HP P2000 G3 MSA Technical White Paper page 6

Technical white paper | HP P2000 G3 MSA
Shared LUN number between controllers with a maximum of 512 LUNs
No duplicate LUNs allowed between controllers
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Either controller can use any unused logical unit number
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ULP recognizes which paths are "preferred"
The preferred path indicates which is the owning controller per ALUA specifications
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"Report Target Port Groups" identifies preferred path
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Performance is slightly better on preferred path
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Write I/O Processing with ULP
Write command to controller A for LUN 1 owned by Controller B
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The data is written to Controller A cache and broadcast to Controller A mirror
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Controller A acknowledges I/O completion back to host
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Data written back to LUN 1 by Controller B from Controller A mirror
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Figure 1. Write I/O Processing with ULP
Read I/O Processing with ULP
Read command to controller A for LUN 1 owned by Controller B:
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– Controller A asks Controller B if data is in Controller B cache
– If found, Controller B tells Controller A where in Controller B read mirror cache it resides
– Controller A sends data to host from Controller B read mirror, I/O complete
– If not found, request is sent from Controller B to disk to retrieve data
– Disk data is placed in Controller B cache and broadcast to Controller B mirror
– Read data sent to host by Controller A from Controller B mirror, I/O complete
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