IBM REDP-4285-00 User Manual
Chapter 1. Understanding the Linux operating system
23
Draft Document for Review May 4, 2007 11:35 am
4285ch01.fm
Figure 1-21 Flushing dirty buffers
1.4.3 Block layer
The block layer handles all the activity related to block device operation (refer to Figure 1-18
on page 20). The key data structure in the block layer is the
on page 20). The key data structure in the block layer is the
bio
structure. The bio structure is
an interface between file system layer and block layer.
When a write is performed, file system layer tries to write to page cache which is made up of
block buffers. It makes up a bio structure by putting the contiguous blocks together, then
sends bio to the block layer. (refer to Figure 1-18 on page 20)
block buffers. It makes up a bio structure by putting the contiguous blocks together, then
sends bio to the block layer. (refer to Figure 1-18 on page 20)
The block layer handles the bio request and links these requests into a queue called the I/O
request queue. This linking operation is called
request queue. This linking operation is called
I/O elevator
. In Linux kernel 2.6
implementations, four types of I/O elevator algorithms are available. These are described
below.
below.
Block sizes
The block size, the smallest amount of data that can be read or written to a drive, can have a
direct impact on a server’s performance. As a guideline, if your server is handling many small
files, then a smaller block size will be more efficient. If your server is dedicated to handling
large files, a larger block size may improve performance. Block sizes cannot be changed on
the fly on existing file systems, and only a reformat will modify the current block size.
direct impact on a server’s performance. As a guideline, if your server is handling many small
files, then a smaller block size will be more efficient. If your server is dedicated to handling
large files, a larger block size may improve performance. Block sizes cannot be changed on
the fly on existing file systems, and only a reformat will modify the current block size.
I/O elevator
Apart from a vast amount of other features, the Linux kernel 2.6 employs a new I/O elevator
model. While the Linux kernel 2.4 used a single, general-purpose I/O elevator, kernel 2.6
offers the choice of four elevators. Because the Linux operating system can be used for a
wide range of tasks, both I/O devices and workload characteristics change significantly. A
model. While the Linux kernel 2.4 used a single, general-purpose I/O elevator, kernel 2.6
offers the choice of four elevators. Because the Linux operating system can be used for a
wide range of tasks, both I/O devices and workload characteristics change significantly. A
Process
Cache
Data
Disk
Data
read
Process
Cache
Disk
Data
write
Data
dirty buffer
• Process read a data from disk
The data on memory and the data on disk are identical at this time.
• Process writes a new data
Only the data on memory has been changed, the data on disk and the data on memory is not the identical.
Process
Cache
Disk
Data
flush
• Flushing writes the data on memory to the disk.
The data on disk is now identical to the data on memory.
Data
•pdflush
•sync()
•sync()