(S2D) S2D with CSV over Fibre Channel

1. SCSI in S2D with CSV over Fibre Channel (FC)

When you use Fibre Channel for the storage connectivity in an S2D setup (specifically with CSV volumes), you're dealing with a high-speed, block-level communication protocol between your storage devices and the servers in the cluster.

• SCSI (Small Computer System Interface) is a protocol used for block-level communication between the host (server) and storage devices. Even when the storage is accessed over a Fibre Channel (which is a high-speed network used to connect storage devices), the underlying protocol for data transfer and block-level access still relies on SCSI commands.
• Fibre Channel (FC) is simply the transport mechanism used to connect the server to the storage devices, but the actual data access happens through SCSI commands.
  • Fibre Channel provides the physical layer (the hardware and network transport) that connects the nodes to storage, and SCSI is the protocol that governs how the storage is accessed at the block level.

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2. How SCSI Works with S2D CSV Over Fibre Channel

In a S2D cluster with CSV, the cluster nodes share access to the same volume. Here’s how the components fit together:

• SCSI commands: When you write or read data from a disk, the operations are performed using SCSI commands. These are low-level instructions sent to the storage devices that instruct them to perform operations like reading, writing, or erasing data blocks.
• Fibre Channel (FC): This is the physical link or network that connects the servers in the cluster to the shared storage devices. It allows block-level access to the disks. Fibre Channel provides high-speed, low-latency access between the servers and storage devices, but the protocol for accessing the data still relies on SCSI commands.
• S2D with CSV: In the context of S2D, when you create a CSV volume, this volume is stored on the S2D pool of drives. While the Fibre Channel provides high-speed connectivity to the shared storage, the block-level operations for accessing the data on these drives are still managed through SCSI commands.
  • CSV is simply a logical layer that enables multiple nodes in the cluster to access the same shared volume simultaneously. It handles access control, locking, and ensuring consistency of the volume across all nodes.
  • When a node needs to access a CSV volume, the data transfer is done using SCSI commands, even if the storage devices are physically connected over Fibre Channel.

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3. SCSI over Fibre Channel in S2D with CSV

Here’s how the SCSI protocol operates in this setup:

• Block-Level Storage Access: SCSI remains the protocol for block-level access to the storage. This is the core protocol that S2D uses to perform read/write operations.
• Transport Layer (Fibre Channel): While SCSI handles the data transfer and operations on the volume, Fibre Channel provides the transport layer that enables high-speed, low-latency connections between the servers and the storage.
  • The Fibre Channel network transmits the SCSI commands between the servers and the storage devices. In essence, the SCSI protocol runs over the Fibre Channel infrastructure. You can think of Fibre Channel as the "high-speed highway", and SCSI as the "rules of the road" for block-level data access.
• Cluster Shared Volumes (CSV): When a volume is exposed via CSV in the cluster, it becomes accessible to multiple nodes simultaneously. The data access still happens at the block level, using SCSI commands, with the added complexity of ensuring data consistency and coordination across the cluster (which CSV and S2D handle).

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4. The Protocol Stack (Simplified)

Let’s visualize how the protocol stack works in S2D with CSV over Fibre Channel:

• Application Layer (e.g., your application or file system)
• SMB3 or iSCSI (used for sharing or accessing files over the network)
• Cluster Shared Volumes (CSV) (manages shared access to the volume)
• SCSI (block-level protocol for read/write commands to storage)
• Fibre Channel (FC) (transport network providing high-speed connections for SCSI commands between servers and storage)

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5. Key Points to Remember

• Fibre Channel provides the physical network transport for accessing storage, but the SCSI protocol handles the block-level communication.
• SCSI commands are used for low-level operations like reading and writing data on disks, even when those disks are connected over Fibre Channel.
• S2D with CSV makes use of SCSI for data access at the block level, and CSV ensures that the shared volume can be accessed by multiple nodes simultaneously.
• Fibre Channel only provides the connection for transmitting SCSI commands over a high-speed network, allowing for low-latency and high-bandwidth data access between the nodes and storage.

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Summary

• Yes, SCSI is still used in an S2D with CSV setup over Fibre Channel, but Fibre Channel only serves as the transport layer for SCSI commands.
• The SCSI protocol governs the low-level data operations between the servers and the storage devices, and Fibre Channel provides the high-speed network for those SCSI operations to take place.
• S2D with CSV enables shared access to volumes across multiple nodes, and all block-level access (including data mirroring and resiliency) relies on SCSI.