POWER SOLUTIONS

More documents

Recommendations

Info

STORAGE TECHNOLOGYThe extended link serviceSequence RetransmissionRequest is sent by the initiator(the host) to the target deviceto request that a targetretransmit data back to theinitiator, or to inform thetarget that data will beretransmitted back to thetarget. The invocation andacceptance of an SRR indicatethat a previous commandor data frame was lost.target during a read operation.The DTC is importantin determining whether alldata was successfully transmittedor received. TheE_STAT word value has twobit values that identify thestate of the exchange. Thefirst value is the Completionbit, which specifies whetherthe exchange is still open;and the second value isthe Sequence Initiative bit,which indicates which portholds the initiative (that is,which end of the transmissionis supposed to make thenext move).Sequence RetransmissionRequest functionThe next part of FC-TAPEerror recovery involves theextended link service SRR.This service is sent by the initiator(the host) to the targetdevice to request that a target retransmit data back to the initiator,or to inform the target that data will be retransmitted back to thetarget. The invocation and acceptance of an SRR indicate that aprevious command or data frame was lost.The SRR payload consists of an OX_ID, a Relative Offsetvalue, and a Routing Control (R_CTL) value, all of which assistin determining which data needs to be retransmitted. The RelativeOffset field identifies an offset value that the initiator requires fordata retransmission, and the R_CTL field defines the frame functiontype that is to be retransmitted. The R_CTL value identifieswhether the retransmission is a data descriptor (FCP_XFER_RDY),command status (FCP_RSP), or solicited data (FCP_DATA). If thetarget accepts the SRR, the target will respond with an ACC, whichwill then allow the data to be retransmitted.Confirmed Completion (FCP_CONF) is used to acknowledgesuccessful receipt of a response frame. In other words, once thetarget sends an FCP_RSP to the initiator, the initiator will send backconfirmation informing the target that the response was successfullyreceived. The confirmation is merely an added protection mechanismto help ensure successful delivery of the response—allowingthe target to safely close the exchange. Confirmed Completion ina Fibre Channel environment is equivalent to the SCSI functionREQ/ACK for command completion.Example error recovery implementation with FC-TAPETo demonstrate the error recovery process related to FC-TAPE,the Figure 2 example shows how a data frame (FCP_DATA), lostduring a SCSI write command (FCP_CMD), is recovered usingFC-TAPE as follows:• Section A: An uninterrupted flow process writes data to atape device. Once the data is successfully transmitted to thetape device, the tape device returns good status back to theinitiator.• Section B: An error causes the third data frame to be lost.In this instance, the initiator is waiting for a response, andthe target is still waiting for more data. During this deadlockperiod, the three-second REC_TOV timer expires. Sensing adelay in the response from the target, the initiator then transmitsa REC requesting information from the target regardingthe status of the exchange. The target responds to the RECby transmitting an ACC back to the initiator. The ACC payloadfrom the target contains the DTC value, indicating thatthe number of bytes successfully received by the target wasincomplete. Next, the E_STAT bit in the ACC payload informsthe initiator that the exchange is still open, and that thetarget does not own the sequence initiative. In other words,the target is telling the initiator to take the next action to continuethe exchange.• Section C: To correct the error condition, the initiator willtransmit an SRR to the target requesting that the targetresend an FCP_XFER_RDY (request to allow data transfer).The specific request for the FCP_XFER_RDY is defined inSection A Section B Section CInitiatorTarget InitiatorTarget InitiatorFCP_CMD (write)FCP_CMD (write)SRRFCP_XFER_RDYFCP_DATAFCP_RSPFCP_XFER_RDYFCP_CMD (write)012 xxxxxxxxxxxxxxREC (REC_TOV)ACCFigure 2. Example FC-TAPE error recovery process012ACCFCP_XFER_RDYFCP_DATAFCP_RSPTarget90DELL POWER SOLUTIONS Reprinted from Dell Power Solutions, August 2005. Copyright © 2005 Dell Inc. All rights reserved. August 2005
STORAGE TECHNOLOGY• Line 4: The initiator issues an SRR.• Line 5: The target replies toFigure 3. Example Fibre Channel trace showing FC-TAPE error recoverythe SRR.• Line 6: The target resends the“lost” FCP_RSP (good status).• Line 7: The initiator sends anFCP_CONF to the target indicatingthat it received the FCP_RSP.the R_CTL payload in the SRR. The target will respond tothe SRR with an ACC and then transmit an FCP_XFER_RDYback to the initiator, indicating that the target is ready toreceive data transmission. Once the initiator receives theFCP_XFER_RDY, the initiator will immediately retransmitthe FCP_DATA data frames.The amount of time to recover from the original lost data frameis no more than three seconds (the REC_TOV), which is typically farless time than the error recovery mechanisms used by tape backupsoftware. For this reason, FC-TAPE can be a valuable asset in linklevelerror recovery procedures for tape backup devices.Fibre Channel trace showing FC-TAPE error recoveryFigure 3 displays a segment from a Fibre Channel trace in whichFC-TAPE error recovery is invoked because of a lost FCP_RSP(good status). The debug trace data shown in Figure 3 correspondsto the following process:• Line 1: The host tape backup server issues a SCSI readcommand.• Line 2: No FCP_RSP is received within the REC_TOV timer(three seconds), and thus a REC is transmitted. (The SCSIread command was issued at the 3.4 second mark, and theREC was issued at the 6.3 second mark.)• Line 3: The target accepts the REC by responding with an ACC.This example demonstrates the efficiency of FC-TAPE in performingerror recovery without incurring overhead on the tapebackup software. FC-TAPE error recovery is conducted at the FibreChannel link level and completed within seconds between the initiatorand the target tape device.Recovering from tape backup errors quickly and efficientlyBest practices for tape backup operations recommend that administratorsenable FC-TAPE error recovery on the Fibre Channel adapteras well as the data verification setting in the tape backup software.By combining FC-TAPE and tape backup software error recovery,administrators can quickly and easily enhance the capability todetect and correct errors on a Fibre Channel link, thereby helpingto preserve data integrity.Richard Golasky is a development engineer senior consultant in theDell/EMC Enterprise Storage Group. His responsibilities include all areasof tape backup, including Fibre Channel storage, high-availability clusters,network attached storage, and performance analysis. He has a B.S. inElectrical Engineering from Florida Atlantic University.FOR MORE INFORMATIONDell tape backup devices:www.dell.com/tapebackupExperienceDell Power Solutionswww.dell.com/powersolutions Reprinted from Dell Power Solutions, August 2005. Copyright © 2005 Dell Inc. All rights reserved. DELL POWER SOLUTIONS 91
Page 2 and 3:
Gigabit EthernetMulti-Gigabit Ether
Page 4 and 5:
TABLE OF CONTENTSPOWERSOLUTIONSTHE
Page 6 and 7:
TABLE OF CONTENTSDATABASE TECHNOLOG
Page 8 and 9:
EDITOR’S COMMENTSRelational Bliss
Page 10 and 11:
SYSTEMS MANAGEMENTTake Control with
Page 12 and 13:
SYSTEMS MANAGEMENTInstrumented plat
Page 14 and 15:
SYSTEMS MANAGEMENTinterface (CLI) a
Page 16 and 17:
SYSTEMS MANAGEMENTIntroducing Dell
Page 18 and 19:
SYSTEMS MANAGEMENTA query definitio
Page 20 and 21:
SYSTEMS MANAGEMENTremote agents. Fo
Page 22 and 23:
SYSTEMS MANAGEMENTError code Error
Page 24 and 25:
SYSTEMS MANAGEMENTFigure 3. Wbemtes
Page 26 and 27:
SYSTEMS MANAGEMENTSoftware Change M
Page 28 and 29:
SYSTEMS MANAGEMENTDell UpdatePackag
Page 30 and 31:
SYSTEMS MANAGEMENTFigure 5. Summary
Page 32 and 33:
SYSTEMS MANAGEMENTIT Assistant UI I
Page 34 and 35:
SYSTEMS MANAGEMENTFigure 3. Configu
Page 36 and 37:
SYSTEMS MANAGEMENTConsole, Altiris
Page 38 and 39:
SYSTEMS MANAGEMENTDeploying Server
Page 40 and 41:
SYSTEMS MANAGEMENTAdvancing the Upd
Page 42 and 43: SYSTEMS MANAGEMENTallows administra
Page 44 and 45: SYSTEMS MANAGEMENTTime-Savings Vali
Page 46 and 47: SYSTEMS MANAGEMENTBENEFITS OF ALTIR
Page 48 and 49: SYSTEMS MANAGEMENTLANDesk Server Ma
Page 50 and 51: SYSTEMS MANAGEMENT• SEL event cre
Page 52 and 53: SYSTEMS MANAGEMENTAdministrators ca
Page 54 and 55: SYSTEMS MANAGEMENTUsing the DRAC 4
Page 56 and 57: SYSTEMS MANAGEMENTVirtual floppy me
Page 58 and 59: SYSTEMS MANAGEMENTUpdating theDell
Page 60 and 61: SYSTEMS MANAGEMENTthe overall Power
Page 62 and 63: SYSTEMS MANAGEMENTFor the firmware
Page 64 and 65: DATA CENTER TECHNOLOGYGuidelines fo
Page 66 and 67: DATA CENTER TECHNOLOGYusing the tem
Page 68 and 69: DATA CENTER TECHNOLOGYInformation L
Page 70 and 71: DATA CENTER TECHNOLOGYdata center:
Page 72 and 73: DATA CENTER TECHNOLOGYresources and
Page 74 and 75: BUSINESS CONTINUITYe-mail systems u
Page 76 and 77: BUSINESS CONTINUITYGalaxy and Backu
Page 78 and 79: STORAGE TECHNOLOGYWhen migrating fr
Page 80 and 81: STORAGE TECHNOLOGYwhich can lead to
Page 82 and 83: STORAGE TECHNOLOGYChannel ports; al
Page 84 and 85: STORAGE TECHNOLOGYMaximum zone sets
Page 86 and 87: STORAGE TECHNOLOGYbenefits includin
Page 88 and 89: STORAGE TECHNOLOGYmemory manager wa
Page 90 and 91: STORAGE TECHNOLOGYLink-Level Error
Page 94 and 95: STORAGE TECHNOLOGYMigrating from De
Page 96 and 97: STORAGE TECHNOLOGYFigure 3. Managin
Page 98 and 99: DATABASE TECHNOLOGYFast, Flexible C
Page 100 and 101: DATABASE TECHNOLOGYScaling Out SQL
Page 102 and 103: DATABASE TECHNOLOGYMSN Communicatio
Page 104 and 105: DATABASE TECHNOLOGYThe MSN CSP appl
Page 106 and 107: DATABASE TECHNOLOGY1,20070Transacti
Page 108 and 109: DATABASE TECHNOLOGYAdvantages of OC
Page 110 and 111: DATABASE TECHNOLOGYASM instance ins
Page 112 and 113: VIRTUALIZATION TECHNOLOGYDomain 0 V
Page 114 and 115: SYSTEM ARCHITECTUREImproving Fault
Page 116 and 117: SYSTEM ARCHITECTUREBIOS Hot Hotopti
Page 118 and 119: SYSTEM ARCHITECTUREinitialized. The
Page 120: If you’re paying unreasonable lic
show all

POWER SOLUTIONS

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?