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

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Date:	Tue, 16 Sep 2014 11:24:31 -0700
From:	Zach Brown <zab@...bo.net>
To:	"Elliott, Robert (Server Storage)" <Elliott@...com>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Milosz Tanski <milosz@...in.com>,
	"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)

On Mon, Sep 15, 2014 at 10:36:46PM +0000, Elliott, Robert (Server Storage) wrote:
> 
> 
> > -----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/.

It's the old {read,write}x proposals:

http://www.mcs.anl.gov/uploads/cels/papers/TM-302-FINAL.pdf

- z
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