Page 455 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Figure 14.34: The Relation between the File Descriptor Table, the Open File
Description Table, and the I-node Table

However, if an unrelated process opens the file, it gets its own open file description entry, with
its own file position, which is precisely what is needed. Thus the whole point of the open file
description table is to allow a parent and child to share a file position, but to provide unrelated
processes with their own values.
Getting back to the problem of doing the read, we have now shown how the file position and
i-node are located. The i-node contains the disk addresses of the first 12 blocks of the file. If the
file position falls in the first 12 blocks, the block is read and the data are copied to the user. For
files longer than 12 blocks, a field in the i-node contains the disk address of a single indirect
block, as shown in Figure 14.34. This block contains the disk addresses of more disk blocks.
For example, if a block is 1 KB and a disk address is 4 bytes, the single indirect block can hold
256 disk addresses. Thus this scheme works for files of up to 268 KB in total. Beyond that, a
double indirect block is used. It contains the addresses of 256 single indirect blocks, each of
which holds the addresses of 256 data blocks. This mechanism is sufficient to handle files up to
10 ??216 blocks (67,119,104 bytes). If even this is not enough, the i-node has space for a triple
indirect block. Its pointers point to many double indirect blocks. This addressing scheme can
handle file sizes of 224 1 KB blocks (16 GB). For 8 KB block sizes, the addressing scheme can
support file sizes up to 64 TB.
14.6.6 Linux Ext3 File System

In order to prevent data loss after system crashes and power failures, the ext2 file system
would have to write out each data block to disk as soon as it was created. The latency incurred
during the required disk head seek operation would be so high that the performance would
be intolerable. Therefore, writes are delayed, and changes may not be committed to disk for
up to 30 sec, which is a very long time interval in the context of modern computer hardware.
To improve the robustness of the file system, Linux relies on journaling file systems. Ext3, a
follow-on of the ext2 file system, is an example of a journaling file system.

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