From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ravi Shankar Subject: Re: mpt2sas: /sysfs sas_address entries do not show individual port sas addresses. Date: Tue, 23 Aug 2011 12:39:28 -0700 Message-ID: <4E5401F0.7030303@oracle.com> References: <4E4B9B04.10302@mpstor.com> <4E4BDFD7.3060109@interlog.com> <4E4D529F.9090908@oracle.com> <4E4D6D78.6040106@interlog.com> <4E4E5780.5000705@mpstor.com> <4E4EB442.20903@oracle.com> <4E5059F5.30908@interlog.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from rcsinet15.oracle.com ([148.87.113.117]:59516 "EHLO rcsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755102Ab1HWTjo (ORCPT ); Tue, 23 Aug 2011 15:39:44 -0400 In-Reply-To: <4E5059F5.30908@interlog.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: dgilbert@interlog.com Cc: Benjamin ESTRABAUD , "linux-scsi@vger.kernel.org" > > spl2r02.pdf section 6.18.2 [link layer, SSP, Full duplex]: > "SSP is a full duplex protocol. An SSP phy may receive > an SSP frame or primitive in a connection while it is > transmitting an SSP frame or primitive in the same > connection. A wide SSP port may send and/or receive > SSP frames or primitives concurrently on different > connections (i.e., on different phys)." > > For a SCSI command like READ(10) a connection consumes > one initiator phy and one target phy plus the pathway > between them until it is closed. Typically a READ > would have two connections: one to send the CDB and a > second connection later to return the data and response > (SCSI status and possibly sense data). For a spinning > disk there could be milliseconds between those two > connections; with an SSD less (do they use only one > connection?). > > Due to the full duplex nature of a connection, DATA > frames associated with a WRITE could overlap with DATA > frames associated with an READ CDB sent earlier. > > In SAS-2, a single READ's maximum data rate is 6 Gbps. > If a 2-phy wide link is available (along the whole pathway > (see Figure 129 in spl2r02.pdf)) then two READs, sent one > after the other or concurrently, could have their DATA > frames returned concurrently. So the combined maximum > data rate of the two READs would be 12 Gbps. > > Expanders don't change what is stated above. Pathways > become an interconnection of links. A small latency is > added to the opening of connections. And there is the > possibility that no links are available to establish a > connection (e.g. target to expander has available link(s) > but all expander to initiator links are occupied). > >> Wondering has anyone measured performance under such scenario ?. It >> would be >> great to see Expanders terminating SSP frames to over come >> some of above limitation. Links between HBA and Expander and Expander >> to Disk >> can be still Class 1. > > Not sure I follow. Expanders come into play when > connections are being established. > If you've single Expander with multiple initiators and disks then there are no issue unless multiple initiators/targets trying to access to same resources. There is no QoS (other than Path way count and Disconnect/Reconnect mode page) enforced by the expander. Once you start daisy chaining expanders pathway between expanders will be bottle neck due to connection oriented nature of SAS protocol.