Background copy and cleaning rates

FlashCopy® mapping copy rate values can be between 1 and 100 and can be changed when the FlashCopy mapping is in any state.

If NOCOPY is specified, background copy is disabled. You can specify NOCOPY for short-lived FlashCopy mappings that are only used for backups, for example. Because the source data set is not expected to significantly change during the lifetime of the FlashCopy mapping, it is more efficient in terms of managed disk (MDisk) I/Os to not perform a background copy.

Note: For the command-line interface (CLI), the value NOCOPY is equivalent to setting the copy rate to 0 (zero).

Table 1 provides the relationship of the copy and cleaning rate values to the attempted number of grains to be split per second. A grain is the unit of data represented by a single bit.

Table 1. Relationship between the *rate*, data rate and grains per second values
User-specified rate attribute value	Data copied/sec	256 KB grains/sec	64 KB grains/sec
1 - 10	128 KB	0.5	2
11 - 20	256 KB	1	4
21 - 30	512 KB	2	8
31 - 40	1 MB	4	16
41 - 50	2 MB	8	32
51 - 60	4 MB	16	64
61 - 70	8 MB	32	128
71 - 80	16 MB	64	256
81 - 90	32 MB	128	512
91 - 100	64 MB	256	1024

The data copied/sec and the grains/sec numbers represent standards that the SAN Volume Controller tries to achieve. The SAN Volume Controller is unable to achieve these standards if insufficient bandwidth is available from the nodes to the physical disks that make up the managed disks (MDisks) after taking into account the requirements of foreground I/O. If this situation occurs, background copy I/O contends for resources on an equal basis with I/O that arrives from hosts. Both tend to see an increase in latency and consequential reduction in throughput with respect to the situation had the bandwidth not been limited.

Degradation runs smoothly. Background copy, stopping copy, and foreground I/O continue to make forward progress and do not stop, hang or cause the node to fail.

The background copy is performed by one of the nodes that belong to the I/O group in which the source VDisk resides. This responsibility is moved to the other node in the I/O group in the event of the failure of the node that performs the background and stopping copy.

The background copy starts with the grain that contains the highest logical block numbers (LBAs) and works in reverse towards the grain that contains LBA 0. The background copy is performed in reverse to avoid any unwanted interactions with sequential write streams from the application.

The stopping copy operation copies every grain that is split on the stopping map to the next map (if one exists) which is dependent on that grain. The operation starts searching with the grain that contains the highest LBAs and works in reverse towards the grain that contains LBA 0. Only those grains that other maps are dependent upon are copied.

Cleaning mode

When you create or modify a FlashCopy mapping, you can specify a cleaning rate for the FlashCopy mapping that is independent of the background copy rate. The cleaning rates shown in Table 1 controls the rate at which the cleaning process operates. The cleaning process copies data from the target VDisk of a mapping to the target VDisks of other mappings that are dependent on this data. The cleaning process must complete before the FlashCopy mapping can go to the stopping state.

Cleaning mode allows you to activate the cleaning process when the FlashCopy mapping is in the copying state. This keeps your target VDisk accessible while the cleaning process is running. When operating in this mode, it is possible that host I/O operations can prevent the cleaning process from reaching 100% if the I/O operations continue to copy new data to the target VDisks. However, it is possible to minimize the amount of data that requires cleaning while the mapping is stopping.

Cleaning mode is active if the background copy progress has reached 100% and the mapping is in the copying state, or if the background copy rate is set to 0.