How to Monitor Linux Disk I/O Performance
Linux's storage system I/O stack consists of three parts: file system layer (file system layer) , general block layer (general block layer)** and **device layer ( device layer).
Among them, General block layer is the core of Linux disk I/O. It provides a standard interface upward for accessing block devices of file systems and applications; downward, it abstracts various heterogeneous disk devices into a unified block device and responds to I/O sent by the file system and applications.
In this article, we will look at disk performance metrics and how to view them.
Linux disk performance indicators
When measuring disk performance, we often mention five common metrics: utilization, saturation, IOPS, throughput, and response time. These five indicators are the basic indicators for measuring disk performance.
- Utilization: The percentage of time the disk handles I/O. Excessive utilization (such as over 80%) usually means there is a performance bottleneck in disk I/O.
- Saturation: Refers to how busy the disk is processing I/O. Oversaturation means the disk has a serious performance bottleneck. When saturation is 100%, the disk cannot accept new I/O requests.
- IOPS (Input/Output Per Second) : Refers to the number of I/O requests per second.
- Throughput: The size of I/O requests per second.
- Response time (Response time): Refers to the interval between sending an I/O request and receiving a response.
It should be noted that regarding utilization, we only consider whether there is I/O, not the size of I/O. That is, when utilization is 100%, it is still possible for the disk to accept new I/O requests.
Generally speaking, when selecting a server for an application, you must first benchmark the I/O performance of the disk to accurately evaluate the disk performance and determine whether it can meet the application needs.
Of course, this requires you to use various application scenarios such as random reading, sequential reading, random writing, sequential writing etc. Test the performance of different I/O sizes (usually between 512B ~ 1MB).
Disk I/O Observation
The first thing to observe is the usage of each disk. iostat is the most commonly used disk I/O performance observation tool. It provides a variety of common performance metrics, such as Utilization, IOPS, and Throughput per disk. Of course, these metrics actually come from /proc/diskstats
.
The following is an example of the output of iostat
:
# -d -x means display all disk I/O performance $ iostat -d -x 1 Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util loop0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 loop1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 sda 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 sdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Among the above indicators, what you need to pay attention to is:
-
%util
是我们前面提到的磁盘 I/O 使用情况 -
r/s
和w/s
是 IOPS -
rkB/s
和wkB/s
是吞吐量 -
r_await
和w_await
是响应时间
您可能已经注意到 iostat
不能直接获取磁盘的饱和度。事实上,通常没有什么简单的方法可以测量饱和度。但是,您可以将观察到的平均请求队列长度或完成读写请求的等待时间与基准测试(例如通过 fio)的结果进行比较,以综合评估磁盘饱和度。
进程 I/O 观察
除了每个磁盘的 I/O 情况,每个进程的 I/O 情况也是大家关注的重点。
上面提到的 iostat
只提供了观察磁盘的整体 I/O 性能数据。缺点是无法知道哪些进程正在读写磁盘。要观察进程的 I/O,还可以使用 pidstat
和 iotop
工具。
例如,要使用 pidstat
$ pidstat -d 1 13:39:51 UID PID kB_rd/s kB_wr/s kB_ccwr/s iodelay Command 13:39:52 102 916 0.00 4.00 0.00 0 rsyslogd
从 pidstat
的输出可以看出,它可以实时查看每个进程的 I/O 情况,这包括以下内容:
-
用户 ID (
UID
) 和进程 ID (PID
)。 -
每秒读取的数据大小 (
kB_rd/s
),以 KB 为单位。 -
每秒发出的写请求数据的大小(
kB_wr/s
),单位为KB。 -
每秒取消写入请求的数据大小 (
kB_ccwr/s
),以 KB 为单位。 -
块 I/O 延迟 (
iodelay
),包括等待同步块(synchronized block)I/O 和换入块(swap-in block)I/O 完成的时间,以时钟周期为单位。
除了使用 pidstat
实时查看进程磁盘 I/O 外,还有一个磁盘性能分析的常用方法是根据 I/O 大小对进程进行排序。为此,我推荐 iotop
工具。它是一个类似于 top
的工具,您可以按 I/O 大小对进程进行排序,并找到具有更大 I/O 的进程。
$ iotop Total DISK READ : 0.00 B/s | Total DISK WRITE : 7.85 K/s Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND 15055 be/3 root 0.00 B/s 7.85 K/s 0.00 % 0.00 % systemd-journald
从该输出可以看到,前两行分别代表进程的磁盘读写总大小和磁盘的实际读写总大小。由于缓存、缓冲区、I/O 合并等因素,它们可能不相等。
剩下的部分从各个角度代表了进程的 I/O 情况,包括 线程 ID、I/O 优先级、每秒磁盘读取大小、每秒磁盘写入大小、换入百分比和等待 I/O 时钟百分比。
结论
在本文中,我介绍了 Linux 磁盘 I/O 的性能指标和查看性能工具。我们通常使用 IOPS、吞吐量、利用率、饱和度和响应时间等几个指标来评估磁盘的 I/O 性能。
可以使用 iostat
获取磁盘的 I/O 情况,也可以使用 pidstat
、iotop
等观察进程的 I/O 情况。但在分析这些性能指标时,要注意结合读写比率、I/O 类型、I/O 大小等综合分析。
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