1.6.6 Managed disks 1.6.7 Quorum disks A  managed disk  (MDisk) refers to the unit of storage that IBM Storwize V5000 Gen2  virtualizes. This unit can be a logical volume on an external storage array that is presented to  the IBM Storwize V5000 Gen2 or a (traditional or distributed) RAID array that consists of  internal drives. The IBM Storwize V5000 Gen2 can then allocate these MDisks into storage  pools.  An MDisk is invisible to a host system on the storage area network because it is internal to the  IBM Storwize V5000 Gen2 system. An MDisk features the following modes: Array mode MDisks are constructed from internal drives by using the RAID functionality.  Array MDisks are always associated with storage pools. LUNs that are presented by external storage systems to IBM Storwize V5000 Gen2 are  discovered as unmanaged MDisks. The MDisk is not a member of any storage pools,  which means that it is not used by the IBM Storwize V5000 Gen2 storage system.  Managed Disks are LUNs, which are presented by external storage systems to an IBM  Storwize V5000 Gen2, that are assigned to a storage pool and provide extents so that  volumes can use them. Any data that might be on these LUNs when they are added is  lost. Image MDisks are LUNs that are presented by external storage systems to an IBM  Storwize V5000 Gen2 and assigned directly to a volume with a one-to-one mapping of  extents between the MDisk and the volume. For more information, see Chapter 6, “Volume  configuration” on page 309.  A  quorum disk  is an MDisk that contains a reserved area for use exclusively by the system. In  the IBM Storwize V5000 Gen2, internal drives can be considered as quorum candidates. The  clustered system uses quorum disks to break a tie when exactly half the nodes in the system  remain after a SAN failure. The clustered system automatically forms the quorum disk by taking a small amount of space  from an MDisk. It allocates space from up to three different MDisks for redundancy, although  only one quorum disk is active. To avoid the possibility of losing all of the quorum disks because of a failure of a single  storage system if the environment has multiple storage systems, you must allocate the  quorum disk on different storage systems. You can manage the quorum disks by using the  CLI. IP quorum base support provides an alternative for Storwize V5000 IBM HyperSwapfi  implementations. Instead of Fibre Channel storage on a third site, the IP network is used for  communication between the IP quorum application and node canisters in the system to cope  with tie-break situations if the inter-site link fails. The IP quorum application is a Java  application that runs on a host at the third site. The IP quorum application enables the use of  a lower-cost IP network-attached host as a quorum disk for simplified implementation and  operation.  22  Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.2.1 GLYPH<SM590000> Image GLYPH<SM590000> Managed GLYPH<SM590000> Unmanaged GLYPH<SM590000> Array