From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57926) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a9YKT-0007UN-Ev for qemu-devel@nongnu.org; Thu, 17 Dec 2015 08:10:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1a9YKS-0001rE-E5 for qemu-devel@nongnu.org; Thu, 17 Dec 2015 08:10:45 -0500 Received: from mx0.arrikto.com ([2a01:7e00::f03c:91ff:fe6e:d7ab]:59516) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a9YKS-0001qu-7W for qemu-devel@nongnu.org; Thu, 17 Dec 2015 08:10:44 -0500 From: Alex Pyrgiotis References: <1450284917-10508-1-git-send-email-apyrgio@arrikto.com> <5671AA78.8050603@redhat.com> <5672768E.70101@arrikto.com> <56728F08.7070702@redhat.com> Message-ID: <5672B451.1050302@arrikto.com> Date: Thu, 17 Dec 2015 15:10:41 +0200 MIME-Version: 1.0 In-Reply-To: <56728F08.7070702@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 0/9] Add full scatter-gather support for SCSI generic devices List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel@nongnu.org Hi Paolo, On 12/17/2015 12:31 PM, Paolo Bonzini wrote: > > > On 17/12/2015 09:47, Alex Pyrgiotis wrote: >>>> Which commands have large payloads and are on the data path, for >>>> scsi-block? Or is the use case just scsi-generic (e.g. tape devices?)? >>>> >>>> (Just trying to understand before I dive into the patches). >> Sure, no problem. The commands that have large payloads and are on the >> data path are the classic SCSI READ/WRITE commands. Usually, these >> commands are implemented with vectored reads/writes, which utilize the >> controller's scatter-gather list. >> >> However, when opening a "scsi-block" device with the default cache >> policy (cache=writeback), QEMU fallbacks to the "scsi-generic" functions >> (i.e, SG_IO ioctl requests) for reading/writing data [1]. In this case, >> the data are copied in a bounce buffer, which is the issue that this >> patch tackles. > > Right, I forgot about that. However, falling back to scsi-generic > effectively means that scsi-block is always O_DIRECT/cache=none. So why > not just specify cache=none? If I understand correctly, what you're saying is that if "scsi-block" is started with "cache=writeback" and internally uses ioctl()s to bypass the page cache, why not set "cache=none" beforehand and use readv()/writev()? This is a valid suggestion, but this patch does not target only the "scsi-block" device type. Its purpose is to allow faster read/writes via ioctl()s, either to a "scsi-block" device or to a "scsi-generic" device. Note that the latter device type can only use ioctl()s, so it cannot benefit from the readv()/writev() DMA interface and currently has to use a bounce buffer. > We can improve the code to print a warning if you don't. (It needs some > care: iscsi never caches, independent of the cache= argument, so we > don't want to warn for it. But it can be done). I wasn't particularly concerned about that issue. I'd may prefer if this was explicitly addressed in the QEMU doc, under the "cache=" section, but that's a different discussion. Thanks, Alex