Multi-level Raid

Multi-level Raid 2 Agenda Background -Definitions -What is it? -Why would anyone want it? Design Issues -Configuration and User Interface -Expansion -Memory Usage -Virtual Driver Implementation

Multi-level Raid 3 Definitions (according to RAB) Logical Disk - a set of consecutively addressed member disk blocks that is part of a single virtual disk-to- member disk mapping. Logical Volume - A virtual disk made up of logical disks. Also called a virtual disk, volume set, partition. Member Disk - A disk that is in use as a member of a disk array. Virtual Disk - synonym for volume set. Volume set - A collection of user data extents presented to an operating environment as a range of consecutive logical block addresses. A volume set is the disk array object most closely resembling a disk when viewed by the operating environment.

Multi-level Raid 4 Background What is ‘Multi-level Raid’? -Multi-level Raid is the combining of two (or more) levels of Raid algorithms into a single addressable logical unit. -Ex. Raid10, Raid50, Raid30, Raid00, Raid 11, Raid 100 Raid 50 Raid 0 layer Raid 5 layer (Virtual Disk) (Logical Disk) Volume Set (Logical Disk)

Multi-level Raid 5 Background (What) Block 0 Block 1 Block 2 Block 3 Block 4 …. Block 3061 Host AddressRaid 0 Level Block Block Block Parity Raid 5 Level

Multi-level Raid 6 Background (Why) Why use Multi-level Raid? For Performance -For the 3x5 Raid 50 shown, performance is 3x a single Raid 5 For Capacity Without Sacrificing Reliability -A 3x5 Raid 50 can survive 3 failures, while a 12+1 Raid 5 with the same capacity can only survive 1 For Capacity Without Host Involvement -Striping units at the host level is difficult or impossible in some clustered environments

Multi-level Raid 7 Design Issues - Configuration What makes up a raid group? -In our current model a raid group is a set of physical disks on which one or more logical disks is created by partitioning. What makes up a logical disk? -Currently, a logical disk is a slice of a raid group. -Currently, a logical disk is equivalent to a volume set. -A partition is a logical disk. What is a volume set? -A volume set could be a single logical disk -A volume set could be a collection of logical disks -A Raid 50 is a type of volume set, and a type of logical disk -A Raid 5 logical disk, that is part of a Raid 50 is not a volume set.

Multi-level Raid 8 Design Issues -Configuration There are at least 2 approaches for creating Raid 50 volume sets. -Stripe across logical disks (including partitions) > The underlying Raid 5 units can be partitions. In effect you could (not that you’d want to) create a 3x5 Raid 50 on 5 drives. > The resulting volume sets can not be further partitioned > Advantages : more flexibility, physical disk units can be part of a Raid 50 and Raid 5 simultaneously, may support a layered architecture better. > Disadvantages : must keep the Raid 5 logical disks from being visible to the host -Build multi-level Raid Groups > Multiple raid groups are combined into a ‘super raid group’. Raid 50 logical disks are then created by taking slices (partitions) of the super raid group. > Advantages : The Raid 5 groups are never visible to the host > Disadvantages : All space on the physical drives can only be used for Raid 50

Multi-level Raid 9 Design Issues - Configuration Striping across arbitrary partitions seems like the best approach. How should this be presented to a user? -How do we keep the component Raid 5 logical disks from being accessed independently? -The resulting Raid 50 will span multiple Raid groups. How does the GUI hierarchically display that? -How does the user specify the combinations?

Multi-level Raid 10 Design Issues - Expansion How do we handle expansion of Raid groups that hold the component Raid 5 logical disks? -In some ways, there is no issue. > The expansion of a Raid group does not affect the capacity of the Raid 5 logical disks. -The issue is geometry > If the Raid 5 geometry changes, the Raid 0 stripe element size will be more or less efficient. > If the Raid 5 logical disks have different geometry, the way to be efficient is to use a least common multiple approach. Is it possible to expand the Raid 50 volume set? -Its possible, but since it cannot be further partitioned, there may not be reason to allow it.

Multi-level Raid 11 Design Issues - Memory A multi-level Raid design requires data to be reorganized at each level. -Raid 0 layer splits data into 1 or more chunks per logical Raid 5 disk -Raid 5 layer must split each chunk into data for each physical disk This reorganization is an extra computational step A bigger problem is allocating ‘extra’ data buffers. -The upper layer can only allocate what it needs for the host data. -The lower layers must get additional buffers for parity and pre- reads.

Multi-level Raid 12 Design Issues - Virtual Drivers Multi-level Raid is an excellent opportunity to take advantage of the Virtual Driver architecture. The current model is: Cache Driver Lun Driver Raid Driver

Multi-level Raid 13 Design Issues - Virtual Drivers A multi-level Raid driver stack could look like this: Cache Driver Lun Driver Raid Driver Raid Driver / Stripe Driver