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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 324
SEC. 4.3
FILE-SYSTEM IMPLEMENTATION
293
in a directory and its subdirectories onto a tape may make multiple copies of a
linked file. Furthermore, if the tape is then read into another machine, unless the
dump program is clever, the linked file will be copied twice onto the disk, instead
of being linked.
4.3.5 Log-Structured File Systems
Changes in technology are putting pressure on current file systems.
In particu-
lar, CPUs keep getting faster, disks are becoming much bigger and cheaper (but not
much faster), and memories are growing exponentially in size. The one parameter
that is not improving by leaps and bounds is disk seek time (except for solid-state
disks, which have no seek time).
The combination of these factors means that a performance bottleneck is aris-
ing in many file systems. Research done at Berkeley attempted to alleviate this
problem by designing a completely new kind of file system, LFS (the
Log-struc-
tured File System
). In this section we will briefly describe how LFS works. For a
more complete treatment, see the original paper on LFS (Rosenblum and Ouster-
hout, 1991).
The idea that drove the LFS design is that as CPUs get faster and RAM memo-
ries get larger, disk caches are also increasing rapidly. Consequently, it is now pos-
sible to satisfy a very substantial fraction of all read requests directly from the
file-system cache, with no disk access needed.
It follows from this observation
that in the future, most disk accesses will be writes, so the read-ahead mechanism
used in some file systems to fetch blocks before they are needed no longer gains
much performance.
To make matters worse, in most file systems, writes are done in very small
chunks. Small writes are highly inefficient, since a 50-
μ
sec disk write is often pre-
ceded by a 10-msec seek and a 4-msec rotational delay. With these parameters,
disk efficiency drops to a fraction of 1%.
To see where all the small writes come from, consider creating a new file on a
UNIX system.
To write this file, the i-node for the directory, the directory block,
the i-node for the file, and the file itself must all be written. While these writes can
be delayed, doing so exposes the file system to serious consistency problems if a
crash occurs before the writes are done. For this reason, the i-node writes are gen-
erally done immediately.
From this reasoning, the LFS designers decided to reimplement the UNIX file
system in such a way as to achieve the full bandwidth of the disk, even in the face
of a workload consisting in large part of small random writes. The basic idea is to
structure the entire disk as a great big log.
Periodically, and when there is a special need for it, all the pending writes
being buffered in memory are collected into a single segment and written to the
disk as a single contiguous segment at the end of the log.
A single segment may
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