Heard of SLER – Sequence Level Error Recovery in FC (Fibre Channel)?

First, what is SLER?

SLER or Sequence Level Error Recovery, as name suggests it is the error recovery process at Transport layer (Sequence Level) without passing on Sequence or Exchange missing or lost errors to Storage Protocol layer (Upper Level).

Why SLER?

When FCP (Fibre Channel Protocol) was designed and developed, in-order was guaranteed end to end. Then EBR (Exchange-Based Routing) (Dynamic Path Selection (DLS) in Brocade switches) feature came in switch fabric which talks about possibility of out of order delivery of exchanges. And this created issues in FCP based Error Detect Recovery – recovery being done at upper SCSI layer, adding up to latencies. Therefore, SLER, which was designed initially for FC-NVMe and introduced in FC-NVMe-2, was backported to FCP-5(latest FCP) – thereby adding an optional faster mechanism for recovery while SCSI doing its own error recovery.

How does SLER work?

Let’s understand this by following example of FCP_CMND IU lost during transmission.

Error detection - Here, upon error detection (lost FCP_CMND IU), FLUSH BLS is sent to check the status of Exchange (timeout value FLUSH_TOV exhausted).

Recovery – FLUSH_RSP response points that Exchange status is closed (as OE=0), hence not reached target. FCP_CMND IU is retransmitted using same(or sometimes different) OX_ID, SLER Qualifier and CRN.

When similar situations occur for losses of FCP_XFER_RDY, FCP_RSP, FCP_DATA IUs, SLER recovery

Ref: Annex. D of FCP-5 - https://www.t10.org/cgi-bin/ac.pl?t=f&f=fcp5r04.pdf?

How to take advantage of SLER?

Emulex HBA LPe35000/LPe36000, HPE SN1600 Series HBAs supports SLER.

References:

-???????https://standards.incits.org/apps/group_public/download.php/114957/livelink??(Section 4.12)

-???????How Fibre Channel Standards Are Made, Part VI: FCP-5 – More Than Just Another Acronym – Fibre Channel Industry Association