last block, and then the third-to-last, and so on until we've
@@ -240,8 +242,8 @@ Exhibit B: A backwards linked-list
```
However, a backwards linked-list does come with a rather glaring problem.
Iterating over a file _in order_ has a runtime of O(n^2). Gah! A quadratic
runtime to just _read_ a file? That's awful. Keep in mind reading files are
Iterating over a file _in order_ has a runtime cost of O(n^2). Gah! A quadratic
runtime to just _read_ a file? That's awful. Keep in mind reading files is
usually the most common filesystem operation.
To avoid this problem, the littlefs uses a multilayered linked-list. For
@@ -266,7 +268,7 @@ Exhibit C: A backwards CTZ skip-list
```
The additional pointers allow us to navigate the data-structure on disk
much more efficiently than in a single linked-list.
much more efficiently than in a singly linked-list.
Taking exhibit C for example, here is the path from data block 5 to data
block 1. You can see how data block 3 was completely skipped:
@@ -289,15 +291,15 @@ The path to data block 0 is even more quick, requiring only two jumps:
We can find the runtime complexity by looking at the path to any block from
the block containing the most pointers. Every step along the path divides
the search space for the block in half. This gives us a runtime of O(logn).
the search space for the block in half. This gives us a runtime of O(log n).
To get to the block with the most pointers, we can perform the same steps
backwards, which puts the runtime at O(2logn) = O(logn). The interesting
backwards, which puts the runtime at O(2 log n) = O(log n). The interesting
part about this data structure is that this optimal path occurs naturally
if we greedily choose the pointer that covers the most distance without passing
our target block.
So now we have a representation of files that can be appended trivially with
a runtime of O(1), and can be read with a worst case runtime of O(nlogn).
a runtime of O(1), and can be read with a worst case runtime of O(n log n).
Given that the the runtime is also divided by the amount of data we can store
in a block, this is pretty reasonable.
@@ -362,7 +364,7 @@ N = file size in bytes
And this works quite well, but is not trivial to calculate. This equation
requires O(n) to compute, which brings the entire runtime of reading a file
to O(n^2logn). Fortunately, the additional O(n) does not need to touch disk,
to O(n^2 log n). Fortunately, the additional O(n) does not need to touch disk,
so it is not completely unreasonable. But if we could solve this equation into
a form that is easily computable, we can avoid a big slowdown.
@@ -379,11 +381,11 @@ unintuitive property:
![mindblown](https://latex.codecogs.com/svg.latex?%5Csum_i%5En%5Cleft%28%5Ctext%7Bctz%7D%28i%29+1%5Cright%29%20%3D%202n-%5Ctext%7Bpopcount%7D%28n%29)
where:
ctz(i) = the number of trailing bits that are 0 in i
popcount(i) = the number of bits that are 1 in i
ctz(x) = the number of trailing bits that are 0 in x
popcount(x) = the number of bits that are 1 in x
It's a bit bewildering that these two seemingly unrelated bitwise instructions
are related by this property. But if we start to disect this equation we can
are related by this property. But if we start to dissect this equation we can
see that it does hold. As n approaches infinity, we do end up with an average
overhead of 2 pointers as we find earlier. And popcount seems to handle the
error from this average as it accumulates in the CTZ skip-list.
@@ -410,8 +412,7 @@ a bit to avoid integer overflow:
![formulaforoff](https://latex.codecogs.com/svg.latex?%5Cmathit%7Boff%7D%20%3D%20N%20-%20%5Cleft%28B-2%5Cfrac%7Bw%7D%7B8%7D%5Cright%29n%20-%20%5Cfrac%7Bw%7D%7B8%7D%5Ctext%7Bpopcount%7D%28n%29)
The solution involves quite a bit of math, but computers are very good at math.
We can now solve for the block index + offset while only needed to store the
file size in O(1).
Now we can solve for both the block index and offset from the file size in O(1).
Here is what it might look like to update a file stored with a CTZ skip-list:
```
@@ -500,16 +501,17 @@ scanned to find the most recent free list, but once the list was found the
state of all free blocks becomes known.
However, this approach had several issues:
- There was a lot of nuanced logic for adding blocks to the free list without
modifying the blocks, since the blocks remain active until the metadata is
updated.
- The free list had to support both additions and removals in fifo order while
- The free list had to support both additions and removals in FIFO order while
minimizing block erases.
- The free list had to handle the case where the file system completely ran
out of blocks and may no longer be able to add blocks to the free list.
- If we used a revision count to track the most recently updated free list,
metadata blocks that were left unmodified were ticking time bombs that would
cause the system to go haywire if the revision count overflowed
cause the system to go haywire if the revision count overflowed.
- Every single metadata block wasted space to store these free list references.
Actually, to simplify, this approach had one massive glaring issue: complexity.
@@ -539,7 +541,7 @@ would have an abhorrent runtime.
So the littlefs compromises. It doesn't store a bitmap the size of the storage,
but it does store a little bit-vector that contains a fixed set lookahead
for block allocations. During a block allocation, the lookahead vector is
checked for any free blocks, if there are none, the lookahead region jumps
checked for any free blocks. If there are none, the lookahead region jumps
forward and the entire filesystem is scanned for free blocks.
Here's what it might look like to allocate 4 blocks on a decently busy
@@ -622,7 +624,7 @@ So, as a solution, the littlefs adopted a sort of threaded tree. Each
directory not only contains pointers to all of its children, but also a
pointer to the next directory. These pointers create a linked-list that
is threaded through all of the directories in the filesystem. Since we
only use this linked list to check for existance, the order doesn't actually
only use this linked list to check for existence, the order doesn't actually
matter. As an added plus, we can repurpose the pointer for the individual
directory linked-lists and avoid using any additional space.
@@ -773,7 +775,7 @@ deorphan step that simply iterates through every directory in the linked-list
and checks it against every directory entry in the filesystem to see if it
has a parent. The deorphan step occurs on the first block allocation after
boot, so orphans should never cause the littlefs to run out of storage
prematurely. Note that the deorphan step never needs to run in a readonly
prematurely. Note that the deorphan step never needs to run in a read-only
filesystem.
## The move problem
@@ -883,7 +885,7 @@ a power loss will occur during filesystem activity. We still need to handle
the condition, but runtime during a power loss takes a back seat to the runtime
during normal operations.
So what littlefs does is unelegantly simple. When littlefs moves a file, it
So what littlefs does is inelegantly simple. When littlefs moves a file, it
marks the file as "moving". This is stored as a single bit in the directory
entry and doesn't take up much space. Then littlefs moves the directory,
finishing with the complete remove of the "moving" directory entry.
@@ -979,7 +981,7 @@ if it exists elsewhere in the filesystem.
So now that we have all of the pieces of a filesystem, we can look at a more
subtle attribute of embedded storage: The wear down of flash blocks.
The first concern for the littlefs, is that prefectly valid blocks can suddenly
The first concern for the littlefs, is that perfectly valid blocks can suddenly
become unusable. As a nice side-effect of using a COW data-structure for files,
we can simply move on to a different block when a file write fails. All
modifications to files are performed in copies, so we will only replace the
@@ -1151,7 +1153,7 @@ develops errors and needs to be moved.
## Wear leveling
The second concern for the littlefs, is that blocks in the filesystem may wear
The second concern for the littlefs is that blocks in the filesystem may wear
unevenly. In this situation, a filesystem may meet an early demise where
there are no more non-corrupted blocks that aren't in use. It's common to
have files that were written once and left unmodified, wasting the potential
@@ -1171,7 +1173,7 @@ of wear leveling:
In littlefs's case, it's possible to use the revision count on metadata pairs
to approximate the wear of a metadata block. And combined with the COW nature
of files, littlefs could provide your usually implementation of dynamic wear
of files, littlefs could provide your usual implementation of dynamic wear
leveling.
However, the littlefs does not. This is for a few reasons. Most notably, even
@@ -1210,9 +1212,9 @@ So, to summarize:
metadata block is active
4. Directory blocks contain either references to other directories or files
5. Files are represented by copy-on-write CTZ skip-lists which support O(1)
append and O(nlogn) reading
append and O(n log n) reading
6. Blocks are allocated by scanning the filesystem for used blocks in a
fixed-size lookahead region is that stored in a bit-vector
fixed-size lookahead region that is stored in a bit-vector
7. To facilitate scanning the filesystem, all directories are part of a
linked-list that is threaded through the entire filesystem
8. If a block develops an error, the littlefs allocates a new block, and
README.md
+9 -9
View File
@@ -16,7 +16,7 @@ of memory. Recursion is avoided and dynamic memory is limited to configurable
buffers that can be provided statically.
**Power-loss resilient** - The littlefs is designed for systems that may have
random power failures. The littlefs has strong copy-on-write guaruntees and
random power failures. The littlefs has strong copy-on-write guarantees and
storage on disk is always kept in a valid state.
**Wear leveling** - Since the most common form of embedded storage is erodible
@@ -88,7 +88,7 @@ int main(void) {
## Usage
Detailed documentation (or at least as much detail as is currently available)
can be cound in the comments in [lfs.h](lfs.h).
can be found in the comments in [lfs.h](lfs.h).
As you may have noticed, littlefs takes in a configuration structure that
defines how the filesystem operates. The configuration struct provides the
@@ -101,12 +101,12 @@ to the user to allocate, allowing multiple filesystems to be in use
simultaneously. With the `lfs_t` and configuration struct, a user can
format a block device or mount the filesystem.
Once mounted, the littlefs provides a full set of posix-like file and
Once mounted, the littlefs provides a full set of POSIX-like file and
directory functions, with the deviation that the allocation of filesystem
structures must be provided by the user.
All posix operations, such as remove and rename, are atomic, even in event
of power-loss. Additionally, no file updates are actually commited to the
All POSIX operations, such as remove and rename, are atomic, even in event
of power-loss. Additionally, no file updates are actually committed to the
filesystem until sync or close is called on the file.
## Other notes
@@ -116,7 +116,7 @@ can be either one of those found in the `enum lfs_error` in [lfs.h](lfs.h),
or an error returned by the user's block device operations.
It should also be noted that the current implementation of littlefs doesn't
really do anything to insure that the data written to disk is machine portable.
really do anything to ensure that the data written to disk is machine portable.
This is fine as long as all of the involved machines share endianness
(little-endian) and don't have strange padding requirements.
@@ -131,9 +131,9 @@ with all the nitty-gritty details. Can be useful for developing tooling.
## Testing
The littlefs comes with a test suite designed to run on a pc using the
The littlefs comes with a test suite designed to run on a PC using the
[emulated block device](emubd/lfs_emubd.h) found in the emubd directory.
The tests assume a linux environment and can be started with make:
The tests assume a Linux environment and can be started with make:
``` bash
make test
@@ -148,7 +148,7 @@ littlefs is available in Mbed OS as the [LittleFileSystem](https://os.mbed.com/d
class.
[littlefs-fuse](https://github.com/geky/littlefs-fuse) - A [FUSE](https://github.com/libfuse/libfuse)
wrapper for littlefs. The project allows you to mount littlefs directly in a
wrapper for littlefs. The project allows you to mount littlefs directly on a
Linux machine. Can be useful for debugging littlefs if you have an SD card
handy.
SPEC.md
+6 -6
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@@ -46,7 +46,7 @@ Here's the layout of metadata blocks on disk:
| 0x04 | 32 bits | dir size |
| 0x08 | 64 bits | tail pointer |
| 0x10 | size-16 bytes | dir entries |
| 0x00+s | 32 bits | crc |
| 0x00+s | 32 bits | CRC |
**Revision count** - Incremented every update, only the uncorrupted
metadata-block with the most recent revision count contains the valid metadata.
@@ -75,7 +75,7 @@ Here's an example of a simple directory stored on disk:
(32 bits) revision count = 10 (0x0000000a)
(32 bits) dir size = 154 bytes, end of dir (0x0000009a)
(64 bits) tail pointer = 37, 36 (0x00000025, 0x00000024)
(32 bits) crc = 0xc86e3106
(32 bits) CRC = 0xc86e3106
00000000: 0a 00 00 00 9a 00 00 00 25 00 00 00 24 00 00 00 ........%...$...
00000010: 22 08 00 03 05 00 00 00 04 00 00 00 74 65 61 22 "...........tea"
@@ -138,12 +138,12 @@ not include the entry type size, attributes, or name. The full size in bytes
of the entry is 4 + entry length + attribute length + name length.
**Attribute length** - Length of system-specific attributes in bytes. Since
attributes are system specific, there is not much garuntee on the values in
attributes are system specific, there is not much guarantee on the values in
this section, and systems are expected to work even when it is empty. See the
[attributes](#entry-attributes) section for more details.
**Name length** - Length of the entry name. Entry names are stored as utf8,
although most systems will probably only support ascii. Entry names can not
**Name length** - Length of the entry name. Entry names are stored as UTF8,
although most systems will probably only support ASCII. Entry names can not
contain '/' and can not be '.' or '..' as these are a part of the syntax of
filesystem paths.
@@ -222,7 +222,7 @@ Here's an example of a complete superblock:
(32 bits) block count = 1024 blocks (0x00000400)
(32 bits) version = 1.1 (0x00010001)
(8 bytes) magic string = littlefs
(32 bits) crc = 0xc50b74fa
(32 bits) CRC = 0xc50b74fa
00000000: 03 00 00 00 34 00 00 00 03 00 00 00 02 00 00 00 ....4...........
00000010: 2e 14 00 08 03 00 00 00 02 00 00 00 00 02 00 00 ................