99573cc053
We've seen cases where after creating a ZVOL, the ZVOL device node in "/dev" isn't generated after 20 seconds of waiting, which is the point at which our applications gives up on waiting and reports an error. The workload when this occurs is to "refresh" 400+ ZVOLs roughly at the same time, based on a policy set by the user. This refresh operation will destroy the ZVOL, and re-create it based on a snapshot. When this occurs, we see many hundreds of entries on the "z_zvol" taskq (based on inspection of the /proc/spl/taskq-all file). Many of the entries on the taskq end up in the "zvol_remove_minors_impl" function, and I've measured the latency of that function: Function = zvol_remove_minors_impl msecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 1 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 1 | | 128 -> 255 : 45 |****************************************| 256 -> 511 : 5 |**** | That data is from a 10 second sample, using the BCC "funclatency" tool. As we can see, in this 10 second sample, most calls took 128ms at a minimum. Thus, some basic math tells us that in any 20 second interval, we could only process at most about 150 removals, which is much less than the 400+ that'll occur based on the workload. As a result of this, and since all ZVOL minor operations will go through the single threaded "z_zvol" taskq, the latency for creating a single ZVOL device can be unreasonably large due to other ZVOL activity on the system. In our case, it's large enough to cause the application to generate an error and fail the operation. When profiling the "zvol_remove_minors_impl" function, I saw that most of the time in the function was spent off-cpu, blocked in the function "taskq_wait_outstanding". How this works, is "zvol_remove_minors_impl" will dispatch calls to "zvol_free" using the "system_taskq", and then the "taskq_wait_outstanding" function is used to wait for all of those dispatched calls to occur before "zvol_remove_minors_impl" will return. As far as I can tell, "zvol_remove_minors_impl" doesn't necessarily have to wait for all calls to "zvol_free" to occur before it returns. Thus, this change removes the call to "taskq_wait_oustanding", so that calls to "zvol_free" don't affect the latency of "zvol_remove_minors_impl". Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: John Gallagher <john.gallagher@delphix.com> Signed-off-by: Prakash Surya <prakash.surya@delphix.com> Closes #9380 |
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README.md
ZFS on Linux is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.
Official Resources
Installation
Full documentation for installing ZoL on your favorite Linux distribution can be found at our site.
Contribute & Develop
We have a separate document with contribution guidelines.
Release
ZFS on Linux is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197
Supported Kernels
- The
METAfile contains the officially recognized supported kernel versions.