linux-kernel - Re: [RFC] Disk shock protection (revisited)

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Message-ID: <87f94c370802280900t6e099ae8q7635c275732c9c3e@mail.gmail.com>
Date:	Thu, 28 Feb 2008 12:00:21 -0500
From:	"Greg Freemyer" <greg.freemyer@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Elias Oltmanns" <eo@...ensachen.de>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, "Jens Axboe" <jens.axboe@...cle.com>,
	lmb@...e.de
Subject: Re: [RFC] Disk shock protection (revisited)

On Thu, Feb 28, 2008 at 6:13 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > > That sounds like a non starter. What if the box is busy, what if the
>  > > daemon or something you touch needs memory and causes paging ?
>  >
>  > The daemon runs mlock'd anyway, so there won't be any need for paging
>
>  mlock does not guarantee anything of that form. A syscall by an mlocked
>  process which causes a memory allocation can cause paging of another
>  process on the system.
>
>
>  > there. As for responsiveness under heavy load, I'm not quite sure I get
>  > your meaning. On my system, at least, the only way I have managed to
>  > decrease responsiveness noticeably is to cause a lot of I/O operations
>
>  It depends a lot on hardware but you can certainly get user space delays
>  in seconds as an extreme worst case.

I don't know the details, but I believe the Linux-HA heartbeat daemons
take significant effort to eliminate unexpected delays.  See
http://www.linux-ha.org/

Lars Marowsky-Bree of Novell is extremely involved in the project and
he at least occasionally posts on LKML.  I've cc'ed him.

Greg
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