Re: Integration of SCST in the mainstream Linux kernel

Erez Zilber
2008-02-18T09:43:49Z

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