Ivan - FreeBSD read cache - ZFS

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Hello Allan and Benedict,

I was reading FreeBSD mastery: ZFS and in the "Using Log Devices" section
there's a following sentence:

Mirroring read cache doesn't make much sense

And that seemed a bit non-intuitive to me so I was thinking, who better to ask
then resident BSD experts from whom one is the co-author of the above mentioned
book ;-)

I can surely understand that for redundancy sake it most certainly does not make
sense to mirror the read cache, but what about performance? Would system benefit
from having two separate NVMe drives as a read cache vs just one or is
(eventual) performance gain in such cases negligible?

Is it merely a case of "wasting perfectly good NVMe when there are better uses
for it"? But I guess there have to be some edge cases where such ferformance
benefit would be useful.

If this is explained somewhere in the book and I missed it, please do feel free
to  point me to the relevant section.

Kind regards,
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