确定在Linux系统中读/写的“逻辑”字节数

我想确定所有进程通过syscalls(如read()和write())逻辑读取/写入的字节数.这与从存储层实际获取的字节数(由iotop等工具显示)不同,因为它包含(例如)读取页面缓存的读取,并且在识别写入时也有所不同：逻辑写入IO立即发生发出写入调用时,实际物理IO可能会在一段时间后发生,具体取决于各种因素( Linux通常会缓冲写入并在一段时间后执行物理IO).

我知道如何在每个进程的基础上执行它(例如,参见this question),但不知道如何获得系统范围的计数.

解决方法

如果你想使用/ proc文件系统来计算总计数(而不是每秒计数),这很容易.

这也适用于相当老的内核(在Debian Squeeze 2.6.32内核上测试).

# cat /proc/1979/io
rchar: 111195372883082
wchar: 10424431162257
syscr: 130902776102
syscw: 6236420365
read_bytes: 2839822376960
write_bytes: 803408183296
cancelled_write_bytes: 374812672

对于系统范围,只需对所有进程中的数字求和,但这只能在短期内完成,因为当进程死亡时,它们的统计信息将从内存中删除.您需要启用进程记帐才能保存它们.

这些文件的含义记录在内核源文件Documentation/filesystems/proc.txt中：

rchar – I/O counter: chars read

The number of bytes which this task has caused
to be read from storage. This is simply the sum of bytes which this
process passed to read() and pread(). It includes things like tty IO
and it is unaffected by whether or not actual physical disk IO was
required (the read might have been satisfied from pagecache)

wchar – I/O counter: chars written

The number of bytes which this task has
caused,or shall cause to be written to disk. Similar caveats apply
here as with rchar.

syscr – I/O counter: read syscalls

Attempt to count the number of read I/O
operations,i.e. syscalls like read() and pread().

syscw – I/O counter: write syscalls

Attempt to count the number of write I/O
operations,i.e. syscalls like write() and pwrite().

read_bytes – I/O counter: bytes read

Attempt to count the number of bytes which
this process really did cause to be fetched from the storage layer.
Done at the submit_bio() level,so it is accurate for block-backed
filesystems.

write_bytes – I/O counter: bytes written

Attempt to count the number of bytes which
this process caused to be sent to the storage layer. This is done at
page-dirtying time.

cancelled_write_bytes

The big inaccuracy here is truncate. If a process writes 1MB to a file and then deletes the file,it will in fact perform no writeout. But it will have been accounted as having caused 1MB of write. In other words: The number of bytes which this process caused to not happen,by truncating pagecache. A task can cause “negative” IO too.