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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
Showing 976 out of 1137
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 976
SEC. 11.7
INPUT/OUTPUT IN WINDOWS
945
a convenient point for making a clean backup of their persistent state on the vol-
ume. Once all the applications are ready, the system initializes the snapshot of the
volume and then tells the applications that they can continue.
The backup is made
of the volume state at the point of the snapshot.
And the applications were only
blocked for a very short time rather than having to go offline for the duration of the
backup.
Applications participate in the snapshot process, so the backup reflects a state
that is easy to recover in case there is a future failure. Otherwise the backup might
still be useful, but the state it captured would look more like the state if the system
had crashed.
Recovering from a system at the point of a crash can be more dif-
ficult or even impossible, since crashes occur at arbitrary times in the execution of
the application.
Murphy’s Law
says that crashes are most likely to occur at the
worst possible time, that is, when the application data is in a state where recovery
is impossible.
Another aspect of Windows is its support for asynchronous I/O.
It is possible
for a thread to start an I/O operation and then continue executing in parallel with
the I/O.
This feature is especially important on servers. There are various ways
the thread can find out that the I/O has completed.
One is to specify an event ob-
ject at the time the call is made and then wait on it eventually.
Another is to speci-
fy a queue to which a completion event will be posted by the system when the I/O
is done.
A third is to provide a callback procedure that the system calls when the
I/O has completed.
A fourth is to poll a location in memory that the I/O manager
updates when the I/O completes.
The final aspect that we will mention is prioritized I/O.
I/O priority is deter-
mined by the priority of the issuing thread, or it can be explicitly set.
There are
five priorities specified:
critical, high
,
normal
,
low
, and
very low
.
Critical is re-
served for the memory manager to avoid deadlocks that could otherwise occur
when the system experiences extreme memory pressure.
Low and very low priori-
ties are used by background processes, like the disk defragmentation service and
spyware scanners and desktop search, which are attempting to avoid interfering
with normal operations of the system.
Most I/O gets normal priority, but multi-
media applications can mark their I/O as high to avoid glitches.
Multimedia appli-
cations can alternatively use
bandwidth reservation
to request guaranteed band-
width to access time-critical files, like music or video.
The I/O system will pro-
vide the application with the optimal transfer size and the number of outstanding
I/O operations that should be maintained to allow the I/O system to achieve the re-
quested bandwidth guarantee.
11.7.2 Input/Output API Calls
The system call APIs provided by the I/O manager are not very different from
those offered by most other operating systems.
The basic operations are
open
,
read
,
write
,
ioctl
, and
close
, but there are also plug-and-play and power operations,
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