Hardware Reference
In-Depth Information
The result shown informs us that the Raspbian Linux used in this example uses a
block (sector) size of 512 bytes, both physically and logically. This precisely matches the
SD card's sector size. Since the
/boot
file system uses the same physical device as
root
,
this also applies to that partition.
Disk Cache
While we're examining mounted SD card file systems, let's also check the type of device
node used:
$ ls −l /dev/mmcblk0p?
brw−rw−−−T 1 root floppy 179, 1 Dec 31 1969 /dev/mmcblk0p1
brw−rw−−−T 1 root floppy 179, 2 Jan 12 19:33 /dev/mmcblk0p2
$
The example output shows a
b
at the beginning of the
brw-rw—T
field. This tells
us that the disk device is a
block
device as opposed to a
character
device.
(The associated character device would show a
c
instead.) Block devices are important
for file systems because they provide a disk cache capability to vastly improve the file
system performance. The output shows that both the
root
(partition 2) and the
/boot
(partition 1) file systems are mounted using block devices.
Capacities and Performance
SD cards allow a configurable data bus width within limits of the media. All SD cards start
with one data bit line until the capabilities of the memory card are known:
The SD bus allows dynamic configuration of the number of data lines.
After power-up, by default, the SD card will use only DAT0 for data
transfer. After initialization, the host can change the bus width (number
of active data lines). This feature allows [an] easy trade-off between
hardware cost and system performance.
18
After the capabilities of the media are known, the data bus can be expanded under
software control, as supported. Given that SD cards with memory capacities up to 2 GB
operate with a 1-bit data bus, it is highly desirable to use a 4 GB or larger card on the
Raspberry Pi, even if the extra storage is not required. More-advanced cards also offer
greater transfer speeds by use of higher data clock rates.
Table
8-1
summarizes SD card capabilities.
19
