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    <p>As documented in <a href="http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/block/switching-sched.txt;hb=HEAD" rel="noreferrer"><code>/usr/src/linux/Documentation/block/switching-sched.txt</code></a>, the I/O scheduler on any particular block device can be changed at runtime. There may be some latency as the previous scheduler's requests are all flushed before bringing the new scheduler into use, but it can be changed without problems even while the device is under heavy use.</p> <pre><code># cat /sys/block/hda/queue/scheduler noop deadline [cfq] # echo anticipatory &gt; /sys/block/hda/queue/scheduler # cat /sys/block/hda/queue/scheduler noop [deadline] cfq </code></pre> <p>Ideally, there would be a single scheduler to satisfy all needs. It doesn't seem to exist yet. The kernel often doesn't have enough knowledge to choose the best scheduler for your workload:</p> <ul> <li><code>noop</code> is often the best choice for memory-backed block devices (e.g. ramdisks) and other non-rotational media (flash) where trying to reschedule I/O is a waste of resources</li> <li><code>deadline</code> is a lightweight scheduler which tries to put a hard limit on latency</li> <li><code>cfq</code> tries to maintain system-wide fairness of I/O bandwidth</li> </ul> <p>The default was <code>anticipatory</code> for a long time, and it received a lot of tuning, but was removed in <strong>2.6.33</strong> (early 2010). <code>cfq</code> became the default some while ago, as its performance is reasonable and fairness is a good goal for multi-user systems (and even single-user desktops). For some scenarios -- databases are often used as examples, as they tend to already have their own peculiar scheduling and access patterns, and are often the <em>most</em> important service (so who cares about fairness?) -- <code>anticipatory</code> has a long history of being tunable for best performance on these workloads, and <code>deadline</code> very quickly passes all requests through to the underlying device.</p>
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    1. COGreat info, thanks! But my basic question still is unanswered, if I plug in a flash drive or my netbook runs off a flash disk as it's main drive is the kernel smart enough to pick noop instead of the default cfq? Or is it completely up to me to do it manually?
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    2. COYou can configure the kernel to use a different scheduler by default. It would be clever to automatically use `noop` on non-rotational media, but the kernel doesn't have that functionality. It kind of does have detection of non-rotational media, but it's not reliable as some disks misreport themselves, and it's not yet wired up to the I/O scheduler code anyhow.
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