1) When you Tune the Volume small blocks are copied to the new CPG, once all blocks are copied successfully it switches over transparently to the copy and deletes the old blocks. If it fails or task is stopped it just reverts to the old blocks I believe. The servers shouldn't notice this happening at all.
2) Been a while since done this in large scale but when I did I'd maybe select 10 Volumes when tuning the COPY (snapshot data) space, as typically smaller and maybe select 2-3 Volumes when tuning the USER (normal volume data) space, as typically larger. Just keep an eye on performance and if no one notices then select a larger group.
3) Very dependant on system and load, easiest way is to try it out on a test Volume. From what I remember selecting more Volumes went slower then single Volume but the combined load increased on the system, it maybe got noticeable when I tried 4-6 Volumes but that will vary per system.
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