Friday, November 16, 2007

COMSTAR - Solaris as a SCSI (not iSCSI, SCSI) Target

COMSTAR (Common Multiprotocol SCSI Target) allows a server running OpenSolaris to look like a SCSI storage array to another server. Aaron Dailey write about this "disruptive technology" in his blog:
COMSTAR is the last link in being able to create a SCSI storage array using Solaris (actually you could do it before with iSCSI, but COMSTAR is flexible enough to support any transport). With Solaris, you can do RAID in software doing ZFS. CPU cycles are increasing due to multicore technology, especially relative to the speed of disks, so it's realistic to do RAID calculations in software, instead of costly dedicated ASICs. You can attach nearly any kind of disk you want. You have a stable development environment. You can use commodity PC hardware. You want to export some disks as NFS or CIFS, you can do that too. And the code is free, along with a pretty good development environment.
When Project COMSTAR was first announced, Sun's storage software group said:
COMSTAR uses a modular approach to break the huge task of handling all the different pieces in a SCSI target subsystem into independent functional modules, which are glued together by the SCSI Target Mode Framework (STMF). The modules implementing functionality at SCSI level (disk, tape, medium changer etc.) are not required to know about the underlying transport. And the modules implementing the transport protocol (Fibre Channel, iSCSI etc.) are not aware of the SCSI-level functionality of the packets they are transporting. The framework hides the details of allocation, providing execution context and cleanup of SCSI commands and associated resources, and simplifies the task of writing the SCSI or transport modules.
With Solaris, servers are looking more and more like storage arrays. Now it's clear why Sun merged its server and storage group into a "systems group".

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