Shared storage support for Db2 pureScale environments

Fast I/O fencing

Before recovering a failed member in the Db2 pureScale instance, Db2 cluster services ensures that it is not possible for the failed member to modify shared data on disk. This precaution is known as I/O fencing. Some storage controllers and multipath I/O driver combinations support specialized capability, known as SCSI-3 Persistent Reserve (PR), with the write exclusive all registrants persistent reservation type (type 7h). This type enables Db2 cluster services to perform fast I/O fencing, in as little as 1 - 2 seconds.

Recovery time is the elapsed time from a host failure until the release of locks for uncommitted transactions. For a typical OLTP workload with short transactions, fast I/O fencing results in recovery times of approximately 20 seconds. Fast I/O fencing is much faster than the alternative method, based on expiry of lock leases. When deciding which category to use, evaluate if fast I/O fencing support is required to meet your service level agreements (SLAs).

Db2 cluster services tiebreaker support

A subcluster of hosts requires operational quorum for IBM Reliable Scalable Cluster Technology (RSCT) to allow the subcluster to continue running the Db2 pureScale instance in the event of host failures. To be granted operational quorum by RSCT, a subcluster must contain a majority of the hosts. If exactly half of the hosts in cluster remain in communication, the tiebreaker can be exclusively reserved by that subcluster to obtain operational quorum.

In Db2 12.1 and later fix packs, on supported Linux® operating systems, the device used as the cluster manager tiebreaker disk must have SCSI-3 PR WRITE EXCLUSIVE REGISTRANTS ONLY reservation type code 0x50 enabled.

On supported AIX® operating systems, the device used as the cluster manager tiebreaker disk can be of SCSI-3 PR WRITE EXCLUSIVE REGISTRANTS ONLY reservation type code 0x50 enabled or SCSI-2 reservation type enabled.

To check SCSI-3 PR status on Linux, use the command sg_persist with syntax shown below. Ensure that Write Exclusive, registrants only has a value of 1.

sg_persist -c /dev/dm-7
    IBM       2145         0000
    Peripheral device type: disk
    Report capabilities response: Compatible
    Reservation Handling (CRH): 1
    Specify Initiator Ports Capable (SIP_C): 0
    All Target Ports Capable (ATP_C): 0
    Persist Through Power Loss Capable (PTPL_C): 1 
    Type Mask Valid (TMV): 1
    Allow Commands: 0
    Persist Through Power Loss Active(PTPL_A): 1
     Support indicated in Type mask:
      Write Exclusive, all registrants: 1
      Exclusive Access, registrants only: 1     
      Write Exclusive, registrants only: 1
      Exclusive Access: 1
      Write Exclusive: 1
      Exclusive Access, all registrants: 1

For details on the command sg_persist, please see Linux Man page for this command.

lsattr -R -l hdisk1 -a reserve_policy

no_reserve
single_path
PR_exclusive
PR_shared

For details on the command lsattr, please see AIX Man page.

The tiebreaker disk is not a requirement. However, if your cluster has an even number of machines and you do not have a tiebreaker disk, and if half of the machines in the cluster fail, operational quorum is lost. You must have operational quorum for the cluster to be up and running. Without operational quorum, the database is not available for use.

Subsystem Device Driver Path Control Module (SDDPCM)

Upgrading SDDPCM to 2.6.3.x might cause device configurations to fail if the algorithm attribute on the upgraded device is set to fail_over. If the device configuration settings fail, the Db2 pureScale cluster might be affected. You must set the timeout_policy attribute to retry_path if you want to use the fail_over option on the algorithm attribute. For more information, see this IBM Support technote.

Multipath service configuration

For configuring the DM-MP multipath service on Linux, refer to Q4.6 of the IBM Storage Scale FAQ page. For more information, see IBM Storage Scale Frequently Asked Questions and Answers.

Category 1 storage device and multipath I/O driver combinations

Category 2 storage device and multipath I/O driver combinations

Category 3 storage device and multipath I/O driver combinations

All storage devices and multipath I/O driver combinations not identified as category 1 or category 2 fall into category 3. Category 3 combinations have not been validated with the Db2 pureScale Feature, though they are supported.