The /proc Filesystem — The Linux Kernel documentation

Description¶ 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. <please add status regarding NFS and CIFS at a later time> 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. If this task truncates some dirty pagecache, some IO which another task has been accounted for (in its write_bytes) will not be happening. We _could_ just subtract that from the truncating task’s write_bytes, but there is information loss in doing that. Note At its current implementation state, this is a bit racy on 32-bit machines: if process A reads process B’s /proc/pid/io while process B is updating one of those 64-bit counters, process A could see an intermediate result. More information about this can be found within the taskstats documentation in Documentation/accounting.

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