IBM SAN Volume Controller

The SAN Volume Controller (SVC) is in-band virtualization and combines disk subsystems from multiple vendors into a single reservoir of capacity that can be managed from a single point. While the initial disk configuration and presentation must be made using the utility unique to each type of subsystem, once defined all the disks are managed centrally.
The SVC has the ability to tranparently migrate, copy and mirror data, as well as providing a local disk cache and automated storage tiering.

The SVC can enable a tiered storage environment by physically connecting the different classes of storage devices together, and configuring them as storage pools with different availability and cost characteristics. The storage tiers are Flash Storage, Enterprise Storage, Cloud and Nearline Storage. It is possible to automate the movement of data between these tiers by purchasing an extra facility called Easytier. If you have it licenced and you have more than one of the above classes of storage, then Easytier will automatically start moving data between tiers depending on how active it is. If you have just one class of storage then Easytier will run in balancing mode.

The image below shows the principal components of the SVC. Managed Disks (MDisks) correspond to LUNs on the disk subsystems, and are combined together into Managed Disk Groups (MDG). Generally each MDG will have different performance and price characteristics. The MDisks are split up into extents, and the extents are combined together to form Virtual Disks (VDisks). An extent is a fixed size block of data, which you specify when you define the MDG. Extents can vary in size from 16KB to 8GB, with a default of 1GB. You pick the extent size that best suits your application needs. The VDisks are then presented to the hosts as SCSI, FCoE or FC disks. VDisks can be full size or they can be space efficient. You can also mirror a VDisk inside the SVC I/O group for resilience. An SVC cluster is a collection of SVC nodes. These are grouped into pairs called I/O groups. A cluster contains between 1 and 4 I/O groups and so between 2 and 8 SVC nodes. Each I/O group owns one or more MDGs and their respective VDisks Each IO group can connect to up to 1024 iSCSI or 2048 FC hosts.
SAN zoning is critical to the operation of an SVC Cluster. The physical disks must be kept in a separate zone from the hosts, and each host should be it its own zone with its VDisks You may also want to define a SAN zone for the SVC nodes in the cluster to allow them to communicate together. This is explained in more detail in the SVC zoning page

The SVC can support advanced copy services between unlike devices, either high-cost to low-cost or between different vendors. The copy services facilities are FlashCopy, Metro Mirror and Global Mirror, referring to copy within an SVC cluster and synchronous and asynchronous copies between remote SVC clusters. The smallest unit of granularity for Flashcopy is the virtual disk. It is possible to flash data between different MDGs and so to different classes of disk, but the virtual disks must be identically sized. Metro Mirror is analogous to PRRC remote copy and supports distances up to 10km apart. Again it is possible to mirror data between different classes of disk, but the virtual disks must be identically sized. It is possible to take FlashCopy snapshots from local disk to the Cloud.

The SVC can insulate host applications from changes to physical storage. This means that physical storage can be added or removed transparently. The SVC includes a dynamic data migration facility to move data between physical volumes and multi-pathing software for failover and load balancing. It is possible to move data around inside the SVC by VDisk, MDisk or at extent level. If you want to migrate data onto the SVC, define a VDisks of the same size as the LUN you want to copy in image mode, then just associate the VDisk with that LUN. The data is now part of the SVC and that is the only way to attach external disks to the SVC without destroying the data.

The SVC is hosted on dedicated, rack mounted SVC 2145 controllers. An I/O group is a pair of servers and up to four I/O groups can be clustered for resilience and bandwidth. Each cluster of two to eight servers supports up to 8192 virtual volumes. It incorporates an SMIS standard API and supports LAN free and Server free backups. The SVC can run in a 'stretched' configuration where the nodes are split between two geographically dispersed sites.

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