Small computer systems interface (SCSI)

bus: half-duplex
- defines protocol, physical interfaces, electrical interfaces
- master/slave
- strict priority
- usually 1 initiator per bus; can be several for clusterization
for spinning disks
logical unit number (LUN): logical ID ≡ virtial controller
- host has exclusive control over LUN, otherwise – race condition (on mount – signature is written at the start of partition)
- does not care about file system ⇒ LUN can be extended, cannot be shrunk
OS addressing: controller ID, SCSI ID, LUN
- SCSI ID: 0-7, 7 = highest
- new SCSI ID: 8-15 added; priority (decreasing) ≡ 7-0, 15-8
1 queue ≡ one magnet head
command queue: 64
modes
- async
  - requires each command to be acknowledged before sending next command
  - lower BW, than in sync
- sync
  - several commands can be not acknowledged before sending next command ≈ TCP window
  - initiator and target negotiate the permitted number of transient commands
  - implemented on electrical interface level, not in software

SCSI commands

0x00: TEST UNIT READY
0x03: REQUEST SENSE
- previous command error code
0x04: FORMAT
- low-level format (disks)
0x12: INQUIRY
- device type: direct access, sequential
- medium type: removable, not-removable
- version
- supported address space
- options
0x15: MODE SELECT (6)
- inform target which parameters to use
0x1a: MODE SENSE (6)
- discover LUNS and parameters within target
0x1b: START STOP UNIT
- eject/insert CD
- spin disk up/down
0x25: READ CAPACITY
0x55: MODE SELECT (10)
- inform target which parameters to use
0x5a MODE SENSE (10)
- discover LUNS and parameters within target
0xa0: REPORT LUNS

SCSI LUN discovery

initiator sequentially sends TEST UNIT READY to every address on the bus
- few addresses ⇒ poll is quick and simple
initiator sends INQUIRY to every address, that responded
initiator sends REPORT LUNS to discover LUNs

SCSI read

process
1. initiator → read cmd (starting block address, number of blocks)
2. target → data
3. target → SCSI status
fragmented data requires several calls, because read is only sequential
no need for flow control: initiator does not request more than it can accept

SCSI write

process
1. initiator → write cmd (starting block address, number of blocks)
2. target → ready to receive
3. initiator → data
4. target → SCSI status
flow control required:
- initiator does not know, how much data can target accept
- performed by SCSI transport protocol

Mode pages

Disconnect-reconnect

   0     1     2     3     4     5     6     7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PS  | SPF |             Page code             |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                  Page length                  |
+-----+-----+-----+-----+-----+-----+-----+-----+
|               Buffer full ratio               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|              Buffer empty ratio               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_            Bus inactivity limit             _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_            Disconnect time limit            _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_              Connect time limit             _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_              Maximum burst size             _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+
|EMDP |        FA       |DIMM |      DTDC       |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                   Reserved                    |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_              First burst size               _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+

PS: parameters savable, = 0
SPF: subpage format, = 0
Page code: 0x02
Page length: bytes without 1st byte, = 15
EMDP: enable modify data pointers, 1 ≡ data overlay support (overrides PRLI)
FA: fair arbitration
DIMM: disconnect immediate
Maximum burst size: 512-byte words, how much can be sent in 1 FCP_DATA
First burst size: 512-byte words, size of unsolicited FCP_DATA
DTDC: data transfer disconnect control

000: not used
001: data within a single interconnect tenancy for cmd
011: data & response for cmd within a single interconnect tenancy for cmd

FC logical unit control

   0     1     2     3     4     5     6     7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PS  | SPF |             Page code             |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                  Page length                  |
+-----+-----+-----+-----+-----+-----+-----+-----+
|        Reserved       |      Protocol ID      |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                Reserved                 |EPDC |
+-----+-----+-----+-----+-----+-----+-----+-----+
|_                                             _|
|_                   Reserved                  _|
|_                                             _|
|                                               |
+-----+-----+-----+-----+-----+-----+-----+-----+

PS: parameters savable, = 0
SPF: subpage format, = 0
Page code: 0x18
Page length: bytes without 1st byte, = 6
EPDC: enable precise delivery checking, 1 ≡ enable in-order command delivery
Protocol ID:

0 ≡ FC protocol

FC port control

   0     1     2     3     4     5     6     7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PS  | SPF |             Page code             |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                  Page length                  |
+-----+-----+-----+-----+-----+-----+-----+-----+
|        Reserved       |      Protocol ID      |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                Reserved                 |EPDC |
+-----+-----+-----+-----+-----+-----+-----+-----+
|DTFD |PLPB |DDIS | DLM | RHA |ALWI |DTIPE|DTOLI|
+-----+-----+-----+-----+-----+-----+-----+-----+
|_                  Reserved                   _|
|                                               |
+_                            +-----+-----+-----+
|                             |  RR_TOV units   |
+-----+-----+-----+-----+-----+-----+-----+-----+
|                 RR_TOV value                  |
+-----+-----+-----+-----+-----+-----+-----+-----+

PS: parameters savable, = 0
SPF: subpage format, = 0
Page code: 0x19
Page length: bytes without 1st byte, = 6
EPDC: enable precise delivery checking, 1 ≡ enable in-order command delivery
Protocol ID:

0 ≡ FC protocol

DTFD: disable target fabric discovery, discover FC_AL
PLPB: prevent loop port bypass, FC_AL LPB and PBE
DDIS: disable discovery, FC_AL, 1 ≡ does not require ADISC/PDISC after loop init
DLM: disable loop master, 1 ≡ cannot become AL master
RHA: require hard address, FC_AL, 1 ≡ if requested address is not allocated – fail
ALWI: allow login without loop initialization, FC_AL
DTIPE: disable target initiated port enable, FC_AL, 1 ≡ wait for LPE before init
DTOLI: disable target originated loop init, FC_AL
RR_TOV units:

011: 0.1s

SCSI accessed fault-tolerant enclosure (SAF-TE)

part of IPMI
enclosure monitoring: SCI controller ≡ initiator → SCSI enclosure ≡ target
- temperature
- voltage
SCSI commands
- INQUIRY
- READ BUFFER
- REQUEST SENSE
- SEND DIAGNOSTIC
- TEST UNIT READY
- WRITE BUFFER

SCSI enclosure services (SES)

ANSI T10
enclosure monitoring: SES ≡ SCSI target
SCSI commands
- SEND DIAGNOSTIC
- RECEIVE DIAGNOSTIC RESULTS

Networking and IT

Uncovering the Why

SCSI