Disk fundamentals

Disk fundamentals
Old edition chapter 14
The virtual layering
• Virtual layering of disk storage system:
1. disk controller firmware – controller chips
or card to map physical disk geometry for
different drive brands and models
2. BIOS – low level functions to read/write
sectors or format tracks
3. The OS API services to open/close files,
set properties, read/write files
Virtual levels of disk access
System bios
Disk controller firmware
Common to all systems
• Physical partitioning of data
• Access to data at the file level
• Map filenames to physical locations
Hardware level
OS level
• OS level view of the disk is in terms of
partitions, directories and files
Assembly access to disk
• Readily available using BIOS under MSDOS for ME, NT, XP, Windows7, etc.
• Store and retrieve data in a special format
(like Hamming or Huffman codes)
• Recover lost data
• Perform diagnostics
• Using NT or XP you must use Win32 API
for disk manipulation…or write device
drivers with high privilege
Tracks, cylinders, sectors
• Disk is made up of multiple platters
• Attached to a spindle which rotates at
constant speed
• Above the surface of each platter is a r/w
head that records magnetic pulses
• The heads move in or out as a group
• See text sketch p 465
Tracks, cylinders, sectors
• Surface of disk is formatted into (invisible) concentric
bands called tracks where data is stored magnetically.
• A disk will have thousands of tracks.
• Moving r/w head from one track to another is called
• (not mentioned) latency is the time it takes a particular
sector to rotate around under the head
• Seek time for a disk is one sort of performance measure
• RPM is another performance measure- usually 7200
• The outermost track is track 0 and numbers increase as
you move toward the center.
Tracks, cylinders, sectors
• All tracks readable from a given r/w head
position together form a cylinder.
• A file would typically be stored on disk using
adjacent cylinders. This reduces seek time.
• A sector is a 512-byte portion of a track
• Physical sectors are magnetically marked at the
factory using low-level formatting. Their size
does not change regardless of the OS used. A
hard disk may have 63 or more sectors per
Photo of hard disk with reflective platter
A platter from a 5.25" hard disk, with 20 concentric tracks drawn
over the surface. Each track is divided into 16 imaginary sectors
Figure 1
Sectors & tracks
• A sector is the basic unit of data storage on a hard disk. The term
"sector" emanates from a mathematical term referring to that pie
shaped angular section of a circle, bounded on two sides by radii
and the third by the perimeter of the circle - See Figure 1. An
explanation in its simplest form, a hard disk is comprised of a group
of predefined sectors that form a circle. That circle of predefined
sectors is defined as a single track. A group of concentric circles
(tracks) define a single surface of a disks platter. Early hard disks
had just a single one-sided platter, while today's hard disks are
comprised of several platters with tracks on both sides, all of which
comprise the entire hard disk capacity. Early hard disks had the
same number of sectors per track location, and in fact, the number
of sectors in each track were fairly standard between models.
Today's advances in drive technology have allowed the number of
sectors per track, or SPT, to vary significantly, but more about that
More about disks
When a hard disk is prepared with its default values, each sector will be able to store 512 bytes of data. Without
elaborating, there are a few operating system disk setup utilities that permit this 512 byte number per sector to be
modified, however 512 is the standard, and found on virtually all hard drives by default. Each sector, however,
actually holds much more than 512 bytes of information. Additional bytes are needed for control structures,
information necessary to manage the drive, locate data and perform other functions. Exact sector structure
depends on the drive manufacturer and model, however the contents of a sector usually include the following
ID Information: Within each sector a small space is left to identify the sector's number and location, which is used
to locate the sector on the disk and provide for status information about the sector itself. For example, a single bit
is used to indicate if the sector has been marked defective and remapped.
Synchronization Fields: These are used internally by the drive controller to guide the read process.
Data: The actual data in the sector.
ECC: Error correcting code used to ensure data integrity.
Gaps: Often referred to as spacers used to separate sector areas and provide time for the controller to process
what it has been read before processing additional data.
Servo Information: In addition to the sectors, each of which contain the items above, space on each track is
allocated for servo information on drives that utilize embedded servo drives. Most, if not all, modern drives not
employ servo technology.
Aside: Zoned Bit Recording
We would be remiss in our discussion of drive sectors, tracks and
performance without mentioning mass improvements such as Zoned Bit
Recording. One of the methods used to increase capacity and data access
speeds on hard disks is by improving the utilization of the larger, outer
tracks of the disk. Early hard disks were extremely primitive, and their
controllers weren't capable of handling complicated arrangements such as
being able to change tracks. As the result of this arrangement, every track
had the same number of sectors, with the standard set at 17 sectors per
As you can see from our sketch above, Figure 1, tracks are concentric
circles, with the ones on the outside of the platter much larger in
circumference than the ones closer to the center. Since there is a constraint
on how tightly the inner circles can be packed with bits, developers packed
them tightly as possible given the state of technology at the time. By
reducing bit density, developers were able to assign the same number of
sectors to the outer circles. Essentially this meant that the inner sectors
were being packed so tightly there was no room for error, and the outer
sectors underutilized, as in theory they could hold many more sectors given
the same linear bit density limitations as were imposed on the inner sectors.
Zoned Bit Recording
Drive developers, in an effort to create larger drive sizes, as well as improve utilization and
performance, developed a technology referred to as zoned bit recording (ZBR). Zoned bit
recording is often referred to as multiple zone recording or just zone recording. With this
technology, tracks are grouped into zones based on their distance from the center of the disk, and
each zone is assigned a number of sectors per track. As you move from the innermost part of the
disk to the outer edge, you move through different zones, each containing more sectors per track
than the one before. This makes more efficient use of the larger tracks on the outside of the disk.
In essence, with ZBR, the size (or length) of a sector remains reasonably constant over the entire
surface of the disk. Stark contrast to very early hard disks that did not employ ZBR, as their tracks
were limited to only 9 sectors regardless of track size.
An interesting added benefit from zoned bit recording is that the raw data transfer rate of the
disk, also referred to as the media transfer rate (a bit of a misnomer), when reading the outside
cylinders is considerably higher than when reading the inside ones. Although the angular velocity
of the platters is constant regardless of which track is being read, the outer cylinders contain more
data. Bear in mind though that angular velocity does not necessarily compensate for the fact that
the outer tracks (periphery of the platter) is moving much faster than the tracks at the core of the
Take note that constant angular velocity is not the case for all drive technologies, such as older
CD-ROM drives.
Since data is written to the outer tracks of a drive first, hence the drive is filled with data from the
outside in. The fastest data transfer occurs when the drive is first used and data retained in the
outer tracks. Many people that perform benchmarks on their systems and their hard drives when
new, then make some tweaks and changes to their system only to return to their benchmarks
weeks or months later only to be unpleasantly surprised that the disk and its benchmarks are
getting slower. Actually, the disk has probably has not changed at all, but the second benchmark
may have been run on tracks closer to the center of the disk. While most people that take
benchmarking seriously defragment their drives before running the tests, fragmentation of the file
system can have impact performance benchmarks.
• Disk storage becomes fragmented over
time just like main memory.
• A fragmented file is not located in
contiguous disk sectors. This slows
access time.
• Translation is the process converting
physical geometry into logical structure
• The drive itself or a card has a controller to
perform this operation.
• The OS works with logical (not physical)
sector numbers.
Logical Block Addressing: aside
Prior to the advent of Logical Block Addressing, all hard drives were
accessed via CHS (Cylinder, Head, Sector) or Extended CHS, which means
that the drive was accessed by specifying its cylinder, head and sector
address. More appropriately, it was referred to as accessing the drive
through its "geometry". Extended CHS was a transition change in the way a
drive was accessed in order to work around the 504 MiB barrier, however,
the addressing was still done in terms of cylinder, head and sector numbers
and then translated one or more times before actually accessing the drive
By contrast, logical block addressing (LBA) involves a completely new
method of addressing sectors. New in that it is new to the EIDE/IDE
interface. LBA was first developed around SCSI hard drives. With LBA,
instead of referring to a drives cylinder, head and sector number geometry
in order to access or "address" it, each sector is assigned a unique "sector
number". In essence, LBA is a means by which a drive is accessed by
linearly addressing sector addresses, beginning at sector 1 of head 0,
cylinder 0 as LBA 0, and proceeding on in sequence to the last physical
sector on the drive, which, for instance, on a standard 540 Meg drive would
be LBA 1,065,456. While this was new it the AT Specification ATA-2, it has
always been the one and only addressing mode in SCSI. AT Attachment
ATA-2 has been subsequently replaced, and the latest AT specification is at
ATA-7. Note also that LBA does not allow you to address more sectors than
CHS style addressing would.
Logical Block Addressing
In order for you to employ LBA support, it must be supported by both the BIOS and the operating system. In
addition, since it is a new method of communicating with the hard drive, the drive itself must support LBA as well.
All newer hard drives do in fact support LBA. Often we review other sites to ensure that we provide you with
accurate information, and with respect to LBA, we came upon a unique, but inaccurate, statement. One purported
authority on computer systems stated that when drives supporting LBA are auto-detected by a BIOS that supports
LBA, it will be set up to use that mode. This is inaccurate and misleading, as there's nothing in the BIOS code that
will set up your drive to use LBA mode. If you have ever used Fdisk, you may recall that during the drive setup
process, you are asked whether you want to enable LBA. Hence, it is a function of the operating system, and
therefore don't expect your BIOS to somehow mysteriously setup your drive.
While it is true that a drive enabled for LBA is not subject to the 504 MiB drive size barrier, there still remains
considerable confusion about Logical Block Address and what it does. Many knowledgeable technicians and users
believe that it is LBA addressing that avoids the 504 MiB barrier, however this is not quite accurate. Logical Block
Addressing isn't getting around the barrier, because it is just another manner in which to address the same
geometry. If you were still limited to 1,024 cylinders, 16 heads and 63 sectors, you would still have logical sectors
beginning with number 0, and progressing sequentially through to 1,032,191, with the 504 MiB still in place. What
does avoid this barrier is that LBA mode automatically enables geometry translation. This translation is required
because the operating system calling the BIOS Int 13h routines knows nothing about LBA. Therefore it is the
translation part of LBA that really gets around the barrier.
When LBA is enabled, the BIOS will enable geometry translation. This translation may be done in the same way
that it is done in Extended CHS or large mode via a drives geometry, or it may be done using a different algorithm
called LBA-assist translation. It is this translated geometry that is presented to the operating system for use in Int
13h calls. Basically, the difference between LBA and ECHS is that when using ECHS the BIOS translates the
parameters used by these calls from the translated geometry to the drive's logical geometry. With LBA, it
translates from the translated geometry directly into a logical block (sector) number.
LBA is currently the dominant form of hard disk addressing. When the 8.4 GB limit of the Int13h interface was
reached in 1998-1999, it became impossible to express the geometry of large hard disks using cylinder, head and
sector numbers, regardless of whether translated or not, while remaining below the Int13h limits of 1,024 cylinders,
256 heads and 63 sectors. This is one of the reasons that today's hard drives no longer indicate their classical
Disk partitioning
• A single harddrive may be partitioned into logical
units named partitions or volumes represented
by a letter, A, B, C, ..
• A partition may be primary or extended and a
drive may contain both types.
• A primary partition is bootable.
• An extended partition may be further divided into
unlimited logical partitions. Each is mapped to a
drive letter and can not be bootable. But each
may be formatted with a different file system.
Multiboot systems
• It is common to create multiple primary
partitions each booting a different OS.
• Mathlab is dual boot
• In industry, you might have primary
partitions for development and production.
• Logical partitions hold data. Different OS
can access the same file systems. Both
Linux and DOS can read FAT32 disks.
FDISK.exe under MS-DOS
• Create and remove partitions
• Does not preserve data
• Later versions (Win2000 and later) have a
disk manager utility
File systems
• Every OS has some disk management
• At the lowest level it manages partitions, at
the next highest, files and dirctories.
• It must keep track of location, size and
attributes for each file.
FAT File-Allocation-Table (see
also later slide)
• Maps logical sectors to clusters (a basic
storage unit)
• Maps files and directories to sequences of
• A cluster is the smallest unit of space used
by a file, consisting of one or more
adjacent disk sectors.
Wikipedia FAT
File Allocation Table (FAT) is a file system developed by Microsoft for MSDOS and was the primary file system for consumer versions of Microsoft
Windows up to and including Windows Me. FAT as it applies to
flexible/floppy and optical disk cartridges (FAT12 and FAT16 without long
file name support) has been standardized as ECMA-107 and ISO/IEC 9293.
The file system is partially patented.
The FAT file system is relatively uncomplicated, and is supported by
virtually all existing operating systems for personal computers. This ubiquity
makes it an ideal format for floppy disks and solid-state memory cards, and
a convenient way of sharing data between disparate operating systems
installed on the same computer (a dual boot environment).
The most common implementations have a serious drawback in that when
files are deleted and new files written to the media, directory fragments tend
to become scattered over the entire media, making reading and writing a
slow process. Defragmentation is one solution to this, but is often a lengthy
process in itself and has to be performed regularly to keep the FAT file
system clean.
Wikipedia NTFS
• NTFS (New Technology File System) is the standard file
system of Windows NT, including its later versions
Windows 2000, Windows XP, Windows Server 2003,
Windows Server 2008, and Windows Vista.[5]
• NTFS replaced Microsoft's previous FAT file system,
used in MS-DOS and early versions of Windows. NTFS
has several improvements over FAT and HPFS (High
Performance File System) such as improved support for
metadata and the use of advanced data structures to
improve performance, reliability, and disk space
utilization plus additional extensions such as security
access control lists and file system journaling. The exact
specification is a trade secret, although (since NTFS
v3.00) it can be licensed commercially from Microsoft
through their Intellectual Property Licensing program.
XP disk management tool
Cluster sizes for 1.25-2gig volume
FAT Type FAT16
Cluster Size 32 kiB
Number of FAT Entries65,526
Size of FAT~ 128 kiB
4 kiB
~ 2 MiB
Clusters used by FAT
• A chain of clusters is referenced by a FAT
that keeps track of all clusters used by a
file. Pictures show cluster chain and
wasted space examples.
1 2 3 4
5 6 7 8
4096 used
4096 used
1000 bytes used
FAT 12
• Still supported by Windows and Linux
• Cluster size is 512 bytes – perfect for
small files
• Each table entry is 12 bits
• A volume holds less than 4087 clusters
FAT 16
• The only system for drives formatted under msdos
• Supported by all versions of windows and linux
• Drawbacks:
– Storage is inefficient on volumes over 1 gig due to
large cluster size
– Each table entry is 16 bits limiting the total number of
clusters that can be accessed
– Volume holds between 4087 and 65,526 clusters
– Boot sector has no backup so a read error can be
– No built in security or individual user permissions
FAT 32
Introduced with OEM2 release of win 95 and later refined
A single file can be up to 4gb (minus 2b)
Each table entry is 32 bits
a volume holds 65,526 up to 268,435,456 clusters
Volume can hold up to 32 gig
Smaller clusters than FAT 16 on volumes 1gb to 8gb resulting in
less waste
• Boot record has a backup of critical information
• Supported under NT, 2000, XP
• Handles large volumes possibly spread over multiple
• For disks>2gig, default cluster is 4kb
• Supports unicode filenames up to 255 chars long
• Permissions
• Built-in encryption
• Change journal can track file revisions
• Disk quotas for individuals or groups of users
• Robust recovery for data error and automatically repairs
• Supports multiple disk mirroring (a mirror is a copy)
ECC and Hamming
• Hamming is a fairly expensive single-error
correction scheme developed by Hamming
at Bell Labs.
• 2-power bits store parity of the other bits
which they correct. So bit 1 is parity for all
the odd bits. Bit 2 is parity for bits 3, 6, 7,
10,11, 14, 15, bit 4 is correction bit for bits
5, 6, 7, 12, 13, 14, 15. Bit 8 corrects 9, 10,
..15, and so on.
Hamming… performance
• To send an 8 bit (ASCII code for example) piece
of data, we will use correcting bits 1, 2, 4, and 8
(4 bits) plus the 8 data bits means we will
“package” 12 bits. Notice this is a 33%
• To send 16 bits of data we would use correction
bits 1,2,4,8 and 16 for a 21-bit package where
overhead has dropped to less than 25%
• We can send up to 247 bits of data using parity
bits 1,2,4,8,16,32,64,and 128 (=8 correction bits)
so the overhead has dropped down to
8/255…smaller than 3%
Hamming…a 12 bit example
• Compute the correcting bits to send 8 bits
of data, like ‘A’ or ‘9’.
• Assume even parity bits.
ECC example
• In bit interleaved parity disk 4 might hold
parity bits for data on the other three disks.
• Bits are read simultaneously off the 4
disks. If data is lost on one of the 3 data
disks it can be recovered from the parity
• For example, if 2 good data bits read (with
X marking lost data) are 1X1 with parity
bit==1 we see lost data (X) must be a 1.
MS DOS boot record
• See text pg 471
• Root directory is the main directory for a
disk volume A directory entry for a file
contains filename, size, attribute and
starting cluster number.
Directory trees
• FAT and NTFS have root directories
containing primary list of files on the disk.
• Subdirectories may be contained in the
Directory trees
Root directory
MS DOS directory structure
• MS-DOS entries are 32 bytes long with
fields shown in table 14-5
MS DOS directory entry
Hex ofs
Start cluster
8-bit bin
16-bit bin
16-bit bin
16-bit bin
32-bit bin
Filename status byte
Status byte
Entry never used
With attr=0fh and status byte 1h, this is the
first entry of a long filename
Entry is for a filename where the file has
been erased
(.) for directory name
First long name entry… with attr=0fh this
marks the end. n=#entries for filename
Attribute field is bit-mapped
subdir Volume label
An entry of 0Fh indicates that the current dir entry is for an extended filename
Date stamp
Year = 0..119 and is added to 1980
Time stamp
MSDOS 32 bit date/time
same as 16 bit, but date is high word of
a double word
Year bits 31-25
Month 24-21
Day 20-16
Hour 15-11
Min 10-5
Sec 4-0
Cluster chain example- just links
are shown
2 3 4 8
9 10 eoc
File starting cluster=1, filesize=7
14 15
Cluster chain example#2- just links
are shown
6 7 11
12 eoc
File starts in cluster 5, size5
14 15
• When a file is create the OS looks for an
available cluster entry in the FAT. Gaps occur if
insufficient contiguous entries are available –
typically as files are deleted & new ones added.
• As files are modified and resaved, their chains
become fragmented.
• As r/w heads jumps between cylinders to locate
all of a file’s clusters, performance degrades.
3 programs
• Previous (5th) edition text contained 3
programs to read sectors, check free
diskspace, and look at clusters.
• But the first two do not run under xp.
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