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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 325
294
FILE SYSTEMS
CHAP. 4
thus contain i-nodes, directory blocks, and data blocks, all mixed together.
At the
start of each segment is a segment summary, telling what can be found in the seg-
ment. If the average segment can be made to be about 1 MB, almost the full band-
width of the disk can be utilized.
In this design, i-nodes still exist and even have the same structure as in UNIX,
but they are now scattered all over the log, instead of being at a fixed position on
the disk. Nevertheless, when an i-node is located, locating the blocks is done in the
usual way.
Of course, finding an i-node is now much harder, since its address can-
not simply be calculated from its i-number, as in UNIX. To make it possible to
find i-nodes, an i-node map, indexed by i-number, is maintained. Entry
i
in this
map points to i-node
i
on the disk.
The map is kept on disk, but it is also cached,
so the most heavily used parts will be in memory most of the time.
To summarize what we have said so far, all writes are initially buffered in
memory, and periodically all the buffered writes are written to the disk in a single
segment, at the end of the log. Opening a file now consists of using the map to
locate the i-node for the file. Once the i-node has been located, the addresses of
the blocks can be found from it. All of the blocks will themselves be in segments,
somewhere in the log.
If disks were infinitely large, the above description would be the entire story.
However, real disks are finite, so eventually the log will occupy the entire disk, at
which time no new segments can be written to the log. Fortunately, many existing
segments may have blocks that are no longer needed. For example, if a file is over-
written, its i-node will now point to the new blocks, but the old ones will still be
occupying space in previously written segments.
To deal with this problem, LFS has a
cleaner
thread that spends its time scan-
ning the log circularly to compact it.
It starts out by reading the summary of the
first segment in the log to see which i-nodes and files are there.
It then checks the
current i-node map to see if the i-nodes are still current and file blocks are still in
use. If not, that information is discarded.
The i-nodes and blocks that are still in
use go into memory to be written out in the next segment. The original segment is
then marked as free, so that the log can use it for new data. In this manner, the
cleaner moves along the log, removing old segments from the back and putting any
live data into memory for rewriting in the next segment. Consequently, the disk is a
big circular buffer, with the writer thread adding new segments to the front and the
cleaner thread removing old ones from the back.
The bookkeeping here is nontrivial, since when a file block is written back to a
new segment, the i-node of the file (somewhere in the log) must be located,
updated, and put into memory to be written out in the next segment. The i-node
map must then be updated to point to the new copy. Nevertheless, it is possible to
do the administration, and the performance results show that all this complexity is
worthwhile. Measurements given in the papers cited above show that LFS outper-
forms UNIX by an order of magnitude on small writes, while having a per-
formance that is as good as or better than UNIX for reads and large writes.
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