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Date: Sun, 9 Sep 2012 16:14:01 -0400
From: Rich Felker <>
Subject: Re: capset() capget() syscalls

On Sun, Sep 09, 2012 at 09:04:02PM +0100, Justin Cormack wrote:
> On Sat, Sep 8, 2012 at 11:14 PM, Rich Felker <> wrote:
> > Is there any legitimate user of this horrible API? Last I checked
> > these functions were only useful for block-aligned IO on block devices
> > and the impression I got was that they were designed solely for
> > Oracle's use. (puke) I suppose they could be useful for FUSE drivers
> > too, however. We can add them if they have legitimate software that
> > needs them.
> It is still the only way to get decent io performance out of an SSD as
> far as I can see, being about a factor of 8 faster than anything else
> for random reads eg posix aio. Assuming of course our reads are block
> aligned.

If the normal Linux block cache system is even 25% slower than
low-level IO on the device without any operating system at all, that's
a major bug they need to fix. Adding hideous APIs to for userspace
apps to do their own low-level IO is not a solution. 8x is just
atrocious. Where does that figure come from?

In fairness, if you're comparing to POSIX aio, that's a pretty bad API
too. A thread performing standard blocking pread/pwrite should give
the ideal performance.


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