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In-Depth Information
allocation and the smallest unit that can be read/written is a sector, which in UNIX
terminology is called a block. UNIX maintains information about file organization
in certain disk blocks called i-nodes.
A group of physically contiguous clusters from the same file is called an extent.
Ideally, all clusters making up a file will be contiguous on the disk (i.e., the file will
consist of one extent), so as to minimize seek time required to access different
portions of the file. If the disk is nearly full when a file is created, there might not
be an extent available that is large enough to hold the new file. Furthermore, if a file
grows, there might not be free space physically adjacent. Thus, a file might consist
of several extents widely spaced on the disk. The fuller the disk, and the more that
files on the disk change, the worse this file fragmentation (and the resulting seek
time) becomes. File fragmentation leads to a noticeable degradation in performance
as additional seeks are required to access data.
Another type of problem arises when the file's logical record size does not
match the sector size. If the sector size is not a multiple of the record size (or
vice versa), records will not fit evenly within a sector. For example, a sector might
be 2048 bytes long, and a logical record 100 bytes. This leaves room to store
20 records with 48 bytes left over. Either the extra space is wasted, or else records
are allowed to cross sector boundaries. If a record crosses a sector boundary, two
disk accesses might be required to read it. If the space is left empty instead, such
wasted space is called internal fragmentation.
A second example of internal fragmentation occurs at cluster boundaries. Files
whose size is not an even multiple of the cluster size must waste some space at
the end of the last cluster. The worst case will occur when file size modulo cluster
size is one (for example, a file of 4097 bytes and a cluster of 4096 bytes). Thus,
cluster size is a tradeoff between large files processed sequentially (where a large
cluster size is desirable to minimize seeks) and small files (where small clusters are
desirable to minimize wasted storage).
Every disk drive organization requires that some disk space be used to organize
the sectors, clusters, and so forth. The layout of sectors within a track is illustrated
by Figure 8.4. Typical information that must be stored on the disk itself includes
the File Allocation Table, sector headers that contain address marks and informa-
tion about the condition (whether usable or not) for each sector, and gaps between
sectors. The sector header also contains error detection codes to help verify that the
data have not been corrupted. This is why most disk drives have a “nominal” size
that is greater than the actual amount of user data that can be stored on the drive.
The difference is the amount of space required to organize the information on the
disk. Even more space will be lost due to fragmentation.
