Hitachi ShadowImage for IBM z/OS User"s Guide (T5213-96002, June 2007)
Preparing for ShadowImage™ for IBM
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z/OS
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Hitachi ShadowImage™ for IBM
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z/OS
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User’s Guide
Note: For details about the calculation of the total number of the differential
tables per a pair, see the expression described just before Table 3-3.
The maximum number of pairs that you can create is the largest number
that meets the equation, Σ(α) ≤ (β), and Σ(γ) ≤ (δ), where:
Σ(α) stands for the total number of differential tables per pair (see Table
3-3), and
(β) stands for the number of differential tables available in the storage
system, and
Σ(γ) stands for the total number of pair tables per pair, and
(δ) stands for the number of pair tables available in the storage system.
(β) and (δ) will differ according to the installed additional shared memory
for differential tables. For details, see Table 3-2.
For example, if you are to create 10 pairs of 3390-3 volumes and 20 pairs
of 3390-L volumes in a storage system that has 26,176 differential tables,
the following equation would be used to calculate Σ(α) and Σ(γ):
Σ(α) : (3 x 10) + (24 x 20) = 510.
Σ(γ) : (1 x 10) + (1 x 20) = 30
Since 510 is smaller than 26,176, and 30 is smaller than 8,192, it meets
the equation, Σ(α) ≤ (β) and Σ(γ) ≤ (δ), thus ensuring you that 10 pairs of
3390-3 volumes and 20 pairs of 3390-L volumes can be created.
Performance Considerations
• When you operate ShadowImage for z/OS:
SIz operations affect the I/O performance of the USP V storage system
because of the additional write operations to the T-VOLs. You should
consider the relative importance of the storage system’s I/O performance
and the SIz operations when you decide the number or structures of pairs
and their copy pace.
For example, assigning three T-VOLs to each S-VOL takes more resources
than assigning only one or two. More T-VOLs you assign on an S-VOL, the
risk of affection on the I/O performance of the storage system increases.
Note that even you assign only one T-VOL on an S-VOL, pace of the initial
copy operations may affect the I/O performance of the storage system.
Using a slower copy pace minimizes the impact of SIz operations on I/O
performance, while a faster copy pace produces point-in-time copies more
quickly but may affect I/O performance.
• Load sharing of parity groups:
SIz S-VOLs or T-VOLs should not be concentrated in the same parity group.
To disperse workloads of the parity groups, each parity group should have
both S-VOLs and T-VOLs evenly distributed. SIz pairs for which an SIz
operation is performed simultaneously should be in different parity groups.
If SIz pairs are concentrated in only a few parity groups, the host I/O
performance may be degraded. To minimize effect on the host I/O
performance, take the following actions: