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Modern Operating Systems by Herbert Bos and Andrew...
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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 336
SEC. 4.4
Figure 4-23.
(a) An almost-full block of pointers to free disk blocks in memory
and three blocks of pointers on disk. (b) Result of freeing a three-block file.
(c) An alternative strategy for handling the three free blocks. The shaded entries
represent pointers to free disk blocks.
Since the bitmap is a fixed-size data structure, if the kernel is (partially) paged, the
bitmap can be put in virtual memory and have pages of it paged in as needed.
Disk Quotas
To prevent people from hogging too much disk space, multiuser operating sys-
tems often provide a mechanism for enforcing disk quotas. The idea is that the sys-
tem administrator assigns each user a maximum allotment of files and blocks, and
the operating system makes sure that the users do not exceed their quotas.
A typi-
cal mechanism is described below.
When a user opens a file, the attributes and disk addresses are located and put
into an open-file table in main memory. Among the attributes is an entry telling
who the owner is.
Any increases in the file’s size will be charged to the owner’s
A second table contains the quota record for every user with a currently open
file, even if the file was opened by someone else. This table is shown in Fig. 4-24.
It is an extract from a quota file on disk for the users whose files are currently
open. When all the files are closed, the record is written back to the quota file.
When a new entry is made in the open-file table, a pointer to the owner’s quota
record is entered into it, to make it easy to find the various limits. Every time a
block is added to a file, the total number of blocks charged to the owner is incre-
mented, and a check is made against both the hard and soft limits. The soft limit
may be exceeded, but the hard limit may not.
An attempt to append to a file when
the hard block limit has been reached will result in an error. Analogous checks also
exist for the number of files to prevent a user from hogging all the i-nodes.
When a user attempts to log in, the system examines the quota file to see if the
user has exceeded the soft limit for either number of files or number of disk blocks.

Page 337
Open file table
Quota table
Soft block limit
Hard block limit
Current # of blocks
# Block warnings left
Soft file limit
Hard file limit
Current # of files
# File warnings left
disk addresses
User = 8
Quota pointer
for user 8
Figure 4-24.
Quotas are kept track of on a per-user basis in a quota table.
If either limit has been violated, a warning is displayed, and the count of warnings
remaining is reduced by one.
If the count ever gets to zero, the user has ignored
the warning one time too many, and is not permitted to log in. Getting permission
to log in again will require some discussion with the system administrator.
This method has the property that users may go above their soft limits during a
login session, provided they remove the excess before logging out. The hard limits
may never be exceeded.
4.4.2 File-System Backups
Destruction of a file system is often a far greater disaster than destruction of a
If a computer is destroyed by fire, lightning surges, or a cup of coffee
poured onto the keyboard, it is annoying and will cost money, but generally a re-
placement can be purchased with a minimum of fuss. Inexpensive personal com-
puters can even be replaced within an hour by just going to a computer store (ex-
cept at universities, where issuing a purchase order takes three committees, five
signatures, and 90 days).
If a computer’s file system is irrevocably lost, whether due to hardware or soft-
ware, restoring all the information will be difficult, time consuming, and in many
cases, impossible. For the people whose programs, documents, tax records, cus-
tomer files, databases, marketing plans, or other data are gone forever, the conse-
quences can be catastrophic. While the file system cannot offer any protection
against physical destruction of the equipment and media, it can help protect the
information. It is pretty straightforward: make backups. But that is not quite as
simple as it sounds. Let us take a look.

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