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Virtual SCSI Raid Adapter Disclosure Number: IPCOM000129162D
Original Publication Date: 2005-Sep-29
Included in the Prior Art Database: 2005-Sep-29
Document File: 2 page(s) / 98K

Publishing Venue



AIX* supports virtual devices, such as a virtual SCSI (Small Computer System Interface) adapter. The vSCSI (virtual SCSI) adapter is a virtualized version of a standard SCSI adapter that sends requests to another partition, which has access to the physical disk I/O resources. The current implementation of the vSCSI adapter does not support RAID(Redundant Array of Independent Disks) capabilities. The only way a vSCSI client partition can have access to a RAID configuration is by having RAID capable hardware on the VIOS(Virtual I/O Server), the target partition. By using software to simulate a physical RAID adapter on the client side of a virtual SCSI configuration, the VIOS does not need to have a physical adapter or storage subsystem with RAID capabilities to support the client’s needs. This is an advantage, because historically, RAID hardware is considerably more expensive than the non-RAID version.

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Virtual SCSI Raid Adapter

From a user's perspective, the virtual SCSI RAID adapter would be very similar to the current virtual SCSI adapter. The client partition would still see the drives exported by the server; however, the drives would be available as array candidate drives and not virtual drives. In other words, the drives are not usable until the user specifies the RAID configuration by which the drives will be used: Raid0, Raid1, Raid10, Raid5...etc.

Because the virtual SCSI raid adapter is virtual, it has no physical memory; it would use a pre-specified amount of the client system's memory as its I/O cache.

The virtual RAID adapter device driver would be responsible for handling the adapter's cache, and implementing the I/O operations, and such RAID operations as calculating parity and array to drive mappings.

The diagram below shows the path of an I/O operation on a non-raid VSCSI configuration.

    vSCSI adapter


vSCSI Client



    SCSI adapter


 physical SCSI disks

virtual disks

This next diagram shows the virtual SCSI raid implementation. The application issues an I/O operation to a raid array. The I/O first accesses the raid cache associated with the virtual SCSI raid adapter. The operation is handled by the vscsi raid adapter, which determines the number of array candidate drives that need to be accessed to handle the I/O request, and calculates the parity if necessary. Multiple I/Os, one per physical disk on the VIOS, are then sen...