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Date:   Sat, 29 Jun 2019 17:39:24 +0100
From:   Luke Kenneth Casson Leighton <>
Subject: Re: bcachefs status update (it's done cooking; let's get this sucker merged)

hey linus, you made news again, all blown up and pointless again.
you're doing great: you're being honest. remember the offer i made to
put you in touch with my friend.

anecdotal story: andrew tridgell worked on the fujitsu sparc
supercomputer a couple decades ago: it had a really weird DMA ring

* memory-to-memory copy (in the same core) was 10mbytes/sec
* DMA memory-to-memory copy (in the same core) was 20mbytes/sec
* memory-memory copy (across the ring bus i.e. to another machine) was
* DMA memory-memory copy (across the ring bus) was *200* mbytes/sec.

when andrew tried asking people, "hey everyone, we need a filesystem
that can work really well on this fast parallel system", he had to
continuously fend off "i got a great idea for in-core memory-to-memory
cacheing!!!!" suggestions, because they *just would never work*.

the point being: caches aren't always "fast".



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