Database Reference
In-Depth Information
Figure 5-2.
RAID 0+1 vs. RAID 1+0
The difference between RAID 1+0 and RAID 0+1 is that if one disk fails in RAID 0+1, the entire striping set
fails. Then, if another disk in the second mirroring set fails, this array is lost. While in the RAID 1+0 configuration,
as long as both disks in the same mirror set fail, the array is fine. Both these options reduce the disk capacity by
half. This configuration is commonly used for OLTP-type database workload, as this option doesn't impose a heavy
performance overhead for the write operation which is very frequent for OLTP type of workload.
•
RAID 5 for striping blocks with parity on all the disks. To provide error protection, distributed
parity blocks are kept on all the disks. In case of a disk failure, the parity blocks are used to
reconstruct the errant sector. Although this option reduces the capacity of only one disk, it
imposes a heavy performance overhead for write operations, as the parity block has to be
recalculated every time. Considering the cost savings, RAID 5 may be an acceptable configuration
option for a data warehouse-type database, which mostly does read operations and which needs
a significant amount of disk capacity, but it is not a good choice for update-heavy databases
engaged in OLTP workloads.
•
RAID 6 for striping blocks with parity on all the disks. It is similar to RAID 5 except for having
double distributed parity. It provides fault tolerance up to two failed drives.
A RAID array can be implemented as either hardware RAID or software RAID. Hardware RAID operates at
the level of the RAID Controller; the OS does not know anything about it. When the data comes from the OS to the
storage, the RAID controller card takes care of striping or mirroring. In software RAID, the OS takes the responsibility
of striping the data and writing to the appropriate disk as needed. Software RAID is low cost as it doesn't require
special hardware, but it costs the host CPU resource. Hardware RAID doesn't consume any host CPU, but there is an
additional cost for the RAID controller card. The external storage used for shared storage in Oracle RAC Database
usually uses SAN or something similar. The storage controller of these storage systems provides hardware RAID
functionality and supports different kinds of disk RAID configuration such as RAID 0, 1, 1+0, and 5, etc. It is a part
of the storage design and configuration to create a disk RAID configuration (called the RAID group in some storage
product terminologies) based on a set of individual physical disks. Then, data volumes or logical unit numbers (LUNs)
can be created on these RAID configurations or groups.
Figure
5-3
shows an example of the storage configuration. Table
5-1
lists the design of the RAID groups and the
storage volumes associated with these RAID groups. In this example, there are three RAID 10 RAID groups—DataG1,
DataG2, and DataG3—and two RAID 1 RAID groups—RedoG1 and RedoG2. There are eight storage volumes or LUNs:
Data1-Data3, OCR1-OCR3, and Redo1-Redo2. These volumes are used as the storage LUNs for database files, OCT,
voting disk files, and Redo logs, as indicated by their names. Figure
5-3
shows an example of the actual configuration of
storage volumes on a storage Controller Management GUI tool called Dell PowerVault Modular Disk Storage Manager
(MDSM). Although different storage vendors have different storage management tools, they provide similar functionality
that allows storage administrators to create storage volumes based on physical disks with certain RAID configurations.
