What’s meaning of await in iostat?
The following is the description provided for await field in iostat man page.
1 2 3 $ man iostat await The average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.
It is a measure of disk I/O latency in milliseconds. The latency is from the front of the I/O scheduler to the I/O completion time.
The I/O path mainly includes the following footprints from block layer to underneath storage device.
- Get the I/O requests from application(filesystem)
- Merge the I/O requests to existing device queue
- Dispatch the I/O requests(by the I/O scheduler) to the device driver
- Hypervisor scheduler in virtualization if any
- Multipathing if any
- Hardware handling
- HBA driver
- FC switch routing if any
- Storage controller queuing, caching and processing
- Actual disk latency
How the await time is calculated?
The await is the average time on a per I/O basis, measured in milliseconds. It mainly includes the time spent in I/O scheduler queue and time spent on storage servicing it if the HBA/SAN latency is relatively marginal.
There are two queues involved in the I/O processing path.
- The queue in I/O scheduler
- The queue in storage side(e.g. controller)
nr_requests limits the maximum number of I/Os in the sorted request queue. The front thread will be blocked if the I/O can’t be merged/inserted into the scheduler queue due to the full occupancy of the queue . Note that the nr_requests is applied to read and write separately.
After the I/O is passed to the driver, it is no longer in the scheduler queue and doesn’t cout to nr_requests limit. However, it will count to avgqu-sz. So, the avgqu-sz could reach the sum of nr_requests and LUN queue_depth.
How the svctm time is measured?
await measures the I/O latency on a per I/O basis while svctm take into account parallel I/O. For example, if 100 I/Os are submitted to the I/O scheduler in parallel and queued onto storage(say queue_depth=50), and the 100 I/Os completes in 10ms, the await time would be 10ms, but the svctm time could be 2ms.
Since await includes the time spent in I/O scheduler and storage queue servicing. We may want to see a breakdown for the two parts by using blktrace. It would tell us the overheads on disk queue(I2D) and actual I/O service latency(D2C). For furhter study of blktrace, you can read this article.