ZFS/ZoL: Consequences of increasing zfs_vdev_async_read_min_active (and max)?

Tavin

4/12/23, 3:55 PM

Vdev async read queues have min=1 and max=3 by default. All sync read/write queues are defaulted to min=max=10, while async writes get min=2 and max=10.

Async reads are described as "prefetch reads" in the docs. So I take it that by design, when i/o is heavy, prefetch activity should be kept relatively low. Yet I have seen a lot of recommended configs which raise zfs_vdev_async_read_min_active and the other min_active tunables to the same number A > 10. The corresponding maximums are typically given the same value B, and sometimes B=A.

I understand the general reason for making the i/o queues bigger, but is it a good idea to change the balance between prefetch reads and other i/o?

Perhaps, it's a good idea, only if the workload has a favorable ratio of prefetch hits to misses?

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ewwhite

4/12/23, 4:07 PM

It depends. Are you asking about a specific situation or configuration in mind?

Tavin

4/12/23, 4:17 PM

Well, I've inherited a configuration where all zfs_vdev_(a)sync_(read|write)_(min|max)_active = 16. We've had some performance issues. Before diving into workload testing with different settings, I wanted to understand the theory behind this parameter.

ewwhite

4/12/23, 5:38 PM

What leads you to believe that the performance issues are related to the settings?

Tavin

4/12/23, 5:42 PM

I don't necessarily believe that, I'm just investigating.

ewwhite

4/12/23, 10:09 PM

There isn't enough information here to assist.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: ZFS/ZoL: Consequences of increasing zfs_vdev_async_read_min_active (and max)?

TH: ZFS/ZoL: ผลที่ตามมาของการเพิ่ม zfs_vdev_async_read_min_active (และสูงสุด)?

RO: ZFS/ZoL: Consecințele creșterii zfs_vdev_async_read_min_active (și max)?

RU: ZFS/ZoL: последствия увеличения zfs_vdev_async_read_min_active (и макс.)?

VI: ZFS/ZoL: Hậu quả của việc tăng zfs_vdev_async_read_min_active (và tối đa)?

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