即将发布的2.6.18内核中，CFQ (Complete Fair Queuing) 将成为默认的I/O调度器，取代长期以来的默认选项——预判式调度器。CFQ的一个显著特点是支持I/O优先级，允许用户通过ionice工具调整进程的I/O优先级。如果新调度器出现问题，用户可以通过运行时切换或启动参数继续使用预判式调度器。
Judging by this commit, CFQ (Complete Fair Queuing) I/O scheduler will become the default one in the upcoming 2.6.18 kernel. For a long time, anticipatory scheduler has been the default, although even back in late 2004 there was some thinking about replacing it with CFQ. And it seems the time has finally come. CFQ scheduler has been gaining adoption since then, to the point that it's the default I/O scheduler for RHEL 4, Suse, and other distros.
One of the coolest things about CFQ is that it features I/O priorities (since 2.6.13). That means you can set the I/O priority of a process so you can avoid that a process that does too much I/O (daily updatedb) starves the rest of the system, or give extra priority to a process that shouldn't be starved by other processes, by using the ionice tool included in schedutils (since version 1.5.0).
If you find any problems with the new default scheduler, you can still continue using the AS scheduler, by switching to it at runtime (echo anticipatory > /sys/block/
/queue/scheduler
) or by using the elevator=as boot option.
Learn more about Linux I/O schedulers from this great whitepapers:
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