From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [PATCH] Blktap: Userspace file-based image support. (RFC) Date: Thu, 29 Jun 2006 06:49:31 -0500 Message-ID: <44A3BE4B.8090905@us.ibm.com> References: <1151552130.14360.10.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1151552130.14360.10.camel@localhost.localdomain> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Rusty Russell Cc: Andrew Warfield , Xen Developers , Julian Chesterfield List-Id: xen-devel@lists.xenproject.org Rusty Russell wrote: > On Mon, 2006-06-19 at 09:19 -0700, Andrew Warfield wrote: > >> Attached to this email is a patch containing the (new and improved) >> blktap Linux driver and associated userspace tools for Xen. In >> addition to being more flavourful, containing half the fat, and >> removing stains twice as well as the old driver, this stuff adds a >> userspace block backend and let you use raw (without loopback), qcow, >> and vmdk-based image files for your domUs. There's also a fun little >> driver that provides a shared-memory block device which, in >> combination with OCFS2, represents a cheap-and-cheerful fast shared >> filesystem between multiple domUs. >> > > Hi Andrew, > > I like the idea of block servers in userspace, but I'm curious. When I > wrote the simple share block server I couldn't see an obvious > justification for multiple outstanding requests (with AIO/threads and > all that entails), Are you thinking of posix-aio? posix-aio is "emulated" with threads and normal read/select calls. The performance isn't that great. I believe blktap is using linux-aio which doesn't use threads (it uses the linux specific interface). I've seen a number of benchmarks where linux-aio is significantly better than posix-aio. Regards, Anthony Liguori > so I went for the trivial single request approach. > It seems to me that the backend doesn't have much information the front > end doesn't have. > > Just wondered if you'd tried a naive approach first... > > Thanks! > Rusty. >