|
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI: MaxBurstSizeI believe, we need to consider this as issue with devices that can be both initiators and targets. Take for example a streaming tape; when it is reading the data from a Disk Storage Controller. Often these device start a long I/O streaming action and have less memory then will contain the data they are requesting. Through various double buffering techniques, they can keep the data coming in, and going out to tape, without back-hitching. So I believe MaxBurstSize has value, even if the disk storage folks think it is minimal. . . . John L. Hufferd Senior Technical Staff Member (STSM) IBM/SSG San Jose Ca Main Office (408) 256-0403, Tie: 276-0403, eFax: (408) 904-4688 Home Office (408) 997-6136, Cell: (408) 499-9702 Internet address: hufferd@us.ibm.com Santosh Rao <santoshr@cup.hp.com>@ece.cmu.edu on 02/21/2002 01:35:49 PM Sent by: owner-ips@ece.cmu.edu To: ips@ece.cmu.edu cc: Subject: Re: iSCSI: MaxBurstSize Pat, The MaxBurstSize login key is a don't care for iscsi initiators. It serves mostly an informational purpose, letting the initiator know at login time as to what is the maximum amount of data it can expect the target to request in 1 R2T request, or the maximum amount of data it can receive from the target in 1 data-in sequence. Most initiators should be negotiating this to the maximum possible value, thereby, indicating to the target to just return the maximum value it supports for MaxBurstSize. The MaxBurstSize itself is inherited as one of the tunables from the scsi Disconnect-Reconnect Mode Page as defined in SPC-2, which is to be re-defined for each SCSI transport as appropriate. IMO, the "MAXIMUM BURST SIZE" tunable of the disconnect-reconnect mode page makes sense in the parallel scsi context alone, where, either side (initiator or target) can specify how long the scsi bus can remain in the data phase before a disconnect must be done. In a fabric based SCSI transport, where, the data transfer does not cause any bus to be tied down, and the initiator's data buffers are allocated up-front before the SCSI command is sent to the target, I don't see any practical use of this key for initiators other than being informational, and allowing them to learn of the target's limits up-front at login time. I don't see why iSCSI needs to define this key, if its informational utility is of little or no value. I am also curious about what value FCP-2 initiators would find in this tunable. (?) Regards, Santosh "THALER,PAT (A-Roseville,ex1)" wrote: > > Julian, > > I still don't see the rational for MaxBurstSize. > > The initiator doesn't need it to limit its required resources. The initiator > has to have its buffers allocated when it issues a command. It has to > control the demand on its resources by limiting the commands it issues based > upon its available resources. > > The target has FirstBurstSize and R2T's to control the demand on its > resources. > > The other use of MaxBurstSize is to limit how often target may set the A > bit, but the utility of this is unclear. > > Regards, > Pat Thaler > > ---Original message---- > From: Julian Satran [mailto:Julian_Satran@il.ibm.com] > Sent: Tuesday, February 12, 2002 12:33 AM > To: Robert D. Russell > Cc: ips@ece.cmu.edu > Subject: Re: UNH Plugfest > +++ > > <Section 3.8.3 should be 10.8.3> > 3. Section 3.8.3 limits the value of the Desired Data Transfer Length in > an R2T to at most MaxBurstSize. What is the rationale for this? > An R2T is sent by the target to the initiator, so why can't the > target specify any size it wants in the R2T? The target already > uses R2Ts to control the flow of Data-Out PDUs from the initiator, > so why impose this restriction on the R2Ts? > > Could someone please explain the benefit to this limitation on R2Ts? > > +++ Is this a plugfest question or one of your own? For your own questions > the channels are always open. The MaxBurstSize is there to enable the > initiator to share resources between several executing commands and limit > the number of "pending buffers" a target will have to keep > in case one of the Data Out PDUS is damaged and transfer to a device is not > possible. > > +++ -- ################################## Santosh Rao Software Design Engineer, HP-UX iSCSI Driver Team, Hewlett Packard, Cupertino. email : santoshr@cup.hp.com Phone : 408-447-3751 ##################################
Home Last updated: Mon Feb 25 00:18:10 2002 8874 messages in chronological order |