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