FlashCopy from IBM

FlashCopy Principles

FlashCopy is part of the Copy Services offering from IBM. It works on enterprise subsystems like the DS and Storwise v700 series, and also the SVC SAN controller. The z/OS version of FlashCopy is emulated by EMC TimeFinder and HDS ShadowImage.
In FlashCopy terminology, you copy from a Source volume to a Target volume.
FlashCopy works on extents, even for a full volume copy. If the data on a volume occupies 5 extents, with free space between the extents, then 5 FlashCopy relationships will be used to copy the data. If the copy would require more than 255 extents, then occupied space and free space will be combined to keep the total number of FlashCopy sessions per volume down to 255. Dataset level FlashCopy works on exactly the same principle, except that the extents can be on more than one volume.
Just to confuse things, the term 'FlashCopy Relationship' is often used to refer to all the individual source / target FlashCopy pairs that are required to form a volume or dataset copy.

FlashCopy has three phases, Establish, Copy and Termination.

The Establish phase is very fast, it just involves setting up pointers to a bitmap mask. Once it is complete, the source and target disks can be accessed independently. From an application's point of view, the copy is complete once the Establish phase is over. This is sometimes called the T0 ot Time0 data position as this was the point in time when both disks were identical.

The Copy phase works differently, depending on if FlashCopy is running in Copy or NoCopy mode (see the next section for details on these).

FlashCopy is controlled by one or more bitmaps, which are used to indicate the copy status of the data. A bitmap entry of '1' indicates that track has not been copied yet, and a '0' entry indicates that it has been copied. While the copy is in progress there are three scenarios to consider

  • data read request from target: if the bitmap is '0' then the data is read from target, otherwise it is read from the source.
  • data update request on source: if the bitmap is '0' then original data has already been copied to the target, so this IO is written straight to source. If the bitmap is '1' then the source data is copied over to the target to preserve the time zero position, the bitmap is changed to a '0', then the source track is updated. As all writes are considered complete in cache, this delay does not affect the writing application. This is a Copy on Write, or CoW snapshot implementation.
  • data update request on target: If the bitmap is '1' then it is set to '0' to prevent the data from being overwritten by source data in future.

The Termination phase can happen automatically once all the source data has been copied over, or as a response to a FCWITHDRAW or rmflash command. At this point, there is no FlashCopy relationship between source and target, they are two independent volumes again. This auto withdrawal can cause problems if you control FlashCopy with GDPS scripts. The scripts do a explicit FlashCopy withdraw, and will get errors if they were previously withdrawn automatically. There is a new 'persist' parameter available which makes NoCopy sessions into Persistent Flashcopy relationships which are are never terminated.

For full volume FlashCopy, a source volume can have up to 12 target volumes, but a target volume can only have one relationship with a source volume. It is possible for a target volume to have many source volumes for dataset level FlashCopy. If you specify rmflash for a target volume, then all FlashCopy relationships that are targeting that volume are withdrawn. You can also request that only specific extents be withdrawn, rather than the full relationship. If you specify both source and target volumes on rmflash, then only relationships between that source / target pair are removed.

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Basic Types of FlashCopy

Volume level FlashCopy

To copy a full volume, the target volume must be the same size or bigger than the source and have the same logical track format. In the original FlashCopy as implemented in the early ESS sharks the volumes needed to be in the same logical subsystem. In current releases of FlashCopy, the source and target volumes must be in the same Storage Complex, but can be in different logical subsystems.

Dataset level FlashCopy

Dataset level FlashCopy only applies to z/OS implementations and is used by FlashCopy aware utilities like DFDSS. It uses one FlashCopy relationship for every contiguous extent occupied by the dataset. These extents can range from a single track to an entire volume. As it tracks the dataset by extent, this means that the source and target volumes do not need to be the same size, and it is also possible to copy a dataset into the same volume. A copied dataset can be also be on multiple volumes.

Copy Mode

Copy will physically copy all the data from the source to the target as a background task whether it is updated or not. Once the data copy is complete, the relationship between source and target is broken and the target disk is a T0 replica of the source disk. You will not get any notification when the copy is complete, you must use the FCQUERY command to check to progress of the copy.

NoCopy Mode

NoCopy mode will only copy data to the target disk when it is updated on the source disk.
When using NoCopy, the relationship between source and target is maintained until it is manually terminated, or all the data on the target disk has been updated (unless you have specified that this will be a persistent Flashcopy). GDPS and FDRinstant both use Nocopy mode. The Flashcopy relationship must be withdrawn manually when it is nolonger required.

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Invoking Basic FlashCopy

There are several ways to invoke FlashCopy. It can be invoked from the storage subsystem itself using the DS or Storwatch GUI, or by using DS CLI commands. In a Z/OS implementation it can also be invoked with TSO, DFDSS or ICKDSF commands. Both methods also have APIs.

CLI commands

To explicitly invoke FlashCopy you can use CLI commands. When using CLI commands, you should set up a profile file for every DS8K, and specify the DS8K machine data in that profile. This is explained in the DS8K configuration page.
The following are examples of CLI commands.

mkflash

You use mkflash to set up a FlashCopy relationship like this.

The first command is the simplest possible. It assumes that you have already defined your storage subsystem device in a profile, and will start a FlashCopy between devices 1E04 and 8E04, on the subsystem defined in that profile. Note that this will set up a FlashCopy between two subsystems, you could not do this with the first release of FlashCopy that ran on the old ESS devices.
The second command will flash the same two volumes, but it explicitly states that the FlashCopy will run on device 75BS072, and will be given a sequence number of 03 for later tracking. The target volume must be offline and it will also run in nocopy mode, copy is the default. If you run either commands, you should see a message "FlashCopy pair 1E04:8E04 successfully created."

lsflash

The first command will display any FlashCopy relationships active on device 1E00. The second command will list only FlashCopy relationships that are active between devices 1E00 and 8E00 and will display extra information about these relationships, because the -l parameter is specified. The output looks like this -

rmflash

Use the rmflash command to delete a FlashCopy relationship.

The first command will delete all FlashCopy relationships between 1E04 and 8E04
The second command will also delete FlashCopy relationships between 1E04 and 8E04, but only those that were defined as sequence number 3. However, the -cp parameter means convert the FlashCopy to a COPY relationship, copy all remaining data from source to target, then delete the relationship.

z/OS TSO commands

On a z/OS mainframe, you can explicitly invoke FlashCopy by using TSO commands. FCESTABL, FCQUERY, FCWITHDR
The command syntax is

FCESTABL

Establish FlashCopy relationship

FCESTABL SDEVN(X'nnnn') TDEVN(X'nnnn') MODE(COPY/NOCOPY) ONLINTGT(NO/YES)

SDEVN is the source UCB, the one you are flashing from
TDEVN is the target UCB, the one you are flashing to
MODE can be COPY or NOCOPY as explained above.
ONLINTGT can be NO or YES. If you specify YES, then the data on the volume will be overwritten, and the volume forced offline, because the VOLSER will be a duplicate. You need to be careful with this.

The XTNTLST parameter specifies a list of source and target extent pairs. Each pair describes a FlashCopy relationship between a source volume begin and end track extent, and the corresponding target volume begin and end track extent. You can specify as many extent pairs as you can fit onto a TSO command line like this.

XTNTLST (01000004 0357000A 13000001 15570007,02AC0006 02FF000C 18A00000 18F30006)

Now, can you honestly see anyone dumping a VTOC to get the extent information for a file, then using that to create an extent list as above? This command will be used by value added applications like DFDSS.

FCQUERY

Query FlashCopy status

FCQUERY DEVN(X'nnnn')

Output is

Which tells you that CCA 00 has 4 active flash sessions. This represents four extents that are in an active relationship.

FCWITHDR

Explicitly withdraw a FlashCopy relationship

FCWITHDR SDEVN(X'nnnn') TDEVN(X'nnnn')

This will withdraw any active sessions which are active to the specified target device, but will keep active sessions to other targets. If no target is specified, all sessions from the source are withdrawn. You can also withdraw FlashCopy sessions to specific extents

FCWITHDR SDEVN(9400)TDEVN(9470)
XTNTLST(00000000 0100000E 20000000 2100000E)

Applications and Utilities

If the conditions are right, a DFDSS copy job will invoke FlashCopy automatically. If this happens, you should see a message

ADR806I (001) - VOLUME COPIED USING A FAST REPLICATION FUNCTION

IBM supplies a list of conditions that need to be met for DFDSS to invoke FlashCopy, basically, nothing must change the track layout of the dataset. Two conditions are that you should use the NOPACKING keyword so that PDS files are not compressed and use the NOREBLOCKING keyword. Some other DFDSS features are -
A full volume DFDSS FlashCopy will only copy used extents.
If you do not want to use FlashCopy, then use the FASTREPLICATION(NONE) keyword.
DFDSS also has a new keyword, FCNOCOPY which will invoke FlashCopy in NOCOPY mode.
If you use DFDSS DEFRAG, then DEFRAG will try to use FlashCopy to move datasets around a disk

Several applications invoke FlashCopy through the API. Examples are GDPS or FDRinstant, see the relevant sections for details.

DS SM GUI

You can use the DS Storage Mamager to manage FlashCopy. Select Real Time Manager then FlashCopy. The GUI should then guide you through the FlashCopy process, but has less functionality than the CLI interface.

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Advanced FlashCopy Functions

Multi-relationship FlashCopy

Its possible to have up to 12 target copies of any source active at a time, though a target can only have one source. This applies to volume level and most implementations of dataset level Flash. At any given time, a volume or data set can be either a source or target, but not both . There are no special commands for this, just start new FlashCopy sessions as normal (unless you are using cascading flash copy).

Consistency Groups

While the FlashCopy Establish phase is fast, if you are copying several hundred disks, it will take a minute or so to do them all. Databases require their data to be at a consistent point-in-time, and single second difference between two database components can make the database unusable. The FlashCopy Consistency Group uses the freeze function to put all the FlashCopy source volumes in an extended long busy state until the FlashCopy completes over all disks. Consistency groups can span multiple storage systems, but the storage subsystems must all be the same type. That is, you cannot have a DS8K and an Storwise V7000 in the same consistency group.
Consistency groups are created by using the mkflash command with the -freeze parameter. This parameter is only available from the GUI or the CLI interface. The DS8000 CLI command below shows 5 volumes being flashcopied within a consistency group. The second command can be used to remove a freeze state. Note that it acts on a subsystem not a volume pair.

Persistent FlashCopy

Persistent FlashCopy is not withdrawn automatically when all tracks are copied from source to target, but remains is place until it is stopped by an explicit command. This has three potential uses
At volume level, it prevents the target from being overwritten by another FlashCopy, as a target can only be in one FlashCopy relationship at a time (but it does not prevent direct updates to the target of course)
Tools like GDPS use command scripts to manage FlashCopy, and these scripts can fail if they issue a command to withdraw a FlashCopy that has already been withdrawn because all the data has been copied over.
Persistant copy is required for some advanced flash functions like incremental copy, as bitmap changes must be preserved. The first command below will create a FlashCopy pair in persistent mode, and will also record changes to the target so the FlashCopy can be incrementally refreshed later. The second command can be used to check the persistence and recording status of the same disks.

Incremental FlashCopy

It can take several hours to make a full Background copy of 10 terabytes of data, depending on how busy your subsystem is. If you do this regularly for rapid recovery, then Incremental FlashCopy lets you just refresh the target data with tracks that have changed since the last copy was made. The subsystem requires three bitmaps to manage incremental flash, one to track changes to the source, one to track changes to the target and a third to indicate target tracks that have not been updated yet. Incremental FlashCopy must be in a persistent relationship to allow the subsystem to continue tracking updates to source and target extents. Incremental flash works at volume level only, not for dataset FlashCopy. It must also be invoked from the GUI, or by using the CLI interface,

If a Flash is running in Copy mode, then an incremental refresh of the Target device will be placed into a new T0, sometimes called T0'. The effect is as if a complete new FlashCopy has happened, so updates to the source will be copied to the target, and also any updates that were made to the target since the previous T0 will be backed out.
If the original FlashCopy was in NoCopy mode, then Refresh converts it to Copy mode.
The first command below will set up two FlashCopy pairs in record mode. If you use the record parameter, then the persist parameter is also added as a default. The second command will refresh the FlashCopy incrementally to a new T0.

mkflash -dev IBM.2107-75BS072 -record -seqnum 01 0101:0501 0103:0503
resyncflash -dev IBM.2107-75BS072 -record -persist -seqnum 01 0101:0501 0103:0503

Reverse Restore

Reverse Restore is an extension of incremental FlashCopy, but instead of creating a new T0 from the source, the source volume is back levelled with updates and orginal data from the target. In this case, the bitmaps are used to identify any tracks that have been updated on the source since T0 so the original data at the target can be copied back, and also any tracks that were updated at the target. The background copy process must be complete before reverse restore can be used. GDPS applications often use a remote FlashCopy session to preserve the remote mirrored data during a Metro Mirror refresh. The problem is that this remote FlashCopy was almost unusable, as GDPS used NOCOPY mode. It would be very useful if this could be changed to persistent FlashCopy and a reverse restore used.
The command below does a reverse restore over a single disk pair. Note that when the FlashCopy completes, the target and source pairing is reversed as indicated in the second lsflash command

reverseflash -dev IBM.2107-75BS072 -record 0103:0503
lsflash -dev IBM.2107-75BS072 -seqnum 02 -persist -record 0503:0103

Fast Reverse Restore

Fast Reverse Restore works with Global Mirror. If you start a FlashCopy session with change recording enabled, then you can reverse the direction of the copy and recover the Flashcopy source disk without needing to wait for background copy to complete.

setflashrevertible -dev IBM.2107-75BS072 0002:0102

Space Efficient

FlashCopy can use space efficient disks, which can be a very cost effective way to get a second copy of data for backup or testing. You need to tell Flascopy that the disks are space efficient by adding the -tgtse parameter.

mkflash -dev IBM.2107-75FA120 -tgtse 0100:0200

FlashCopy with other copy services products

A FlashCopy source disk can be either a Metro Mirror primary or secondary disk, A Metro Mirror primary disk can now be a FlashCopy Target. Either the Flash or the Mirror relationship can be started first, but be aware that a FlashCopy will not be valid until the mirror is complete.

Remote Control

FlashCopy used to require a dedicated network link if you wanted to send commands to a remote storage subsystem. Commands can be sent over a Metro Mirror or Global Copy PPRC link, which also means that there is no requirement for an operating system at the remote site. The remote FlashCopy Source volume must be a Metro Mirror or a Global Copy secondary volume.

Remote FlashCopy CLI commands just include the word remote in the command as shown in the examples below

mkremoteflash -dev IBM.2107-75BS072 0002:0102
rmremoteflash -dev IBM.2107-75BS072 0002:0102
lsremoteflash -dev IBM.2107-75BS072 0002:0102
resynchremoteflash -dev IBM.2107-75BS072 0002:0102

Cascaded Flash Copy

With Cascaded FlashCopy, a target device from one flashcopy relashionship can be a source flashcopy for another relationship, essentially a 'copy of a copy'. In this way, a cascade of relationships can be created, with up to 256 relationships in the chain. For example:

S1 -> T1=S2-> T2=S3 -> T3

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Tivoli Storage FlashCopy Manager versions

FlashCopy Manager is a set of applications that use Flashcopy to manage instant backups of different types of databases and other applications. FlashCopy Manager utilities include:

,
  • FlashCopy Manager for DB2
  • FlashCopy Manager for Oracle
  • FlashCopy Manager for Oracle SAP
  • FlashCopy Manager for MS SQL
  • FlashCopy Manager for MS Exchange
  • FlashCopy Manager for MS VSS
  • FlashCopy Manager for VMware