FTL(3) FTL(3)
NAME
ftl - flash translation layer
SYNOPSIS
bind -a '#X' /dev
/dev/ftlctl
/dev/ftldata
DESCRIPTION
The flash translation layer device provides the interface
for management of rewritable blocks on a flash memory
device. Flash memory differs from normal disc or memory in
that it is organised in large blocks (erase units), typi-
cally 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 rewrit-
able disc blocks. The translation layer manages the details
of block remapping, copying erase units to reclaim obsolete
physical versions 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 format 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 avail-
able 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
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FTL(3) FTL(3)
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 equivalent 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 erasesize. Make the newly for-
matted device's contents available on ftldata.
init device [ offset [ n [ erasesize ] ] ]
Make available on ftldata the logical blocks (with
existing content) 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 similar properties to ftldata, but which con-
strains 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
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FTL(3) FTL(3)
tracing is done if n is zero. Larger values of n
increase the level of detail.
SOURCE
/os/*/devftl.c
SEE ALSO
flash(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.
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