linux-kernel - RE: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

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Date:	Mon, 15 Sep 2014 22:36:46 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Andreas Dilger <adilger@...ger.ca>,
	Milosz Tanski <milosz@...in.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Christoph Hellwig" <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-aio@...ck.org" <linux-aio@...ck.org>,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>
Subject: RE: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)



> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
> owner@...r.kernel.org] On Behalf Of Andreas Dilger
> Sent: Monday, 15 September, 2014 4:34 PM
> To: Milosz Tanski
> Cc: linux-kernel@...r.kernel.org; Christoph Hellwig; linux-
> fsdevel@...r.kernel.org; linux-aio@...ck.org; Mel Gorman; Volker Lendecke;
> Tejun Heo; Jeff Moyer
> Subject: Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)
> 
> On Sep 15, 2014, at 2:20 PM, Milosz Tanski <milosz@...in.com> wrote:
> 
> > This patcheset introduces an ability to perform a non-blocking read
> > from regular files in buffered IO mode. This works by only for those
> > filesystems that have data in the page cache.
> >
> > It does this by introducing new syscalls new syscalls readv2/writev2
> > and preadv2/pwritev2. These new syscalls behave like the network sendmsg,
> > recvmsg syscalls that accept an extra flag argument (O_NONBLOCK).
> 
> It's too bad that we are introducing yet another new read/write
> syscall pair that only allow IO into discontiguous memory regions,
> but do not allow a single call to access discontiguous file regions
> (i.e. specify a separate file offset for each iov).
> 
> Adding syscalls similar to preadv/pwritev() that could take a iovec
> that specified the file offset+length in addition to the memory address
> would allow efficient scatter-gather IO in a single syscall.  While
> that is less critical for local filesystems with small syscall latency,
> it is more important for network filesystems, or in the case of
> NVRAM-backed filesystems.
> 
> Cheers, Andreas

That sounds like the proposed WRITE SCATTERED/READ GATHERED 
commands for SCSI (where are related to, but not necessarily
tied to, atomic writes).  We discussed them a bit at 
LSF-MM 2013 - see http://lwn.net/Articles/548116/.


---
Rob Elliott    HP Server Storage





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