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spa: make read/write queues configurable

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We are finding that as customers get larger and faster machines
(hundreds of cores, large NVMe-backed pools) they keep hitting
relatively low performance ceilings. Our profiling work almost always
finds that they're running into bottlenecks on the SPA IO taskqs.
Unfortunately there's often little we can advise at that point, because
there's very few ways to change behaviour without patching.

This commit adds two load-time parameters `zio_taskq_read` and
`zio_taskq_write` that can configure the READ and WRITE IO taskqs
directly.

This achieves two goals: it gives operators (and those that support
them) a way to tune things without requiring a custom build of OpenZFS,
which is often not possible, and it lets us easily try different config
variations in a variety of environments to inform the development of
better defaults for these kind of systems.

Because tuning the IO taskqs really requires a fairly deep understanding
of how IO in ZFS works, and generally isn't needed without a pretty
serious workload and an ability to identify bottlenecks, only minimal
documentation is provided. Its expected that anyone using this is going
to have the source code there as well.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
This commit is contained in:
Rob Norris
2023-10-25 15:11:37 +11:00
committed by Brian Behlendorf
parent d530d5d8a5
commit db2db50e37
2 changed files with 306 additions and 1 deletions
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man/man4/zfs.4
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@@ -2280,6 +2280,16 @@ If
.Sy 0 ,
generate a system-dependent value close to 6 threads per taskq.
.
.It Sy zio_taskq_read Ns = Ns Sy fixed,1,8 null scale null Pq charp
Set the queue and thread configuration for the IO read queues.
This is an advanced debugging parameter.
Don't change this unless you understand what it does.
.
.It Sy zio_taskq_write Ns = Ns Sy batch fixed,1,5 scale fixed,1,5 Pq charp
Set the queue and thread configuration for the IO write queues.
This is an advanced debugging parameter.
Don't change this unless you understand what it does.
.
.It Sy zvol_inhibit_dev Ns = Ns Sy 0 Ns | Ns 1 Pq uint
Do not create zvol device nodes.
This may slightly improve startup time on