Wednesday 2 October 2013

Intro to SRDF - 1 - Getting Started

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Intro to SRDF

1 - Getting Started

provides a brief introduction to SRDF and the SRDF component of the Solutions Enabler SYMCLI. It also explains which SRDF documents to read, considerations to be aware before invoking any SRDF operation, and a detailed overview of SRDF pair states.

Using the Solutions Enabler SYMCLI

The Solutions Enabler SYMCLI is a specialized library consisting of commands that you can invoke from a host command line, or within scripts. These commands perform control operations on devices and data objects within your managed storage complex. The Solutions Enabler SRDF component extends the basic SYMCLI command set to include SRDF commands for performing control operations on remotely located SRDF devices.
Table 2 Command summary
Command Description For more information
symrdf Performs the control operations on SRDF devices, such as: • Establishes (mirrors) an SRDF pair by initiating a data copy from the source (R1) side to the target (R2) side. This operation can be a full or incremental establish.
• Restores remote mirroring. Initiates a data copy from the target (R2) side to the source (R1) side. This operation can be a full or incremental restore.
• Splits an SRDF pair, which stops mirroring for the SRDF pairs in a device group.
• Fails over and back from the source (R1) side to the target (R2) side, switching data processing to the target (R2) side.
• Updates the source (R1) side after a failover, while the target (R2) side may still be operational to its local host(s).
• Swaps the source (R1) and target (R2) destinations between the target and the source.
• Creates, deletes, or swaps dynamic SRDF device pairs.
• Performs dynamic RDF group controls to add, modify, and remove dynamic groups.
• Enables link domino locally or remotely when creating dynamic groups.
• Enables auto link recovery locally or remotely when creating dynamic groups.
• Enables/disables consistency for SRDF/A capable devices operating in asynchronous mode that are managed by a device group or file.
• Activates/deactivates the SRDF/A Delta Set Extension (DSE) feature. 		See page 33 and the symrdf man page.
symreplicate Invokes a replicate session that generates automated recurrent, background copies of the standard data following a path across SRDF links and cascading BCVs. You can start a replicate session, stop it, and restart the replicate session. This command is used for SRDF/Automated Replication. See page 169 and the symreplicate man page.
symstar Uses concurrent and cascaded SRDF/Synchronous and SRDF/Asynchronous links to replicate source data synchronously to a nearby regional site and asynchronously to a distant remote site. See page 233 and the symstar man page.
symrecover Provides a session state monitoring tool that attempts to restart a group session if it enters the suspended or partitioned state. See page 299 and the symrecover man page.

Understanding SRDF pair states and links

SRDF devices that are operational are always in an SRDF pair state. The SRDF pair
state encompasses the SRDF state on the source (R1) side, the SRDF link state, and the
SRDF state on the target (R2) side.
NR (Not ready) Disabled for both reads and writes
RW (Ready) Enabled for both reads and writes
WD (Write disabled) Enabled for reads but not writes
NA (Not available) Unable to report on correct state

SRDF pair state descriptions

State Description
SyncInProg A synchronization is currently in progress between the R1 and the R2. There are existing invalid tracks between the two pairs and the logical links between both sides of an SRDF pair are up.
Synchronized The R1 and the R2 are currently in a synchronized state. The same content exists on the R2 as the R1. There are no invalid tracks between the two pairs.
Split The R1 and the R2 are currently ready to their hosts, but the links are not ready or write disabled.
Failed Over The R1 is currently not ready or write disabled and operations have been failed over to the R2.
R1 Updated The R1 is currently not ready or write disabled to the host, there are no local invalid tracks on the R1 side, and the links are ready or write disabled.
R1 UpdInProg The R1 is currently not ready or write disabled to the host, there are invalid local (R1) tracks on the source side, data is being copied from the R2 to the R1 device, and the links are ready.
Suspended The SRDF links have been suspended and are not ready or write disabled. If the R1 is ready while the links are suspended, any I/O will accumulate as invalid tracks owed to the R2.
Partitioned The SYMAPI is currently unable to communicate through the corresponding SRDF path to the remote Symmetrix. Partitioned may apply to devices within an RA group. For example, if SYMAPI is unable to communicate to a remote Symmetrix from an RA group, devices in that RA group will be marked as being in the Partitioned state. A half pair and a duplicate pair are also reported as Partitioned.
Mixed A composite SYMAPI device group SRDF pair state. There exist different SRDF pair states within a device group.
Invalid This is the default state when no other SRDF state applies. The combination of the R1 device, the R2 device, and the SRDF link states do not match any other pair state. This state may occur if there is a problem at the disk director level.
Consistent The R2 SRDF/A capable devices are in a consistent state. Consistent state signifies the normal state of operation for device pairs operating in asynchronous mode.
Transmit Idle The SRDF/A session cannot push data in the transmit cycle across the link because the link is down.

Verifying Pair States

You can verify the state of the SRDF pairs by including the expected state option (such as, -synchronized, -split, -failover, and so on) in the command line. For example, to verify that the SRDF pair DEV007, in device group prod, is in the synchronized pair state, enter:

symrdf -g prod verify DEV007 -synchronized

Invalid tracks in SRDF pairs

Invalid tracks in SRDF configurations indicate that the data is not synchronized between the two devices in an SRDF pair. On both the source and target sides of an SRDF configuration, the Symmetrix array keeps an account of the tracks that are "owed" to the other side. The owed tracks are known as remote invalids. For example, consider the case of an R1 device whose logical connection to its R2 has been suspended. If both devices are made write-accessible, hosts on both sides of the SRDF links can write to their respective devices, creating R2 invalids on the R1 side and R1 invalids on the R2 side. Each invalid track represents a track of data that has changed since the two sides were split. To re-establish the logical links between the R1 and R2, the invalid tracks must be resolved.
The resolution of invalid tracks depends on which control operation you perform. For instance, you can have remote invalids on both sides prior to an establish or a restore operation. If so, performing an establish operation indicates to SRDF that you want to copy the modified R1 tracks to the R2 side. In the process, any tracks that were modified on the R2 side are overwritten with data from corresponding tracks on the R1 side. Performing a restore operation indicates the opposite—that you want to copy modified R2 tracks to the R1 side. In the process, any tracks that were modified on the R1 side are overwritten with data from corresponding tracks on the R2 side. For information on control options that perform synchronization, refer to Chapter 2, page 2-37.

SRDF device and link state combinations

When you invoke a control action on an SRDF pair, the SRDF pair state may be changed. This depends on whether the SRDF state of the source (R1) side, the SRDF links, or the SRDF state of the target (R2) side has changed. Additionally, the state of a device can change if its front-end or back-end Integrated Directors change in the SRDF links. Table 4 shows each SRDF pair state that results from the combination of the states of the source and target devices and the SRDF links. This table also indicates the possible R1 or R2 invalid tracks for each SRDF pair state.
SRDF pair state Source (R1) SRDF state SRDF link state Target (R2) SRDF state R1 or R2 invalid tracks
Synchronized Ready (RW) Ready (RW) Not Ready or WD 0
Failed Over Not Ready or WD Not Ready Ready (RW) —
R1 Updated Not Ready or WD Ready (RW) or WD Ready (RW) 0a
R1 UpdInProg Not Ready or WD Ready (RW) or WD Ready (RW) > 0a
Split Ready (RW) Not Ready or WD Ready (RW) —
SyncInProg Ready (RW) Ready (RW) Not Ready or WD > 0
Suspended Any statusb Not Ready or WD Not Ready or Write Disabled —
Partitionedc Any status Not Ready Not Available —
Partitionedd Not Available Not Ready Any status —
Mixed e e e —
Invalide Any statusf Any status Any status —
Consistent Ready (RW)f Ready (RW) Not Ready or WD 0
Transmit Idle Ready (RW)f Ready (RW) Not Ready or WD —
a. Refers to invalid local (R1) tracks on source.
b. Any status value is possible (Ready, Not Ready, Write Disabled, or Not Available).
c. Viewed from the host locally connected to the source (R1) device.
d. Viewed from the host locally connected to the target (R2) device.
e. When no other SRDF states apply, the state defaults to Invalid.
f. The combination of source SRDF, SRDF links, and target SRDF statuses do not match any other SRDF state; therefore, the SRDF state is considered Invalid.

Pinging Symmetrix arrays through SRDF links

The Symmetrix arrays are pinged through SRDF links. The symrdf -rdf ping option determines if a Symmetrix array using SRDF links is up and running. Based on return codes, you can determine whether some or all of the Symmetrix arrays were successfully pinged. For more information on return codes, refer to the EMC Solutions Enabler Symmetrix CLI Command Reference.
For example, to ping Symmetrix array 123 through the SRDF links, enter:

symrdf -rdf -sid 123 ping

Enabling SRDF software and hardware compression

Solutions Enabler provides SRDF software and hardware compression for devices operating in asynchronous and adaptive copy modes. Compression minimizes the amount of data to be transmitted over an SRDF link. You enable and report on software and hardware compression at the group level. Both software and hardware compression require Symmetrix arrays running Enginuity 5875 and higher. Although you can enable software and hardware compression on the R2 side, you must first enable it on the R1 side using the following parameters in the symconfigure set command.

set [ra | rdf] group GroupNum
[rdf_sw_compression = ]
[rdf_hw_compression = ]
To determine if SRDF software and hardware compression is enabled for an RDF
group, issue the symcfg list -rdfg command. For example, to view if software or
hardware compression is enabled for RDF group 12 on Symmetrix 432, enter:

symcfg list -sid 432 -rdfg 12

Considerations

SRDF operations and copy sessions

Certain SRDF operations are not allowed within Symmetrix arrays employing either EMC TimeFinder®/Snap or TimeFinder/Clone operations, which use copy session pairs. The availability of some SRDF actions depends on the current pair state of the TimeFinder/Snap or TimeFinder/Clone copy session devices. Refer to Appendix A for a description of the TimeFinder/Snap and TimeFinder/Clone pair states, and which SRDF operations are available within each state.

Migrating data from R1 to a larger R2 device

You can copy data from an R1 device to a larger R2 device but the following
restrictions apply:
  • All swap and SRDF/Star operations are blocked.
  • If SYMAPI_RDF_CREATEPAIR_LARGER_R2 is set to DISABLE in the options file, all createpair operations are blocked.
  • Data mirrored to a larger R2 device cannot be restored back to its R1 device.
  • Concatenated metadevices are not supported but striped metadevices are supported.
Note: Depending on the type of file system and attached host, certain host-dependent
operations may be required to access data migrated to a larger R2 device.

Preventing synchronization actions

For some sites, it may be desirable to block users on a host from performing either anestablish or restore operation on any of the Symmetrix devices. The sync direction parameter (SYMAPI_SYNC_DIRECTION) in the options file allows you to confine SRDF and TimeFinder operations to only establish or restore actions. You can block a user on a host from executing a restore or an establish action using the following form:
SYMAPI_SYNC_DIRECTION=ESTABLISH | RESTORE | BOTH
ESTABLISH confines the possible operations to just establish actions.
RESTORE confines the possible operations to just restore actions, which includes (allows) restore, failback, R1 update actions.
BOTH is the default, which does not restrict any SRDF or TimeFinder actions.

Device external locks

SYMAPI/SYMCLI uses device external locks in the Symmetrix array to lock BCV pairs during TimeFinder control operations and to lock SRDF device pairs during SRDF control operations. To list a range of Symmetrix devices (0000 to 000A) that have a device external lock, enter:
symdev list -sid 870 -devs 0000:000A -lock

SRDF/A and the consistency exempt option

When an RDF group is supporting an active SRDF/A session, control operations must be targeted at all device pairs in the session, or the session must first be made inactive by suspending the links between all device pairs in the session before attempting to control a subset of the device pairs.
The consistency exempt feature, available with EMC Enginuity® 5773.150 and higher, relaxes that restriction. Devices marked consistency exempt can, in some cases, be controlled independently of other devices in the active SRDF/A session. The -cons_exempt option flags devices targeted by the command as consistency exempt. Enginuity automatically clears the consistency exempt status when the affected device pairs have become consistent and two cycle switches have subsequently occurred.

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