187 lines
8.3 KiB
ReStructuredText
187 lines
8.3 KiB
ReStructuredText
=====
|
|
NXFFS
|
|
=====
|
|
|
|
This page contains information about the implementation of the NuttX
|
|
wear-leveling FLASH file system, NXFFS.
|
|
|
|
General NXFFS organization
|
|
==========================
|
|
|
|
The following example assumes 4 logical blocks per FLASH erase block. The
|
|
actual relationship is determined by the FLASH geometry reported by the MTD
|
|
driver::
|
|
|
|
ERASE LOGICAL Inodes begin with a inode header. inode may
|
|
BLOCK BLOCK CONTENTS be marked as "deleted," pending re-packing.
|
|
n 4*n --+--------------+
|
|
|BBBBBBBBBBBBBB| Logic block header
|
|
|IIIIIIIIIIIIII| Inodes begin with a inode header
|
|
|DDDDDDDDDDDDDD| Data block containing inode data block
|
|
| (Inode Data) |
|
|
4*n+1 --+--------------+
|
|
|BBBBBBBBBBBBBB| Logic block header
|
|
|DDDDDDDDDDDDDD| Inodes may consist of multiple data blocks
|
|
| (Inode Data) |
|
|
|IIIIIIIIIIIIII| Next inode header
|
|
| | Possibly a few unused bytes at the end of a block
|
|
4*n+2 --+--------------+
|
|
|BBBBBBBBBBBBBB| Logic block header
|
|
|DDDDDDDDDDDDDD|
|
|
| (Inode Data) |
|
|
4*n+3 --+--------------+
|
|
|BBBBBBBBBBBBBB| Logic block header
|
|
|IIIIIIIIIIIIII| Next inode header
|
|
|DDDDDDDDDDDDDD|
|
|
| (Inode Data) |
|
|
n+1 4*(n+1) --+--------------+
|
|
|BBBBBBBBBBBBBB| Logic block header
|
|
| | All FLASH is unused after the end of the final
|
|
| | inode.
|
|
--+--------------+
|
|
|
|
General operation
|
|
=================
|
|
|
|
Inodes are written starting at the beginning of FLASH. As inodes are
|
|
deleted, they are marked as deleted but not removed. As new inodes are
|
|
written, allocations proceed to toward the end of the FLASH -- thus,
|
|
supporting wear leveling by using all FLASH blocks equally.
|
|
|
|
When the FLASH becomes full (no more space at the end of the FLASH), a
|
|
re-packing operation must be performed: All inodes marked deleted are
|
|
finally removed and the remaining inodes are packed at the beginning of
|
|
the FLASH. Allocations then continue at the freed FLASH memory at the
|
|
end of the FLASH.
|
|
|
|
Headers
|
|
=======
|
|
|
|
``BLOCK HEADER``
|
|
The block header is used to determine if the block has every been
|
|
formatted and also indicates bad blocks which should never be used.
|
|
|
|
``INODE HEADER``
|
|
Each inode begins with an inode header that contains, among other things,
|
|
the name of the inode, the offset to the first data block, and the
|
|
length of the inode data.
|
|
|
|
At present, the only kind of inode support is a file. So for now, the
|
|
term file and inode are interchangeable.
|
|
|
|
``INODE DATA HEADER``
|
|
Inode data is enclosed in a data header. For a given inode, there
|
|
is at most one inode data block per logical block. If the inode data
|
|
spans more than one logical block, then the inode data may be enclosed
|
|
in multiple data blocks, one per logical block.
|
|
|
|
NXFFS Limitations
|
|
=================
|
|
|
|
This implementation is very simple as, as a result, has several limitations
|
|
that you should be aware before opting to use NXFFS:
|
|
|
|
1. Since the files are contiguous in FLASH and since allocations always
|
|
proceed toward the end of the FLASH, there can only be one file opened
|
|
for writing at a time. Multiple files may be opened for reading.
|
|
|
|
2. Files may not be increased in size after they have been closed. The
|
|
O_APPEND open flag is not supported.
|
|
|
|
3. Files are always written sequential. Seeking within a file opened for
|
|
writing will not work.
|
|
|
|
4. There are no directories, however, '/' may be used within a file name
|
|
string providing some illusion of directories.
|
|
|
|
5. Files may be opened for reading or for writing, but not both: The O_RDWR
|
|
open flag is not supported.
|
|
|
|
6. The re-packing process occurs only during a write when the free FLASH
|
|
memory at the end of the FLASH is exhausted. Thus, occasionally, file
|
|
writing may take a long time.
|
|
|
|
7. Another limitation is that there can be only a single NXFFS volume
|
|
mounted at any time. This has to do with the fact that we bind to
|
|
an MTD driver (instead of a block driver) and bypass all of the normal
|
|
mount operations.
|
|
|
|
Multiple Writers
|
|
================
|
|
|
|
As mentioned in the limitations above, there can be only one file opened
|
|
for writing at a time. If one thread has a file opened for writing and
|
|
another thread attempts to open a file for writing, then that second
|
|
thread will be blocked and will have to wait for the first thread to
|
|
close the file.
|
|
|
|
Such behavior may or may not be a problem for your application, depending
|
|
(1) how long the first thread keeps the file open for writing and (2) how
|
|
critical the behavior of the second thread is. Note that writing to FLASH
|
|
can always trigger a major FLASH reorganization and, hence, there is no
|
|
way to guarantee the first condition: The first thread may have the file
|
|
open for a long time even if it only intends to write a small amount.
|
|
|
|
Also note that a deadlock condition would occur if the SAME thread
|
|
attempted to open two files for writing. The thread would would be
|
|
blocked waiting for itself to close the first file.
|
|
|
|
ioctls
|
|
======
|
|
|
|
The file system supports to ioctls:
|
|
|
|
``FIOC_REFORMAT``
|
|
Will force the flash to be erased and a fresh, empty NXFFS file system to
|
|
be written on it.
|
|
|
|
``FIOC_OPTIMIZE``
|
|
Will force immediate repacking of the file system. This will avoid the
|
|
delays to repack the file system in the emergency case when all of the
|
|
FLASH memory has been used. Instead, you can defer the garbage collection
|
|
to time when the system is not busy. Calling this function on a thrashing
|
|
file system will increase the amount of wear on the FLASH if you use this
|
|
frequently!
|
|
|
|
Things to Do
|
|
============
|
|
|
|
- The statfs() implementation is minimal. It should have some calculation
|
|
of the f_bfree, f_bavail, f_files, f_ffree return values.
|
|
- There are too many allocs and frees. More structures may need to be
|
|
pre-allocated.
|
|
- The file name is always extracted and held in allocated, variable-length
|
|
memory. The file name is not used during reading and eliminating the
|
|
file name in the entry structure would improve performance.
|
|
- There is a big inefficiency in reading. On each read, the logic searches
|
|
for the read position from the beginning of the file each time. This
|
|
may be necessary whenever an lseek() is done, but not in general. Read
|
|
performance could be improved by keeping FLASH offset and read positional
|
|
information in the read open file structure.
|
|
- Fault tolerance must be improved. We need to be absolutely certain that
|
|
any FLASH errors do not cause the file system to behavior incorrectly.
|
|
- Wear leveling might be improved (?). Files are re-packed at the front
|
|
of FLASH as part of the clean-up operation. However, that means the files
|
|
that are not modified often become fixed in place at the beginning of
|
|
FLASH. This reduces the size of the pool moving files at the end of the
|
|
FLASH. As the file system becomes more filled with fixed files at the
|
|
front of the device, the level of wear on the blocks at the end of the
|
|
FLASH increases.
|
|
- When the time comes to reorganization the FLASH, the system may be
|
|
unavailable for a long time. That is a bad behavior. What is needed,
|
|
I think, is a garbage collection task that runs periodically so that
|
|
when the big reorganization event occurs, most of the work is already
|
|
done. That garbage collection should search for valid blocks that no
|
|
longer contain valid data. It should pre-erase them, put them in
|
|
a good but empty state... all ready for file system re-organization.
|
|
NOTE: There is the FIOC_OPTIMIZE IOCTL command that can be used by an
|
|
application for force garbage collection when the system is not busy.
|
|
If used judiciously by the application, this can eliminate the problem.
|
|
- And worse, when NXFSS reorganization the FLASH a power cycle can
|
|
damage the file system content if it happens at the wrong time.
|
|
- The current design does not permit re-opening of files for write access
|
|
unless the file is truncated to zero length. This effectively prohibits
|
|
implementation of a proper truncate() method which should alter the
|
|
size of a previously written file. There is some fragmentary logic in
|
|
place but even this is conditioned out with __NO_TRUNCATE_SUPPORT__.
|