Hello everyone, the current throttling code in QEMU allows limiting the I/O rate on block devices. Limits can be set in operations per second (IOPS) or bytes per second, allowing separate limits for read and write operations on both cases.
In its basic usage the user can set a limit of, say, 100 IOPS on a block device passing this option to -drive: throttling.iops-total=100 In addition to that, QEMU can also allow the user to do I/O bursts exceeding that limit up to a configurable rate: throttling.iops-total=100,throttling.iops-total-max=2000 With this, the user can do a burst of 2000 IOPS before they're throttled down to 100 IOPS. Then, after a sufficiently long period of unused I/O they will be able to do a burst again. This patch series introduces the possibility to do bursts for longer period of times. A new setting called throttling.iops-total-max-length is used to define for how long bursts can be sustained. So adding throttling.iops-total-max-length=60 to the previous configuration allows the user to do I/O at a rate of 2000 IOPS for 1 minute before going down to the base rate of 100 IOPS. This is essentially the same as described in this AWS blog post: https://aws.amazon.com/blogs/aws/new-ssd-backed-elastic-block-storage/ As described in the article, a use case for this feature is to allow better performance when booting the OS or restarting a service while keeping the average I/O limits lower the rest of the time. Comments: - There are 6 different settings for setting I/O limits: iops-total, iops-read, iops-write, bps-total, bps-read, bps-write. This series adds one new setting to set the length for each one of those. I don't know if there's a good use case that requires such fine-grained control. It's of course also possible to make it simpler and add just one 'burst-length' setting that would apply for all cases, but the current solution is IMHO simple enough and consistent with the current API, and if we need to extend it later the result is probably going to be ugly. - With this series we set "a maximum of X operations/second for a period of T seconds". If would also be possible to make it "a maximum of X operations/second up to a total of Y operations". It would be equivalent (Y = X * T) but I thought the current proposal makes a more clear API. And I think that's all. Thanks! Berto Alberto Garcia (13): throttle: Make throttle_compute_timer() static throttle: Make throttle_conflicting() set errp throttle: Make throttle_max_is_missing_limit() set errp throttle: Make throttle_is_valid() set errp throttle: Set always an average value when setting a maximum value throttle: Merge all functions that check the configuration into one throttle: Use throttle_config_init() to initialize ThrottleConfig throttle: Add support for burst periods throttle: Add command-line settings to define the burst periods qapi: Add burst length parameters to block_set_io_throttle qapi: Add burst length fields to BlockDeviceInfo throttle: Check that burst_level leaks correctly throttle: Test throttle_compute_wait() during bursts block/qapi.c | 20 ++++++++ blockdev.c | 99 ++++++++++++++++++++++++++---------- hmp.c | 12 +++++ include/qemu/throttle.h | 55 ++++++++++++++------ qapi/block-core.json | 90 ++++++++++++++++++++++++++++----- tests/test-throttle.c | 88 ++++++++++++++++++++++++-------- util/throttle.c | 132 ++++++++++++++++++++++++++++++++---------------- 7 files changed, 377 insertions(+), 119 deletions(-) -- 2.7.0
