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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 326
SEC. 4.3
FILE-SYSTEM IMPLEMENTATION
295
4.3.6 Journaling File Systems
While log-structured file systems are an interesting idea, they are not widely
used, in part due to their being highly incompatible with existing file systems.
Nevertheless, one of the ideas inherent in them, robustness in the face of failure,
can be easily applied to more conventional file systems. The basic idea here is to
keep a log of what the file system is going to do before it does it, so that if the sys-
tem crashes before it can do its planned work, upon rebooting the system can look
in the log to see what was going on at the time of the crash and finish the job. Such
file systems, called
journaling file systems
, are actually in use. Microsoft’s NTFS
file system and the Linux ext3 and ReiserFS file systems all use journaling. OS X
offers journaling file systems as an option.
Below we will give a brief introduction
to this topic.
To see the nature of the problem, consider a simple garden-variety operation
that happens all the time: removing a file. This operation (in UNIX) requires three
steps:
1.
Remove the file from its directory.
2.
Release the i-node to the pool of free i-nodes.
3.
Return all the disk blocks to the pool of free disk blocks.
In Windows analogous steps are required.
In the absence of system crashes, the
order in which these steps are taken does not matter; in the presence of crashes, it
does. Suppose that the first step is completed and then the system crashes.
The i-
node and file blocks will not be accessible from any file, but will also not be avail-
able for reassignment; they are just off in limbo somewhere, decreasing the avail-
able resources.
If the crash occurs after the second step, only the blocks are lost.
If the order of operations is changed and the i-node is released first, then after
rebooting, the i-node may be reassigned, but the old directory entry will continue
to point to it, hence to the wrong file. If the blocks are released first, then a crash
before the i-node is cleared will mean that a valid directory entry points to an i-
node listing blocks now in the free storage pool and which are likely to be reused
shortly, leading to two or more files randomly sharing the same blocks. None of
these outcomes are good.
What the journaling file system does is first write a log entry listing the three
actions to be completed. The log entry is then written to disk (and for good meas-
ure, possibly read back from the disk to verify that it was, in fact, written cor-
rectly). Only after the log entry has been written, do the various operations begin.
After the operations complete successfully, the log entry is erased.
If the system
now crashes, upon recovery the file system can check the log to see if any opera-
tions were pending.
If so, all of them can be rerun (multiple times in the event of
repeated crashes) until the file is correctly removed.
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