Available data for I/O requests
Meaning of the information produced by blkparse or btt.
- blkparse Reads summary
- The output of blkparse contains a summary of the analyzed data. The "Reads" columns shows the number of read requests processed ("Queued", "Dispatched" and "Completed") together with the total amount of data read with these requests. This replaces the items virt_hba_input and virt_hba_received from the old statistics interfaces.
- blkparse Writes summary
- The same information as for the read requests is also provided for the write requests. he available data shows the number of write requests and the amount of data written.
- Request sizes
- The average size of read and write requests can be obtained from the blkparse summary by dividing the total amount of data by the number of requests. A histogram that shows the request sizes is available from the btt analysis tool.
- Request latencies
- The latencies of requests can be retrieved from the btt analysis tool. The D2C ("dispatched" to "completion") latency tracks the time from the request being issued to the device driver to the time of the request completion.
- Queue depth
- The listing per CPU in the blkparse summary also shows the maximum number of pending read and write requests in the "Read depth" and "Write depth" field.
Note: Only block devices like disks and CD-ROMs
can be traced with blktrace. If the data was captured from a tape
drive, then the data analysis with btt is not available: btt uses
the sector number of each I/O request for mapping the blktrace events
to the originating requests. With tape drives, there are no sector
numbers and the Linux block layer simply passes "0" for the sector
of the blktrace events.