When you have 2 copies of a LV (in mirrored configuration), you can choose and change dynamicaly the read copy of the logical volume corresponding to the copy on local storage, instead going through SAN ISL:

On each logical volume, search which copy of data is on local disks Ex:

[root@testaix]/root# lslv -m hd8
hd8:N/A
LP    PP1  PV1               PP2  PV2               PP3  PV3
0001  0290 hdisk11           0290 hdisk12

[root@testaix]/root# lspv -u
hdisk11         00fa343c89c7eec0          rootvg          active      33213600507681234567890000000000001404214503IBMfcp         f9afa638-691d-25d0-0495-5d6f902ea8f3
hdisk12         00fa343c89c8037e          rootvg          active      33213600507681111111110000000000000204214503IBMfcp         f3ffdf11-1f8c-cc5b-49de-b74f8bbcf573

My preferred read copy is automaticaly selected (copie 1) (the value can be 0, 1, 2, or 3)

[root@testaix]/root# lslv hd8
LOGICAL VOLUME:     hd8                    VOLUME GROUP:   rootvg
...
INFINITE RETRY:     no                     PREFERRED READ: 0

As my local disk has the storage ID 60050768111111111, then I 'll choose hdisk12 which correspond to copy number 2

[root@testaix]/root# chlv -R 2 hd8
[root@testaix]/root# lslv hd8
LOGICAL VOLUME:     hd8                    VOLUME GROUP:   rootvg
...
INFINITE RETRY:     no                     PREFERRED READ: 2

Different parameters can be changed to increase disk performance:

• Split a VG on multiple disks (for big capacity)
• change the LV parameter to use a maximum of disks
• change the queue_depth parameter to 10 and check during high IO to see if iostat report a value lower than 100% of disk use

To be sure to split a VG (for intersite mirroring) between 2 DS8000, you can use mirrorpool, to specify the right disk location.