2 - Raspberry PI Community Projects

Configuration of the bootloader must identify the different hard drives and their partitions.
Linux uses a special filesystem (in “block” mode) stored in the /dev/ directory, for this purpose.
Historically, /dev/hda was the master disk on the first IDE controller, and /dev/hdb its first
slave, /dev/hdc and /dev/hdd being, respectively, the master and slave disks on the second IDE
controller, and so on down for any others. /dev/sda corresponded to the first SCSI drive, /dev/
sdb being the second, etc. This naming scheme has been unified with the Linux kernel present
in Squeeze, and all hard drives (IDE/PATA, SATA, SCSI, USB, IEEE 1394) are now represented by
/dev/sd*.
Each partition is represented by its number on the disk on which it resides: for instance, /dev/
sda1 is the first partition on the first disk, and /dev/sdb3 is the third partition on the second
disk.
The PC architecture (or “i386”) is limited to four “primary” partitions per disk. To go beyond
this limitation, one of them must be created as an “extended” partition, and it can then contain
additional “secondary” partitions. These secondary partitions must be numbered from 5. Thus
the first secondary partition could be /dev/sda5, followed by /dev/sda6, etc.
It is not always easy to remember what disk is connected to which SATA controller, or in third
position in the SCSI chain, especially since the naming of hotplugged hard drives (which includes
among others most SATA disks and external disks) can change from one boot to another.
Fortunately, udev creates, in addition to /dev/sd*, symbolic links with a fixed name, which you
could then use if you wished to identify a hard drive in a non-ambiguous manner. These symbolic
links are stored in /dev/disk/by-id. On a machine with two physical disks, for example,
one could find the following:
mirexpress:/dev/disk/by-id# ls -l
total 0
lrwxrwxrwx 1 root root 9 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP -> ../../
➥ sda
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP-part1 ->
➥ ../../sda1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP-part2 ->
➥ ../../sda2
[...]
lrwxrwxrwx 1 root root 9 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-
➥ WCAT00241697 -> ../../sdb
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-
➥ WCAT00241697-part1 -> ../../sdb1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-
➥ WCAT00241697-part2 -> ../../sdb2
[...]
lrwxrwxrwx 1 root root 9 23 jul. 08:58 scsi-SATA_STM3500418AS_9VM3L3KP ->
➥ ../../sda
lrwxrwxrwx 1 root root 10 23 jul. 08:58 scsi-SATA_STM3500418AS_9VM3L3KP-part1
➥ -> ../../sda1
162 The Debian Administrator's Handbook