linux-kernel - Re: Approaches to making io

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Message-ID: <20110902130835.GB4110@thunk.org>
Date:	Fri, 2 Sep 2011 09:08:35 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Daniel Ehrenberg <dehrenberg@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Approaches to making io_submit not block

On Thu, Sep 01, 2011 at 04:54:15PM +1000, Dave Chinner wrote:
> Apples to oranges - there's orders of magnitude of difference in the
> number of operations that the different stacks do. Allocation in XFS
> when it does not block can still take milliseconds of CPU time; in
> comparison, the networking stack is expected to process thousands of
> packets in that same time frame.  IOWs, the scale of processing per
> item of work is -vastly- different - that's why working in process
> context matters a great deal to the networking stack but not to
> allocation in XFS.

That may be true for hard drives, but PCIe attached flash can support
millions of IOP's per second --- i.e., at least hundreds of IOP's in
milliseconds.  Yes, these devices are expensive, but so are the
thousand-disk RAID arrays that some people attach via XFS.  :-)

There are people in the ext4 development community interested in
looking at such devices.  We've made some improvements, we (and by
this I mean the whole kernel) are a long, long way from supporting
such beasts properly....

							- Ted
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