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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 338
SEC. 4.4
FILE-SYSTEM MANAGEMENT AND OPTIMIZATION
307
Most people do not think making backups of their files is worth the time and
effort—until one fine day their disk abruptly dies, at which time most of them
undergo a deathbed conversion. Companies, however, (usually) well understand the
value of their data and generally do a backup at least once a day, often to tape.
Modern tapes hold hundreds of gigabytes and cost pennies per gigabyte. Neverthe-
less, making backups is not quite as trivial as it sounds, so we will examine some
of the related issues below.
Backups to tape are generally made to handle one of two potential problems:
1. Recover from disaster.
2. Recover from stupidity.
The first one covers getting the computer running again after a disk crash, fire,
flood, or other natural catastrophe.
In practice, these things do not happen very
often, which is why many people do not bother with backups. These people also
tend not to have fire insurance on their houses for the same reason.
The second reason is that users often accidentally remove files that they later
need again. This problem occurs so often that when a file is ‘‘removed’’ in Win-
dows, it is not deleted at all, but just moved to a special directory, the
recycle bin
,
so it can be fished out and restored easily later. Backups take this principle further
and allow files that were removed days, even weeks, ago to be restored from old
backup tapes.
Making a backup takes a long time and occupies a large amount of space, so
doing it efficiently and conveniently is important. These considerations raise the
following issues.
First, should the entire file system be backed up or only part of
it? At many installations, the executable (binary) programs are kept in a limited
part of the file-system tree.
It is not necessary to back up these files if they can all
be reinstalled from the manufacturer’s Website or the installation DVD. Also,
most systems have a directory for temporary files. There is usually no reason to
back it up either.
In UNIX, all the special files (I/O devices) are kept in a directory
/dev
.
Not only is backing up this directory not necessary, it is downright dangerous
because the backup program would hang forever if it tried to read each of these to
completion. In short, it is usually desirable to back up only specific directories and
everything in them rather than the entire file system.
Second, it is wasteful to back up files that have not changed since the previous
backup, which leads to the idea of
incremental dumps
.
The simplest form of
incremental dumping is to make a complete dump (backup) periodically, say
weekly or monthly, and to make a daily dump of only those files that have been
modified since the last full dump. Even better is to dump only those files that have
changed since they were last dumped. While this scheme minimizes dumping time,
it makes recovery more complicated, because first the most recent full dump has to
be restored, followed by all the incremental dumps in reverse order.
To ease recov-
ery, more sophisticated incremental dumping schemes are often used.
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