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Modern Operating Systems by Herbert Bos and Andrew...
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Page 349
benefit of the doubt and put in sequential-access mode. However, whenever a seek
is done, the bit is cleared.
If sequential reads start happening again, the bit is set
once again. In this way, the file system can make a reasonable guess about wheth-
er it should read ahead or not.
If it gets it wrong once in a while, it is not a disas-
ter, just a little bit of wasted disk bandwidth.
Reducing Disk-Arm Motion
Caching and read ahead are not the only ways to increase file-system perfor-
mance. Another important technique is to reduce the amount of disk-arm motion
by putting blocks that are likely to be accessed in sequence close to each other,
preferably in the same cylinder. When an output file is written, the file system has
to allocate the blocks one at a time, on demand.
If the free blocks are recorded in a
bitmap, and the whole bitmap is in main memory, it is easy enough to choose a free
block as close as possible to the previous block. With a free list, part of which is on
disk, it is much harder to allocate blocks close together.
However, even with a free list, some block clustering can be done. The trick is
to keep track of disk storage not in blocks, but in groups of consecutive blocks. If
all sectors consist of 512 bytes, the system could use 1-KB blocks (2 sectors) but
allocate disk storage in units of 2 blocks (4 sectors).
This is not the same as having
2-KB disk blocks, since the cache would still use 1-KB blocks and disk transfers
would still be 1 KB, but reading a file sequentially on an otherwise idle system
would reduce the number of seeks by a factor of two, considerably improving per-
formance. A variation on the same theme is to take account of rotational posi-
tioning. When allocating blocks, the system attempts to place consecutive blocks
in a file in the same cylinder.
Another performance bottleneck in systems that use i-nodes or anything like
them is that reading even a short file requires two disk accesses: one for the i-node
and one for the block. The usual i-node placement is shown in Fig. 4-29(a). Here
all the i-nodes are near the start of the disk, so the average distance between an i-
node and its blocks will be half the number of cylinders, requiring long seeks.
One easy performance improvement is to put the i-nodes in the middle of the
disk, rather than at the start, thus reducing the average seek between the i-node and
the first block by a factor of two. Another idea, shown in Fig. 4-29(b), is to divide
the disk into cylinder groups, each with its own i-nodes, blocks, and free list
(McKusick et al., 1984).
When creating a new file, any i-node can be chosen, but
an attempt is made to find a block in the same cylinder group as the i-node.
none is available, then a block in a nearby cylinder group is used.
Of course, disk-arm movement and rotation time are relevant only if the disk
has them.
More and more computers come equipped with
solid-state disks
which have no moving parts whatsoever. For these disks, built on the same technol-
ogy as flash cards, random accesses are just as fast as sequential ones and many of
the problems of traditional disks go away. Unfortunately, new problems emerge.

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