linux-kernel - Re: raid io requests not parallel?

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Message-ID: <e0e4cb3e0607160938k70819e40g4172f5917045ebf8@mail.gmail.com>
Date:	Sun, 16 Jul 2006 09:38:25 -0700
From:	"Jonathan Baccash" <jbaccash@...il.com>
To:	"Avi Kivity" <avi@...o.co.il>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: raid io requests not parallel?

> Each head has to service 1024 write requests (compared to just 512 read
> requests).

By that logic, it would take twice as long for my writes to finish.
Why is it taking 4x as long in my parallel test? I don't think any of
the repliers understood my question. What I want to know is, during a
single direct IO write request, I would expect the raid software to
issue the write request to both disks simultaneously. But the evidence
indicates that isn't what is happening. I would expect a raid-1 write
to take about as long to write a single block as a single write to a
single disk (assuming no other disk activity), because I would expect
two writes to happen concurrently.
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