Return-Path: linux-nfs-owner@vger.kernel.org Received: from mx1.redhat.com ([209.132.183.28]:6978 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754058Ab3IZTGo (ORCPT ); Thu, 26 Sep 2013 15:06:44 -0400 Date: Thu, 26 Sep 2013 12:06:11 -0700 From: Zach Brown To: Miklos Szeredi Cc: "J. Bruce Fields" , Anna Schumaker , Kernel Mailing List , Linux-Fsdevel , "linux-nfs@vger.kernel.org" , Trond Myklebust , Bryan Schumaker , "Martin K. Petersen" , Jens Axboe , Mark Fasheh , Joel Becker , Eric Wong Subject: Re: [RFC] extending splice for copy offloading Message-ID: <20130926190611.GP30372@lenny.home.zabbo.net> References: <1378919210-10372-1-git-send-email-zab@redhat.com> <20130925183828.GA30372@lenny.home.zabbo.net> <20130925190620.GB30372@lenny.home.zabbo.net> <20130925195526.GA18971@fieldses.org> <20130925210742.GG30372@lenny.home.zabbo.net> <20130926153359.GE704@fieldses.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Sender: linux-nfs-owner@vger.kernel.org List-ID: On Thu, Sep 26, 2013 at 08:06:41PM +0200, Miklos Szeredi wrote: > On Thu, Sep 26, 2013 at 5:34 PM, J. Bruce Fields wrote: > > On Thu, Sep 26, 2013 at 10:58:05AM +0200, Miklos Szeredi wrote: > >> On Wed, Sep 25, 2013 at 11:07 PM, Zach Brown wrote: > >> >> A client-side copy will be slower, but I guess it does have the > >> >> advantage that the application can track progress to some degree, and > >> >> abort it fairly quickly without leaving the file in a totally undefined > >> >> state--and both might be useful if the copy's not a simple constant-time > >> >> operation. > >> > > >> > I suppose, but can't the app achieve a nice middle ground by copying the > >> > file in smaller syscalls? Avoid bulk data motion back to the client, > >> > but still get notification every, I dunno, few hundred meg? > >> > >> Yes. And if "cp" could just be switched from a read+write syscall > >> pair to a single splice syscall using the same buffer size. > > > > Will the various magic fs-specific copy operations become inefficient > > when the range copied is too small? > > We could treat spice-copy operations just like write operations (can > be buffered, coalesced, synced). > > But I'm not sure it's worth the effort; 99% of the use of this > interface will be copying whole files. And for that perhaps we need a > different API, one which has been discussed some time ago: > asynchronous copyfile() returns immediately with a pollable event > descriptor indicating copy progress, and some way to cancel the copy. > And that can internally rely on ->direct_splice(), with appropriate > algorithms for determine the optimal chunk size. And perhaps we don't. Perhaps we can provide this much simpler data-plane interface that works well enough for most everyone and can avoid going down the async rat hole, yet again. - z