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Does mfi write-back cache work for anyone?

I have a number of Dell servers with PERC5 (MegaRAID SAS) controllers.

On each controller on each machine, I get, after booting NetBSD, "mfiX:
critical limit on 'mfiX BBU'".  Disk performance subsequently is exactly
what one would expect from a MegaRAID controller in write-through mode:
great for reads, awful for writes unless one feeds huge numbers of
sequential writes to the card, for example by dd bs=1m onto the raw
device so the kernel keeps splatting out 64K chunks as fast as it can.

I assumed I had bad BBUs but this condition persists even with brand
new replacement batteries from Dell.  And, in fact, having obtained a
known-bad BBU, I now know that the controller BIOS reports this -- and
that it's forcing write-through mode -- at boot time.  Which it does
not with any of my BBUs.

I have to assume our driver is somehow always sensing bad BBU and disabling
(or not enabling) write-back cache.

Does anyone actually have a PERC (or any controller using the mfi driver)
card with BBU that is actually successfully using the write-back cache
under NetBSD?

 Thor Lancelot Simon                                          

        It's very complicated.  It's very cumbersome.  There's a
        lot of numbers involved with it.

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