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