Little Alice grows again - the I/O scheduler dance

• The I/O scheduler decides who goes into the elevator - and when.
• It may do I/O re-ordering - to reduce seek time for the disk device.
• It may do I/O merging (or coalescing) to increase disk bandwidth utilization, based on /sys/block/<device>/queue/ parameters:
  • max_sectors_kb - maximum number of sectors to send in a single request.
  • max_hw_sectors_kb - maximum number of sectors to that the (disk) driver can support in a single request.
  • nr_requests - maximum number of in-flight requests to the device.
• We can use the "iostat" command to see what's going on during I/O.
• For full details: iostat -k 1 -x.
• See the iostat man-page for the details.
• Of note: transaction sizes, queue sizes, average request wait time...

Originally written by guy keren