Re: ISCSI: Immediate data extending beyond a single PDU

[Pierre Labat <pierre_labat@hp.com>]

Barry Reinhold wrote:

> Julian,
>         I  think one could carry on a debate about what the draft currently states
> about the coupling of immediate and unsolicited data - but it is probably
> more productive to figure out if we want to have both the concept of
> immediate data and unsolicited data and then worry about the wording in the
> draft.
>         I am of the opinion that immediate data (data within a single PDU) is a
> useful distincition from unsolicited data (iSCSI DATA PDU sent without R2T).
> The reasoning behind this is that there are a number of write IOs that can
> be satisfied by providing a buffer of 8K or less. If this data is contained
> in the command PDU the context can be created for the command, and then the
> data just happens to be sitting there with the context to process it. Thus
> the target can send the iSCSI response PDU without creating any special
> context or mechansims - should have nice low latency characteristics for
> small writes.
>         For devices that don't want to use the initiator task tag for establishing
> context of an iSCSI DATA PDU, immediate data is a nice thing. An iSCSI DATA
> PDU without a target task tag may not be such a nice thing.
>         Question here - What would you lose if you supported only immediate data?

Barry,

Initiators that want to use small iSCSI PDUs (less than 8k) cannot send the 8k
data as immediate. They have to send it in several small unsolicited data PDUs.
It 's why we must keep this alternative to immediate data.

Regards,

Pierre


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