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Partitions-Mass-Storage-Definitions-Naming-HOWTO
Jean-Daniel Dodin
<jdd@dodin.org>
Revision History
Revision V0.1 2009-05-09 Revised by: jdd
------------------------------------------------------------------------
Table of Contents
1.
2. Copyright and Licence
3. Mass Storage Involved Here
4. Definitions
4.1. Warning
4.2. Bytes
4.3. Sectors
4.4. Heads
4.5. Tracks
4.6. Cylinders
4.7. Disks
4.8. Partitions
4.9. Partition Table
4.10. File Systems
4.11. Files and Nodes
5. Drive Naming in Linux
5.1. Naming Convention
6. Partition Naming in Linux
6.1. Numbers
6.2. Meaning of the Numbers
7. Device Major and Minor Numbers
8. Partition Types
8.1. Linux Partition Types
8.2. Foreign Partition Types
8.3. Swap Partitions
8.4. Complete List
9. How Many Partitions
1.
Partitions-Mass-Storage-Definitions-Naming-HOWTO, copyright (c) 2009
Jean-Daniel Dodin
------------------------------------------------------------------------
2. Copyright and Licence
The copyright of this document is to the author, Jean-Daniel Dodin,
according to the following licence.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.2 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts and
no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".
[http://wiki.tldp.org/LdpWikiDefaultLicence#GNUFreeDocumentationLicense]
GNU Free Documentation License
------------------------------------------------------------------------
3. Mass Storage Involved Here
Mass storage involved in the present HOWTO are rewritable random access
ones. Most of them are magnetic rotating disks (floppies, Hard Drive) or
flash memory (USB key or any kind of memory card).
For example, cdroms and dvds are notconcerned by this HOWTO ( see
Wikipedia). Tapes are not either.
Mass storage are used by the kernel, so the basic doc can be found on
[http://kernel.org/pub/linux/docs/device-list/devices-2.6+.txt] the
kernel Web site
Reference site should be the International Disk drive Equipment and
Materials Association. Shouldbecause this Web site is not very friendly.
------------------------------------------------------------------------
4. Definitions
4.1. Warning
Many definitions about drives are only virtual. That is they are used,
but the hardware is often quite different from the expected description.
Usually this have no odd result, any mass storage have to be seen as a
black box.
------------------------------------------------------------------------
4.2. Bytes
Computers counts with binaries, 1 & 0, 1111100001110... To be able to
read this better, humans uses nibbles (4 bits) often shown as
Hexadecimal numbers from 0 to f (0123456789abcdef). Nibbles are usually
grouped by two and this gives a byte. The most used memory unit is byte
and it's multiples, KiB (Kilo Bytes), MiB (Mega Bytes), GiB (Gigabytes).
The "i" denotes the binary use (0ne Ki is 1024, not 1000), the uppercase
"B" denotes Bytes, not bits.
------------------------------------------------------------------------
4.3. Sectors
Sometime, the word blockis used in place of sectors.
Mass storage devices (at least the ones we are dealing with here) store
bytes in "Sectors" of 512 Bytes. This is uneven, because any sector
count have to be divided by two to have the KiB number, so most
partitonning software accepts letters k (KiB), m (MiB), g... as options.
Wise ones do not make any case difference.
Sector size is the available byte count. The true sector is bigger, as
it have to include housekeeping data. You don't have to worry about
that.
Notice that as of 03-22-2006, the IDEMA annouced a new sector size of
4kiB (4096 Bytes):
[http://www.idema.org/_smartsite/modules/local/data_file/show_file.php?cmd=download&data_file_id=1446]
http://www.idema.org/_smartsite/modules/local/data_file/show_file.php?cmd=download&data_file_id=1446-
doc file, can be openned with OpenOffice.org.
------------------------------------------------------------------------
4.4. Heads
Rotating mass storage devices uses heads. True heads are the physical
electromechanical device that writes and read the magnetic track. Drives
being made of rotating plates, the plates have two sides, so disks can
have two head by plate. Having two plates (frequent) you have four
heads.
Heads are writing through very complex system, see detailed info here:
[http://www.spmtips.com/library/data_storage]
http://www.spmtips.com/library/data_storage.
------------------------------------------------------------------------
4.5. Tracks
Plates are rotating. When a head is still, the plate rotation and the
width of the head are defining a track.
Heads are moving from the external part of the plate to the inner part,
step by steps. Each step defines a new track.
------------------------------------------------------------------------
4.6. Cylinders
Heads are moving together, all at the same time. They may rotate - on
they own center, not the plate center, of course. They also may have a
linear move. You can see an example of linear moving head in any
cdreader, looking at the move of the laser head. Most disks are as shown
by this wikipedia image
[http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Hard_disk_platters_and_head.jpg/200px-Hard_disk_platters_and_head.jpg]
http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Hard_disk_platters_and_head.jpg/200px-Hard_disk_platters_and_head.jpg.
When you think of all the tracks defined by each head at the same time,
you have a cylinder. So on a rotating drive, all the tracks of the same
cylinder are read or written at the same time. The actual data is spread
on all the plates. The way the data is actually written is up to the
drive manufacturer, not the user.
------------------------------------------------------------------------
4.7. Disks
Small disks are used directly as a hole bunch of sectors. Basic programs
can access data directly on sectors. Many do (like dd or any
partitionning programm).
But we live in a world of extremely high capacity mass storage.
Terabytes is normal nowaday (2009), when a complete Linux system can
live on a floppy (1440 bytes). So there is a need of making several
parts from a mass storage device, though the partitions.
------------------------------------------------------------------------
4.8. Partitions
Partitioning is a means to divide a single drive into many logical
drives. A partition is a contiguous set of sectors. To lessen the heads
travel, partitions can be "aligned" on the cylinder size, that is use an
integer number of cylinder. This is not always done, but should as it
have many other advantages for recovery.
------------------------------------------------------------------------
4.9. Partition Table
As you can have many partitions, you need to have a partition table.
This partition table is stored in the very beginning of the drive. It's
very unlikely that you will have to change this table directly writing
bytes with an hexadecimal editor, so we wont say more on the position of
the table.
There are many Operating Systems all around that all share similar
hardware and as many partition systems. We will look only at what one
can find in a PC, even if it's not easy to define that nowaday. Say, for
us, a PC is any computer able to run Linux (I know, it's not always
true).
Each of these partition kinds are noted in the table by a special flag
called "type" ("t" in fdisk). Most known are type 83 for Linux
partitions and 82 for Linux swap (hex numbers).
Notice that most Operating Systems can share partition tables. At least,
if a disk is hardware compatible with several systems, these systems
should be able to see what the others have done, not to erase a drive by
accident. I can't say for sure that its true in the real life.
------------------------------------------------------------------------
4.10. File Systems
Partitions can be accessed directly as sectors, as any part of the disk,
but are usually filled with a file system. File system and partitions
are related only because a file system is in a partition, but that's
all. You can have a disk without partition but with a file system or
have partitions without file system (the swap partition beeing the most
well known). For details on file systems, see Wikipedia.
In summary, file systems allow storing data in files with human readable
names and to sort the files in a friendly way, for example as
directories, subdirectories, text, images...
------------------------------------------------------------------------
4.11. Files and Nodes
Nearly all what you can find on a mass storage partition, beside
sectors, from an user point of view, is a file. But computers are
curious geeks and you can treat files like disks if you want. Using the
"loop" system, default in most Linux kernels, one can partition the
inside of the file, create file systems on it and mount it. This is
specially handy for experiments.
Some of these files are devicesor nodes. Partitions are not files and
are accessed via special nodes we will see later. These nodes are not
created by touch but by mknode. Use with caution. Nodes need a type (
cfor "character" or "b" for block) and major and minor numbers. For what
we need, major numbers are disk numbers and minor numbers are partition
numbers. The list is visible in /proc/partitions
cat /proc/partitions
major minor #blocks name
8 0 488386584 sda
8 1 52436128 sda1
8 2 1 sda2
8 5 2104483 sda5
8 6 20972826 sda6
8 7 52436128 sda7
8 8 360434308 sda8
#mknod b 8 9 /dev/sda9
Creates a /dev/sda9 node of no nuse, given this don't create partition,
only the node. In a usual Linux distribution, nodes are dynamically
created at boot time, so nobody should have to do so. However, sometime
the automatic system fails.
------------------------------------------------------------------------
5. Drive Naming in Linux
There is a special nomenclature that linux uses to refer to mass storage
that must be understood.
------------------------------------------------------------------------
5.1. Naming Convention
Linux used to deal with two kind of drives, depending of the electronic
interface (controller), IDE and SCSI. Oldtimers remember the day where
cdwriters where acccessed through "SCSI emulation". In fact IDE and SCSI
use mostly the same low level commands and for 2007 up, with the new
"SATA" interface, the naming was unified and, in new ditributions, all
the drives have the same naming. For this part, CD or DVD
readers/writers are seen like Hard Drives.
------------------------------------------------------------------------
5.1.1. Old IDE Names
By convention, IDE drives where given device names /dev/hdato /dev/hdd.
Hard Drive A( /dev/hda) is the first drive and Hard Drive C( /dev/hdc)
is the third.
A typical PC has two IDE controllers, each of which can have two drives
connected to it. For example, /dev/hdais the first drive (master) on the
first IDE controller and /dev/hddis the second (slave) drive on the
second controller (the fourth IDE drive in the computer).
So, typically, a computer with IDE controller can accomodate 4 drives:
/dev/hda (primary master), /dev/hdb (primary slave), /dev/hdc (secondary
master), /dev/hdd (secondary slave). Some (rare) Mother Boards have more
than two controllers, some addition cards can also have controllers,
these are numbered following the alphabet, but one have to figure out
what real names are given for his particular hardware.
You can have drives where ever you want, it's not mandatory to fill the
gaps. You may have interest to read about what drive/cdrom connect to
what place, but it's out of this document scope.
------------------------------------------------------------------------
5.1.2. New Hard Drives Names
Now all the rotating hard drives uses the same names as the old SCSI
controllers, that is "s" in place of "h", so /dev/sda, and so on. The
number of drives depends on the number of controllers on the Mother
Board or the extended boards. Usually 4 are available. What will be the
number of a drive is up to the controller card and the way it's read by
the kernel, so difficult to say at first.
------------------------------------------------------------------------
5.1.3. Flash Drives Names
Flash drives are usually not connected through IDE or SATA interfaces
and so don't uses the same names. Several interfaces are used with each
different names. The kernel documentations gives the names.
------------------------------------------------------------------------
5.1.4. Low level Devices and Extra naming
You will find in some apps references to lowlevel SCSI devices and
various naming conventions, for example (wodim is the command line cd
burner):
wodim --scanbus
scsibus1:
1,0,0 100) *
1,1,0 101) 'TSSTcorp' 'CD/DVDW TS-L632D' 'ac00' Removable CD-ROM
1,2,0 102) *
1,3,0 103) *
1,4,0 104) *
1,5,0 105) *
1,6,0 106) *
1,7,0 107) *
And you may have to use some sort of SCSI:1,1,0option to access the
CDROM. try to avoid using this as much as possible, as it's very error
prone and should be let to programmers only. I only mention it because
you can't always avoid it.
If you do "cat /dev/ | more", you can see:
lrwxrwxrwx 1 root root 3 mars 9 07:56 scd0 -> sr0
(...)
crw-r----- 1 root disk 21, 0 mars 9 07:56 sg0
crw-rw----+ 1 root disk 21, 1 mars 9 07:56 sg1
These scd, sr, sg devices are lowlevel interface (notice the "c" for
"character"). Try not using them. dmesgand more /var/log/boot.msgshould
give you the usable sdxx device, like (short summary):
<5>sd 0:0:0:0: [sda] 976773168 512-byte hardware sectors: (500GB/465GiB)
<5>sd 0:0:0:0: [sda] Write Protect is off
<7>sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
This mean the drive is /dev/sda.
However these files (given by dmesgand more /var/log/boot.msg) used to
be easy to read but are no more. Now the kernel starts in parallel
several drivers, so the messages are mixed, you can have
<6> sda:<6>USB Universal Host Contr'ller Interface driver v3.0
This don't mean that your sda drive is an usb one, but the usb module
was started at the same time as the drive one and send it's messages
simultaneously. You still have a /dev/sdadrive.
------------------------------------------------------------------------
5.1.5. New Media Names
Here the dmesg content for inserting an USB key:
scsi7 : SCSI emulation for USB Mass Storage devices
usb 5-3: New USB device found, idVendor=0951, idProduct=160e
usb 5-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 5-3: Product: DataTraveler 2.0
usb 5-3: Manufacturer: Kingston
usb 5-3: SerialNumber: 200706200000000059188185
usb-storage: device found at 9
usb-storage: waiting for device to settle before scanning
scsi 7:0:0:0: Direct-Access Kingston DataTraveler 2.0 1.00 PQ: 0 ANSI: 2
sd 7:0:0:0: [sdb] 3930112 512-byte hardware sectors: (2.01GB/1.87GiB)
sd 7:0:0:0: [sdb] Write Protect is off
sd 7:0:0:0: [sdb] Mode Sense: 23 00 00 00
sd 7:0:0:0: [sdb] Assuming drive cache: write through
sd 7:0:0:0: [sdb] 3930112 512-byte hardware sectors: (2.01GB/1.87GiB)
sd 7:0:0:0: [sdb] Write Protect is off
sd 7:0:0:0: [sdb] Mode Sense: 23 00 00 00
sd 7:0:0:0: [sdb] Assuming drive cache: write through
sdb: sdb1
sd 7:0:0:0: [sdb] Attached SCSI removable disk
sd 7:0:0:0: Attached scsi generic sg2 type 0
usb-storage: device scan complete
You see there all what we where speaking about right now! SCSI
emulation, scsi, sd and sg names, but also the sdb that is most
important for us.
Here are the messages for a high speed SDHC card:
tifm_core: MMC/SD card detected in socket 0:1
mmc1: new SDHC card at address d555
mmcblk0: mmc1:d555 SD04G 3.79GiB
mmcblk0: p1
/dev/mmcblk0p1 on /media/H2SD type vfat (rw,nosuid,nodev,noatime,flush,uid=1000,utf8,shortname=lower)
When the two cards are probably the same flash memory chip, the USB key
uses the USB interface and SCSI emulation, the SDHC card uses the PCMCIA
slot of the laptop, with a special device naming (/dev/mmcblk0). The
use, as far as partitionning is involved is the same.
------------------------------------------------------------------------
5.1.6. Disk ID
In a world where disks are many and removable, it's impossible to track
what device is used by what disk. So there are now many way of using a
disk name. This makes it extremely difficult to work with basic tools.
These are "Disk labels" and "Disk UUID", also "Partition Labels". See
fstab man page for details.
------------------------------------------------------------------------
6. Partition Naming in Linux
6.1. Numbers
Partition naming is thanksfully simpler than drive one. Partitions are
simply given a number from 0 up (decimal). Sometime a "p" is appended on
front of the number:
cat /proc/partitions
major minor #blocks name
8 0 488386584 sda
8 1 52436128 sda1
8 2 1 sda2
8 5 2104483 sda5
8 6 20972826 sda6
8 7 52436128 sda7
8 8 360434308 sda8
179 0 3979776 mmcblk0
179 1 3975680 mmcblk0p1
As you see, partition devices are listed in /proc/partition. This
file... is not a real file but is created on the fly. Don't worry, for
what we need it's a file.
Notice the "p1" partition name for the SDHC card.
Max number of partitions is 15 for SCSI and all the drives using the new
SATA driver, 63 for IDE drives (0 is the full drive, 0 to 15 is four
bits 0 to 64, 6 bits)
------------------------------------------------------------------------
6.2. Meaning of the Numbers
Not all the numbers have the same meaning. This mess come from the PC
history. One can divide floppies with partitions, but then 4 ones seems
sufficient. But then come Hard drives :-). So the partitons numbers 1,
2, 3 and 4 are primarypartitions. One drive can only have 4 primaries.
To go further, we have to use one of these primary as a big one and
sub-partition this one, so to have logicalpartitions. The big
extendedpartition can be any of the 4.
So, remember, the primary partitions are inside the drive and the
logical partitions are inside one of the primary, called the
extendedpartition.
Once the logical partitions are created, it's no more recommended to
write directly to the extended one. Writing to an extended partition
would erase the logical ones like writing directly to a hard drive erase
the partitons. Beware, it's possible!!
If, after creating 4 primary partitions, all the disk space is not used,
the remaining space is lost (unusable), so most of the time, create the
desired primaries, then at last the extended one with all the remaining
room.
It's not necessary to create 4 primaries. You could use only one
extended (Linux only), but there are some advantages of using primaries.
Primaries being 4, the first logical partition is always 5. So any
partition with number of five and up is a logical one.
------------------------------------------------------------------------
7. Device Major and Minor Numbers
The only important thing with a device file are its major and minor
device numbers, which are shown instead of the file size:
$ ls -l /dev/hda
brw-rw---- 1 root disk 8, 0 mars 9 07:56 /dev/sda
Shows permissions ( brw-rw----), owner (root), group (disk), major
device number (8), minor device number (0), date (mars 9 - french, no
year), hour (07:56) and device name (guess :-).
When accessing a device file, the major number selects which device
driver is being called to perform the input/output operation. This call
is being done with the minor number as a parameter and it is entirely up
to the driver how the minor number is being interpreted. The driver
documentation usually describes how the driver uses minor numbers.
------------------------------------------------------------------------
8. Partition Types
8.1. Linux Partition Types
A partition is labeled to host a certain kind of file system (not to be
confused with a volume label. Such a file system could be the linux
standard ext3 file system or linux swap space, or even foreign file
systems like (Microsoft) NTFS or (Sun) UFS. There is a numerical code
associated with each partition type. For example, the code for ext2 is
0x83and linux swap is 0x82(0x mean hexadecimal).
------------------------------------------------------------------------
8.2. Foreign Partition Types
The partition type codes have been arbitrarily chosen (you can't figure
out what they should be) and they are particular to a given operating
system. Therefore, it is theoretically possible that if you use two
operating systems with the same hard drive, the same code might be used
to designate two different partition types. OS/2 marks its partitions
with a 0x07 type and so does Windows NT's NTFS. MS-DOS allocates several
type codes for its various flavors of FAT file systems: 0x01, 0x04 and
0x06 are known. DR-DOS used 0x81 to indicate protected FAT partitions,
creating a type clash with Linux/Minix at that time, but neither
Linux/Minix nor DR-DOS are widely used any more.
------------------------------------------------------------------------
8.3. Swap Partitions
Every process running on your computer is allocated a number of blocks
of RAM. These blocks are called pages. The set of in-memory pages which
will be referenced by the processor in the very near future is called a
"working set." Linux tries to predict these memory accesses (assuming
that recently used pages will be used again in the near future) and
keeps these pages in RAM if possible.
If you have too many processes running on a machine, the kernel will try
to free up RAM by writing pages to disk. This is what swap space is for.
It effectively increases the amount of memory you have available.
However, disk I/O is about a hundred times slower than reading from and
writing to RAM. Consider this emergency memory and not extra memory.
If memory becomes so scarce that the kernel pages out from the working
set of one process in order to page in for another, the machine is said
to be thrashing. Some readers might have inadvertenly experienced this:
the hard drive is grinding away like crazy, but the computer is slow to
the point of being unusable. Swap space is something you need to have,
but it is no substitute for sufficient RAM.
------------------------------------------------------------------------
8.4. Complete List
From the fdisk help:
0 Vide 1e Hidden W95 FAT1 80 Old Minix bf Solaris
1 FAT12 24 NEC DOS 81 Minix / old Lin c1 DRDOS/sec (FAT-
2 XENIX root 39 Plan 9 82 Linux swap / So c4 DRDOS/sec (FAT-
3 XENIX usr 3c PartitionMagic 83 Linux c6 DRDOS/sec (FAT-
4 FAT16 <32M 40 Venix 80286 84 OS/2 hidden C: c7 Syrinx
5 Extended 41 PPC PReP Boot 85 Linux extended da Non-FS data
6 FAT16 42 SFS 86 NTFS volume set db CP/M / CTOS / .
7 HPFS/NTFS 4d QNX4.x 87 NTFS volume set de Dell Utility
8 AIX 4e QNX4.x 2nd part 88 Linux plein tex df BootIt
9 AIX bootable 4f QNX4.x 3rd part 8e Linux LVM e1 DOS access
a OS/2 Boot Manag 50 OnTrack DM 93 Amoeba e3 DOS R/O
b W95 FAT32 51 OnTrack DM6 Aux 94 Amoeba BBT e4 SpeedStor
c W95 FAT32 (LBA) 52 CP/M 9f BSD/OS eb BeOS fs
e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a0 IBM Thinkpad hi ee GPT
f W95 Etendu (LBA 54 OnTrackDM6 a5 FreeBSD ef EFI (FAT-12/16/
10 OPUS 55 EZ-Drive a6 OpenBSD f0 Linux/PA-RISC b
11 Hidden FAT12 56 Golden Bow a7 NeXTSTEP f1 SpeedStor
12 Compaq diagnost 5c Priam Edisk a8 UFS Darwin f4 SpeedStor
14 Hidden FAT16 <3 61 SpeedStor a9 NetBSD f2 DOS secondary
16 Hidden FAT16 63 GNU HURD or Sys ab Amorce Darwin fb VMware VMFS
17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fc VMware VMKCORE
18 AST SmartSleep 65 Novell Netware b8 BSDI swap fd Linux raid auto
1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fe LANstep
1c Hidden W95 FAT3 75 PC/IX be Amorce Solaris ff BBT
------------------------------------------------------------------------
9. How Many Partitions
The exact number of partitions allowed on a drive is fixed by the
kernel. So you can find the exact number is the kernel documentation,
the last version is maintained here
[http://kernel.org/pub/linux/docs/device-list/]
http://kernel.org/pub/linux/docs/device-list/If you have the kernel
source installed, you can find your version on your computer at
/usr/src/linux/Documentation/devices.txt.
Look at "limit on partition". Find yours. Common SATA number is 31, SCSI
is 15, some are less.
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