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From: Martijn Heil <m.heil375@gmail.com>
Date: Sun, 16 Jan 2022 16:27:04 +0100
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Subject: [9front] devfs performance
Reply-To: 9front@9front.org
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Hello,

Lately I've been playing around with devfs, doing block interleaving
on two identical USB 3.0 HDD's. I was expecting to have a bit more
throughput compared to using one of those disks, but when I measured
read throughput of the block-interleaved virtual device with tput, the
throughput was exactly the same as measured on a single disk.
The read throughput of a single disk as measured with tput is about 60
MiB/s, so I don't think this would saturate the USB 3.0 SuperSpeed
bus.

Taking a look at /sys/src/9/port/devfs.c, and especially interio() at
/sys/src/9/port/devfs.c:1134,
it seems that the internal read and write operations to the files that
make up the virtual device are done synchronously instead of
asynchronously like I (perhaps naively) expected.
As I understand it, it reads one block from the first file, and only
when that block is done reading it starts reading a block from the
other file, and so on.

I think my interpretation here is correct, but I'm not that familiar
with the kernel source, so is what I'm saying here indeed correct?

It might be worthwhile to optimize this so that it reads
asynchronously from the files that the virtual device is built from.
Is this possible? And how could we achieve this?

Thank you for your time
Martijn Heil
aka
ninjoh