IBM Magstar Tape Drives -- AIX High Availability SAN Failover for 3590

Transcription

1 IBM Magstar Tape Drives - SAN Failover Capability June 12, 2001 IBM Magstar Tape Drives -- AIX High Availability SAN Failover for 3590 By Rob Basham & Jim Ayres, IBM Corporation, Tucson, Arizona, U.S.A.

2 Page 2 The IBM Magstar 3590 tape drive continues to experience strong success in the marketplace, with more than 75,000 units in use worldwide. IBM s Magstar family of tape drive and automation solutions continues to meet the high standard of excellence set by IBM s most demanding customers. This family is designed to deliver the highest reliability and performance to enterprise tape environments. For more information, visit the 3590 Web site at:

3 Page 3 Table of Contents 3 Preface 4 Tape on SANs Today 5 Failover for Disk 5 The New Failover for Tape Drives 9 Alternate Pathing Configuration Notes 9 Failover Availability 9 Summary Preface Fibre Channel on a SAN, or Storage Area Network, delivered some time ago on its promise of highly reliable delivery of information and failover capability for disk. That failover capability is now available for IBM Magstar 3590 Tape Drives on RS/6000 and IBM eserver pseries servers. This solution is designed to allow users to configure Storage Area Networks with redundant paths such that component failures--even on the network edges--are overcome in a way completely transparent to the application. This has been made possible on RS/6000 using a failover mechanism in the 3590 tape device driver by configuring it with information about the multiple paths to a device.

4 Page 4 Tape on SANs Today Although Storage Area Networks offer multiple path configurations today, the redundancy and reliability they provide have generally only been enabled for disk drives 1. For tapes, more than one path may be configured but normally only a single path to a tape drive is used. That does not mean there is no redundancy within the network. As long as the connection at the host and the connection at the drive do not fail, intermediate failures are hidden by the switches in the network. However, most systems today only have a few switches so the likelihood that a failure in the network will be on the edge where there currently is no provision for recovery is quite high. The figure below shows an example of which paths are recoverable within the network and which are unrecoverable on an edge. Host Tape Unrecoverable Paths Recoverable Paths Figure 1. Recoverable and Unrecoverable Paths Tape drives complying with the FCP-2 Fibre Channel tape standard take advantage of the error recovery mechanism for Fibre Channel. At the protocol layer, retries are allowed and recovery is often possible. However, the retries are all on the same path. This recovery mechanism keeps transient and temporary error conditions from causing problems. However, if there is a true failure at the edge of the SAN, there is 1 Sometimes referred to as Multipath I/O (MPIO).

5 Page 5 no recourse in tape drives. The job will fail. This is comparable to how recovery works on SCSI Parallel interfaces. Tape drives rely on recovery at the protocol layer. Failover For Disk Disk drives have the advantage that a permanent error in the transport layer (in this case, Fibre Channel) can simply be retried down an alternate path by reissuing the command. For this reason, it is commonly done. This is much more difficult with the tape command set because each command builds on the prior command so a much more complex algorithm is needed for failover. The New Failover For Tape Drives What s new for tape drives is adding an awareness of the multiple paths into the device driver so when a command fails, the device driver knows to look down another path. Furthermore, the device driver maintains position information so that it can restore positioning and reissue the command down the other path. The advantage of this approach is that the failover is done without any application involvement so any application running gets the failover capability.

6 Page 6 The first release of AIX High Availability SAN Failover is on AIX using the IBM AIX Enhanced Tape and Medium Changer Device Driver (Atape) 2. The Atape device driver alternate pathing support is designed to provide two basic functions. First, the ability to configure multiple physical paths, one primary and up to fifteen alternate paths to a 3590 Fibre Channel Tape Drive. The physical path can be any combination of HBA, switches, or drive ports. Second, it is intended to provide automatic error recovery and failover to an alternate physical path when a permanent error occurs, transparent to the running application. For example, consider a simple multi-path connection that consists of two Fibre Channel Protocol (FCP) Host Bus Adapters (HBA) in an RS/6000 connected to a RS/6000 HBA 1 HBA 2 Port 1 Port Figure 2. Tape Failover Example 3590 Fibre Channel enabled tape drive. The first HBA is connected to port 1 of the drive and the second HBA is connected to port 2 of the drive. This connection 2 The IBM tape and medium changer device driver is designed specifically to take advantage of the features provided by the IBM 3590 tape drives. This driver provides allows applications access to the functions required for basic tape operations (such as backup and restore), as well as to the advanced functions needed by full tape management systems. Whenever possible, the driver is designed to take advantage of the device features transparent to the application.

7 Page 7 provides two logical paths to same physical tape drive for redundancy in case one path from an HBA to the drive fails. Without the Atape alternate pathing support, each HBA will detect a 3590 tape drive and two logical devices will be configured, port 1 and port 2, even though both devices are the same physical device. A backup/restore application can only open and use one logical device at a time, either port 1 or port 2. If a path error occurs, the application will fail. Manual failover may be able to be initiated from the host RS/6000 by restarting the application on an alternate logical device, but the application will have to be restarted from the beginning and it could have been a long way into the backup/restore operation on tape when it failed. Sometimes manual failover could require operator intervention to reset the drive since a SCSI Reservation could still exist on the failing HBA path. With the Atape alternate pathing support enabled and configured on both drive ports, only a single port is used by the application. When the application opens the logical device, for example port 1, the Atape device driver will initialize both ports for use. If a permanent error occurs on the first path, port 1, the Atape driver automatically initiates error recovery and continues the operation on the next logical path, port 2, without interrupting the application. In order to ensure data integrity, the Atape device driver constantly keeps track of the device state, SCSI reservation, and current tape position. When failover occurs, the Atape device driver will first restore the previous device state, SCSI Reservation, and tape position prior to retrying the failing operation. This new function is designed to eliminate any single point of failure in the network or at the edge of the network that can cause a permanent failure. Moreover, because this function is managed at the device driver level, any application using the

8 Page 8 RS/6000 driver gets the high-availability capability. Also, this capability allows flexibility in SAN configuration and management. If a switch or HBA fails, it can simply be replaced with no interruption to applications that are in progress. An example of a SAN configuration with failover capability is given in figure 3. Host HBA 1 HBA 2 Port 1 Port 2 Tape Unrecoverable Paths Recoverable Paths Figure 3. SAN Configuration Example In this example, AIX will create four devices for each drive. (IN AIX at a very high level you can think of a drive port as being used to represent a tape device). Each RS/6000 has two Host Bus Adapters and each drive has two Fibre Channel ports. Multiplying the number of Host Bus Adapters times the number of Fibre Channel ports determines how many devices will be created, regardless of how many other redundant paths exist in the SAN. Remember that the SAN manages non-edge type failovers on its own. Now with the addition of AIX High Availability SAN Failover for 3590, edge of network failures are eliminated. This function is designed to provide for failovers in every possible place, including all switches, host bus adapters, cables, and drive ports. A single failure and many double failures can be tolerated without the application going down.

9 Page 9 Alternate Pathing Configuration Notes The details of alternate pathing configuration are described in the Atape device driver manual, entitled: IBM SCSI Tape Drive, Medium Changer, and Library Device Drivers Installation and Planning Guide, Document Number GC This document is available at: Ftp://ftp.software.ibm.com/storage/devdrvr/ and download IBM_scsi_tape_IUG.pdf Failover Availability This failover support will be generally available on June 22, Please refer to the following IBM Web site for the latest information regarding what is supported: Summary AIX High Availability Failover is designed to provide a new failover mechanism in the AIX tape device driver that allows you to configure multiple redundant paths in a SAN environment to Magstar 3590 E Model Tape Drives having Fibre Channel Attachment. In the event of a path or component failure, the failover mechanism is designed to automatically seek to retry the current operation using an alternate, pre-configured path without aborting the current job in progress. This can provide you with improved flexibility in SAN configuration, availability and management.

10 For More Information Please contact your IBM marketing Representative or an IBM Business Partner. For more information about IBM Storage Solutions, visit: storage/ibm.com Copyright IBM Corporation 2001 IBM Storage Systems Group 5600 Cottle Road San Jose,California Produced in the United States of America All Rights Reserved IBM,the IBM logo,aix, zseries, RS/6000, and Magstar are trade-marks IBM Corporation in the United States,other countries,or both. Other company,product,and service names may be trademarks or registered trademarks of their respective companies. The information in this paper is provided by IBM on an AS IS basis without any warranty, guarantee or assurance of any kind.ibm also does not provide any warranty,guarantee or assurance that the information in this paper is free from any errors or omissions.ibm undertakes no responsibility to update any information contained in this paper. Please send comments to George Mower, Contributing writer IBM hardware products are manufactured from new parts,or new and used parts.in some cases, the hardware product may not be new and may have been previously installed.regardless,ibm warranty terms apply.