Linux 如何更改内核 I/O 缓冲区大小
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How to change kernel i/o buffer size
提问by jitihsk
I am running some experiments with I/O intensive applications and am trying to understand the effects of varying the kernel i/o buffer size, different elevator algorithms, and so on.
我正在运行一些 I/O 密集型应用程序的实验，并试图了解改变内核 I/O 缓冲区大小、不同电梯算法等的影响。
How can I know the current size of the i/o buffer in the kernel? Does the kernel use more than one buffer as need arises? How can I change the size of this buffer? Is there a config file somewhere that stores this info?
如何知道内核中 i/o 缓冲区的当前大小？内核是否根据需要使用多个缓冲区？如何更改此缓冲区的大小？是否有存储此信息的配置文件？
(To be clear, I am not talking about processor or disk caches, I am talking about the buffer used by the kernel internally that buffers reads/writes before flushing them out to disk from time to time).
（明确地说，我不是在谈论处理器或磁盘缓存，我在谈论内核内部使用的缓冲区，它在不时将读/写刷新到磁盘之前缓冲它们）。
Thanks in advance.
提前致谢。
采纳答案by Nemo
The kernel does not buffer reads and writes the way you think... It maintains a "page cache" that holds pages from the disk. You do not get to manipulate its size (well, not directly, anyway); the kernel will always use allavailable free memory for the page cache.
内核不会像您想象的那样缓冲读取和写入...它维护一个“页面缓存”，用于保存磁盘中的页面。你不能操纵它的大小（好吧，无论如何不能直接操纵）；内核将始终将所有可用的空闲内存用于页面缓存。
You need to explain what you are really trying to do. If you want some control over how much data the kernel pre-fetches from disk, try a search for "linux readahead". (Hint: blockdev --setra XXX)
你需要解释你真正想要做什么。如果您想控制内核从磁盘预取的数据量，请尝试搜索“linux readahead”。（提示：blockdev --setra XXX）
If you want some control over how long the kernel will hold dirty pages before flushing them to disk, try a search for "linux dirty_ratio".
如果您想在将脏页刷新到磁盘之前控制内核将保留脏页的时间，请尝试搜索“linuxdirty_ratio”。
A specific application can also bypass the page cache completely by using O_DIRECT, and it can exercise some control over it using fsync, sync_file_range, posix_fadvise, and posix_madvise. (O_DIRECTand sync_file_rangeare Linux-specific; the rest are POSIX.)
特定的应用程序还可以绕过页面缓存完全使用O_DIRECT，并且它可以使用实行某种控制权fsync，sync_file_range，posix_fadvise，和posix_madvise。（O_DIRECT并且sync_file_range是特定于 Linux 的；其余的是 POSIX。）
You will be able to ask a better question if you first educate yourself about the Linux VM subsystem, especially the page cache.
如果您首先自学 Linux VM 子系统，尤其是页面缓存，您将能够提出更好的问题。
回答by Zan Lynx
I think you mean the disk IO queues. For example:
我想你的意思是磁盘 IO 队列。例如：
$ cat /sys/block/sda/queue/nr_requests
128
How this queue is used depends on the IO scheduler that is in use.
如何使用此队列取决于正在使用的 IO 调度程序。
$ cat /sys/block/sda/queue/scheduler
noop anticipatory deadline [cfq]
cfqis the most common choice, although on systems with advanced disk controllers and in virtual guest systems noopis also a very good choice.
cfq是最常见的选择，尽管在具有高级磁盘控制器的系统和虚拟来宾系统noop中也是一个非常好的选择。
There is no config file for this information that I am aware of. On systems that I need to change the queue settings on I put the changes into /etc/rc.local although you could use a full-up init script instead and place it into an RPM or DEB for mass distribution to a lot of systems.
我知道没有此信息的配置文件。在我需要更改队列设置的系统上，我将更改放入 /etc/rc.local 中，尽管您可以改用完整的 init 脚本并将其放入 RPM 或 DEB 中，以便向许多系统大规模分发。
