From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Alex Aizman" Subject: RE: [Ksummit-2005-discuss] Summary of 2005 Kernel Summit Proposed Topics Date: Sun, 27 Mar 2005 13:53:54 -0800 Message-ID: <200503272154.j2RLsHp6010460@oss.sgi.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: , , , , , "'David S. Miller'" , To: In-Reply-To: Sender: netdev-bounce@oss.sgi.com Errors-to: netdev-bounce@oss.sgi.com List-Id: netdev.vger.kernel.org Alex Aizman wrote: > > 1) There are iSCSI connections that should be "protected", > resources-wise. Examples: remote swap device, bank accounts > database on RAID accessed via iSCSI, etc. > > 2) There are two ways to protect the "protected" connections. > One "Big Brother" like way is a centralized Resource Manager > that performs a fully deterministic resource accounting > throughout the system, all the way from NIC descriptors and > on-chip memory up to iSCSI buffers for Data-Out headers. > > 3) The 2nd way is *awareness* of the "protected" connections > propagated throughout the system, along with incremental > implementation of more sophisticated recovery schemes. > > 4) The Resource Manager could be used in the following way. > At session open time iSCSI control plane calculates iSCSI and > TCP resources that should be available at all times. The > calculation is done based on: the number of SCSI commands to > be processed in parallel (the 'can_queue'), the maximum size > of the SCSI payload in the SG, the negotiated maximum number > of outstanding R2Ts, sizes of Immediate and FirstBurst data. > > 5) If Resource manager says there is not enough resources, > iSCSI fails session open. This is better than to get in > trouble well into runtime. > > 6) For example: to transmit 'can_queue' commands, iSCSI needs > N skbufs. Let's say, all can_queue commands transmitted in a burst, > and just one of these gets ack-ed by the Target (via > StatSN). In the fully deterministic system this does not > necessarily mean that the scsi-ml can now send one command - > because the full condition involves also recycling of > skbuf(s) used for transmitting this one completed command. > And although it is hard to imagine that the command gets > fully done by the remote target without Tx buffers getting > recycled, the theoretical chance exists (e.g., the NIC is > slow or the driver has a bad Tx recycling implementation), > and the fully deterministic scheme should take it into account. > > 7) Therefore, prior to calling scsi_done() iSCSI asks > Resource Manager whether all the TCP etc. resources used for > this command are already recycled. Just so that it does not look too complicated: there is *no* need to ensure recycling of exactly *the* recources (skbufs, descriptors, on-board memory) that were used to transmit the now-completed command. What's needed is to ensure that the share of resources provisioned at the connection open time for the transmit side of this (protected) connection is back to (can_queue - 1). > If not, the scsi_done() > gets postponed. In addition, iSCSI "complains" to Resource > Manager that it enters slow path because of this, which could > prompt the latter to take an action. (End of the example). > > 8) If we agree to declare some connections > "resource-proteced", it would immediately mean that there are > possibly other connections that are not (resource-protected). > Which in turn gives the Resource Manager a flexibility to > OOM-kill those unprotected connections and cannibalize the > corresponding resources for the protected ones. > > 9) Without some awareness of the resource-protected > connections, and without some kind of resource counting at > runtime (let it be partial and incomplete for starters) - the > only remaining way for customers that require HA (High > Availability) is to over-engineer: use 64GB RAM, TBs of disk > space, etc. Which is probably not the end of the world as > long as the prices go down.. > > Alex