|
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: Unsolicited Data.Sanjeev, > why would initiator choose not to send unsolicited immediate data when > it has negotiated for IntialR2T= Yes?? I assume you mean InitialR2T=no. I would do this because I think that the real benefit of unsolicited data is for writes where ALL the write data can be sent unsolicited, and I'm happy to have the target schedule the transfer of (ALL) data for larger writes. Of course, others would say I'm a moron. Assuming the overwhelming majority of storage use is file access, small writes are exclusively file system metadata writes. Unsolicited data is a huge win for metadata writes because the latency of these metadata writes is frequently not covered by transferring other data (within a single access thread). On the other hand, the file system (usually) generates enough concurrent demand for real data writes that the flow control (R2T) latency is well hidden. You might argue that if you're running a file system optimized fors today's SANs over a link with a HUGE bandwidth * delay product (greater than the expected demand created by the file system), sending large amounts of unsolicited write data will substantially improve the link utilization. That is true, but I don't think many targets are going to offer unsolicited data bursts remotely that large. In a hardware accelerate implementation, unsolicited data will be handled differently than solicited data. The target can chose where the solicited data is delivered, but unsolicited data arrives through a general-delivery path. Therefore, if the offered unsolicited data burst size approaches the regular burst size, an implementation will need TWICE as much buffer memory (half in the general-delivery area and half in the flow-controlled buffer area) to support the same operation load. Maybe this tradeoff is OK, but given the cost sensitivity of storage targets, I doubt it. As always, I welcome comments from those who see things differently. Steph
Home Last updated: Tue Sep 04 01:04:33 2001 6315 messages in chronological order |