2.FTL for FAT ?
It’s very nice to use FAT and FTL. I know FAT has a index table to mapping
logical and physical area. But there is one problem Where should I put it ?
If you put it in Nand Then this area may ware out, if you put in ram then when
should it be synced to Nand and how often is suitable ?
FAT should work fine with an appropriate FTL layer. The wear-leveling
will assure that all physical FLASH blocks receive the same amount of
wear even thought the usage of logic blocks is not at all even. If the
wear on the allocation tables gets high, the FTL should replace it with
a block that has lower were.
A good NAND FTL layer should also detect any read or write errors and
spare that bad block, replacing it with a free block.
FAT itself keeps backup allocation tables (although I don't think that
the NuttX implementation is capable of using them).
However, I don't think I would recommend FAT even with the appropriate
FTL layer because I would suspect that the FTL overhead for frequent
leveling and sparing of the allocation table might high.
what if my device unexpectedly shutdown and lose all updated index ? Obviously
SD-card or SSD supplier has their solution in their firmware but I don’t have
it.
FAT is better than some file systems in that it only occasionally
trashes the file structure. If files are left open, then buffered,
unwritten data will be lost however.
3.Journal filesystem advantage.
Journal filesystem with some node data in the nand page or in the spare area is
robust, it has no central index table to avoid wear out and safe for unexcepted
shutdown,
Next time you power up, filesystem scan all data node and make a new ram
structure. And it naturally support ware-leveling.
TizenRT uses a very old port of NuttX (although they do bring in fixes
and updates from the NuttX mainline). Samsung has modified the version
of SmartFS that they have to include journaling:
https://github.com/Samsung/TizenRT/tree/master/os/fs/smartfs
This is controlled the selections in the Kconfig file of that directory
(see below).
Now both NuttX and TizenRT share an Apache license, so it should be
possible to bring this journaling feature into NuttX if you are so
inclined. I am not sure about grant-related issues. Apache may have
some addition requirements to make that more difficult.
Greg
config SMARTFS_JOURNALING
bool "Enable filesystem journaling for smartfs"
default n
---help---
Enables logging of write operations on filesystem,
so that in
case of power failure the filesystem can be brought
to a stable
state with minimal data loss.
Enabling journaling will increase the delay in
filesystem
operations, because before doing any fs operation,
journaling
will first log it in journal area. The journal area
will also
take up space in the flash, leaving lesser space
for filesystem.
The space reserved for journal area is proportional
to max number
of file descriptors allowed and sector size
defined. Thus, to
minimize the area reserved for journaling, it is
advised to keep
sector size small.
if SMARTFS_JOURNALING
config SMARTFS_JOURNAL_VERIFY
bool "Verify Journal logging"
default n
---help---
The read & verify joural value after right
Sometimes electronic power grow up after power off, in this
case
FS decide write finished properly, but it doesn't finished
properly.
To prevent this, verifying needed.
On the other hands, it takes more time for most of file
operation that
using journal Logging.
endif
config SMARTFS_SECTOR_RECOVERY
bool "Enable recovery of lost sectors in Filesystem"
depends on MTD_SMART
default n
---help---
Enables recovery of lost sectors after power failure. Lost
sectors are the sectors which are allocated but not reachable
from root directory.
endmenu
