Книга: Embedded Linux Primer: A Practical, Real-World Approach

9.2. ext2

9.2. ext2

Building on the example of Listing 9-1, we need to format the partitions created with fdisk. To do so, we use the Linux mke2fs utility. mke2fs is similar to the familiar DOS format command. This utility makes a file system of type ext2 on the specified partition. mke2fs is specific to the ext2 file system; other file systems have their own versions of these utilities. Listing 9-2 captures the output of this process.

Listing 9-2. Formatting a Partition Using mke2fs

# mke2fs /dev/sdb1 -L CFlash_Boot_Vol
mke2fs 1.37 (21-Mar-2005)
Filesystem label=CFlash_Boot_Vol
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
2880 inodes, 11504 blocks
575 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=11796480
2 block groups
8192 blocks per group, 8192 fragments per group
1440 inodes per group
Superblock backups stored on blocks:
         8193
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 39 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
#

Listing 9-2 contains a great deal of detail relating to the ext2 file system and provides an excellent way to begin to understand its operational characteristics. Note that this partition was formatted as type ext2 with a volume label of CFlash_Boot_Vol. It was created on a Linux partition (OS Type:) with a block size of 1024 bytes. Space was allocated for 2,880 inodes, occupying 11,504 blocks. An inode is the fundamental data structure representing a single file. For more detailed information about the internal structure of the ext2 file system, the reader is directed to Section 9.11.1 at the end of this chapter.

Looking at the output of mke2fs in Listing 9-2, we can ascertain certain characteristics of how the storage device is organized. We already know that the block size is 1024 bytes. If necessary for your particular application, mke2fs can be instructed to format an ext2 file system with different block sizes. Current implementations allow block sizes of 1,024, 2,048, and 4,096 blocks.

Block size is always a compromise for best performance. On one hand, large block sizes waste more space on disks with many files because each file must fit into an integral number of blocks. Any leftover fragment above block_size * n must occupy another full block, even if only 1 byte. On the other hand, very small block sizes increase the file system overhead of managing the metadata that describes the block-to-file mapping. Benchmark testing on your particular hardware implementation is the only way to be sure you have selected an optimum block size.

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