Messages in this thread |  | | | Date | Mon, 18 Feb 2008 11:43:31 +0200 | | From | Erez Zilber <> | | Subject | Re: Integration of SCST in the mainstream Linux kernel |
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Bart Van Assche wrote: > On Feb 5, 2008 6:01 PM, Erez Zilber <erezz@voltaire.com> wrote: > >> Using such large values for FirstBurstLength will give you poor >> performance numbers for WRITE commands (with iSER). FirstBurstLength >> means how much data should you send as unsolicited data (i.e. without >> RDMA). It means that your WRITE commands were sent without RDMA. >> > > Sorry, but I'm afraid you got this wrong. When the iSER transport is > used instead of TCP, all data is sent via RDMA, including unsolicited > data. If you have look at the iSER implementation in the Linux kernel > (source files under drivers/infiniband/ulp/iser), you will see that > all data is transferred via RDMA and not via TCP/IP. > >
When you execute WRITE commands with iSCSI, it works like this:
EDTL (Expected data length) - the data length of your command
FirstBurstLength - the length of data that will be sent as unsolicited data (i.e. as immediate data with the SCSI command and as unsolicited data-out PDUs)
If you use a high value for FirstBurstLength, all (or most) of your data will be sent as unsolicited data-out PDUs. These PDUs don't use the RDMA engine, so you miss the advantage of IB.
If you use a lower value for FirstBurstLength, EDTL - FirstBurstLength bytes will be sent as solicited data-out PDUs. With iSER, solicited data-out PDUs are RDMA operations.
I hope that I'm more clear now.
Erez
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