To explain this clearly, it helps to think of the ESXi host as a translator. The Virtual Machine speaks "File Language" (I want to save this document), and the Storage Array speaks "Block Language" (I want to change these specific bits on this specific shelf).
Here is the step-by-step breakdown of how that translation happens.
1. The "Write" Process (Saving Data)
When an application saves a file, the data travels down a "stack" of layers.
Step A: The Virtual Machine (Guest OS)
The application sends a write request to the Guest OS (Windows/Linux). The OS thinks it is writing to a physical SCSI hard drive. It sends the data to its virtual disk controller.
Step B: The Hypervisor (ESXi Host)
The ESXi host intercepts this request. It looks at the .vmdk file (the virtual disk) and determines where that data belongs within the VMFS Datastore.
Step C: The Storage Stack (PSA)
The host's Pluggable Storage Architecture (PSA) decides which physical path (cable/switch) to use. It then wraps the data in a transport protocol like iSCSI (TCP/IP) or Fibre Channel.
Step D: The Fabric (The Network)
The data travels through switches to the Storage Array.
Step E: The Storage Array (LUN)
The Array Controller receives the data, acknowledges it (often placing it in a high-speed cache first), and finally writes it to the physical disks that make up the LUN.
2. The "Read" Process (Retrieving Data)
Reading is about finding the exact "address" of data and bringing it back.
Step A: The Request
The VM asks for a specific block of data.
Step B: The Metadata Lookup
ESXi checks the VMFS Metadata. Since a LUN is just a big pool of storage, ESXi needs to know exactly which physical blocks on that LUN correspond to the "Address 500" the VM is asking for.
Step C: The Command
The host sends a SCSI "Read" command across the network to the Storage Array.
Step D: The Retrieval
The Storage Array pulls the data from its disks or its internal cache.
Step E: Delivery
The data is sent back to the ESXi host, which "unwraps" it and presents it to the VM's operating system as if it just came off a local spinning disk.
Key Concepts to Remember
To keep it simple, remember these three roles:
The VM: Asks for "Files."
The ESXi Host: Translates "Files" into "Blocks" and handles the traffic.
The LUN: The physical "Storage Bin" where the blocks actually live.
Why does this matter?
If any layer in this stack is slow (the network is congested, or the disks in the LUN are too slow), the application inside the VM will experience Latency. This is why "Storage Performance" is usually the biggest bottleneck in virtual environments.
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