FTL(3)FTL(3)NAMEftl - flash translation layer
SYNOPSIS
bind -a '#X' /dev
/dev/ftlctl
/dev/ftldata
DESCRIPTION
The flash translation layer device provides the interface for manage‐
ment of rewritable blocks on a flash memory device. Flash memory dif‐
fers from normal disc or memory in that it is organised in large blocks
(erase units), typically 64k bytes or more in size, and although writes
can reset bits they cannot set them; instead an entire erase unit must
be erased at once. These properties make it unsuitable for direct use
by a conventional block-oriented file system. The flash translation
layer compensates by implementing a logical to physical mapping that
allows 512-byte blocks to be read or written in the same way as
rewritable disc blocks. The translation layer manages the details of
block remapping, copying erase units to reclaim obsolete physical ver‐
sions of rewritten logical blocks, erase unit load wearing, etc.
The flash translation device serves a one-level directory, giving
access to two files. The control file ftlctl receives commands to for‐
mat a flash device or initialise access to an already formatted device.
Ftldata is the data file, giving access to the logical blocks on the
formatted flash. For example, it can be given to kfs(3) for use as a
file system. The length of the ftldata file as returned by Sys->stat
shows the total logical (formatted) space available for use by the
driver's clients.
The target flash device is identified to this driver by name (eg,
#F/flash) in a control message defined below. The flash device must
have the following properties:
1. It must have a corresponding control file devicectl (eg,
#F/flashctl), which must be writable.
2. The flash control file must accept a command of the form erase
offset which must cause the flash erase unit starting at the
given byte offset to be erased.
3. The device must allow reads and writes of any number of bytes on
arbitrary byte boundaries (file offset). (In other words, the
flash driver must hide alignment restrictions.)
4. A write request must allow previously-written regions to be
updated provided the new data does not change any 0 bit to 1
(ie, writes can clear bits to 0 but will not change any 0 bits
to 1).
The following control messages can be written to ftlctl:
format device [ offset [ n [ erasesize ] ] ]
Erase n bytes of the given flash device starting at the given
byte offset, and format the erased region for use by the flash
translation layer. Omitting the optional parameters is equiva‐
lent to setting them to 0xffffffff. Erasesize is the number of
bytes in the flash device's erase unit; setting to 0xffffffff
takes the value from the underlying device. If offset is
0xffffffff, then the underlying device is searched from the
start for an existing flash translation layer header, and the
remaining parameters are taken from there. If n is 0xffffffff,
then everywhere from offset to the end of the underlying device
is erased. Otherwise, offset and n must be multiples of erase‐
size. Make the newly formatted device's contents available on
ftldata.
init device [ offset [ n [ erasesize ] ] ]
Make available on ftldata the logical blocks (with existing con‐
tent) of a previously-formatted device. The parameters are as
defined for the format command, above.
part name start [ limit ]
Add a partition. This creates a new data file ftlname with simi‐
lar properties to ftldata, but which constrains the range of the
formatted data accessed to begin at start, and end at limit-1,
or the last byte of the formatted data if limit is omitted.
delpart name
Removes a partition.
detach Stop flash translation on the corresponding flash device, and
close it. An error results instead if ftldata is open.
scavenge
Force scavenging of reusable blocks (mainly intended to be used
when testing flash or debugging the driver).
trace n
Trace the actions of the flash translation driver. No tracing
is done if n is zero. Larger values of n increase the level of
detail.
SOURCE
/os/*/devftl.c
SEE ALSOflash(3)BUGS
Reads and writes of ftldata must be multiples of 512 bytes in length
and start on a 512-byte block boundary.
Only one flash device can be active.
FTL(3)
