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Disk Mirroring tips

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Storage Array or Hardware provides mirroring of disks or disk partitions and protects against the possible loss of disks and disruption to storage availability. With hardware mirroring, the operating system sees only one device and performs only one write to it. The hardware is responsible for ensuring that the write is directed to all mirrors and is logically atomic.

Mirroring can also be implemented at host level by using the volume management software. When implemented at the host level, with software mirroring, there is the possibility of a system failure occurring after a write has completed on one side of the mirror, but before it has reached the other(s). Therefore, to ensure that mirror consistency can be re-established reasonably quickly after a crash, software mirroring maintains a dirty region log on disk. This identifies the mirror regions that have been written to recently, and thus might be inconsistent in the event of a system failure. The maintenance of a dirty region log adds a major performance overhead to software mirroring. In the past, Oracle has worked with some LVM vendors to eliminate the need for dirty region logs on mirrored Oracle Database files, but this facility is only available on some platforms and introduces extra complexity.

When hardware mirroring is not available, ASM provides the best possible mirroring. No dirty region logs are needed, as Oracle manages to restore consistency between the mirrors using its own redo log data.

Load balancing (SAME)

Oracle has been promoting the concept of SAME ? Stripe and Mirror Everything which aimed at using the full I/O capacity of the all disks. Many database users have implemented SAME sets, each of about 4 mirrored disk pairs. However the SAME functionality does not completely eliminate the disk imbalances. There are different types of files where I/O activity is not uniform, and the configuration of SAME is not the cure for all I/O ills.

ASM takes an additional step in balancing I/O activity. Rebalancing distributes file data evenly across all the disks of the ASM Disk Group. ASM automatically rebalances a disk group whenever disks are added or dropped. ASM ensures that a file is evenly spread across all disks in a disk group when the file is allocated, so rebalancing is not required except when the storage configuration changes. With the I/O balanced whenever files are allocated and whenever the storage configuration changes, there is no need to search for hot spots in a disk group and manually move data to restore a balanced I/O load. It also allows the addition of disks online. There are extra background processes that conduct the rebalancing activity in a non-intrusive way. Thus, ASM makes manual load balancing unnecessary because the dynamic rebalancing overcomes the major drawback of SAME. When new disks are added to a disk group, rebalancing happens automatically without an outage.

ASM is the new best practice for Storage Management

The use of ASM files provides all above mentioned features without any cost and administrative inconvenience. Above mentioned characteristics are derived from the ASM methodology inherently.

Another important benefit that accrues is the use of ASM for RAC database. ASM can very well avoid the use of Cluster File System as the ASM resources (disks and files) are sharable among the RAC instances and fits into the shared storage architecture.

Besides above advantages, there are other benefits that include:

* Ease of administration -- ASM removes the need to specify and manage filenames since ASM knows to place specific data files into specific disk groups. In ASM, every new file automatically gets a new unique name, which eliminates the concern about possibly using the same filename in two different databases.

* ASM replaces external volume managers and file systems. ASM includes storage reliability features, such as mirroring. The storage reliability policy is applied on a file basis, rather than on a volume basis. Hence, the same disk group can contain a combination of files protected by mirroring, parity, or not protected at all.

* ASM provides the performance of raw disk I/O without the problems of managing raw disks. Unlike logical volume managers, ASM maintenance operations do not require that the database be shut down.


Mirroring is the simplest way to gain data redundancy to safeguard data against disk failure.  In mirroring, two or more (minimum of three in LVM) disks or partitions are combined and identical data is written to each device.  The space available is only as large as the smallest device in the mirror.

Some implementations of mirroring offer increased read performance, but write performance is typically similar to traditional disk performance.  What mirroring does offer is the ability to keep running or recover more quickly as a redundant copy of the data is kept online and up-to-date.

RAID and RAID Levels

Mirroring and striping are both forms of RAID(Redundant Array of Inexpensive Disks) methods which allow more than one physical device to be combined to achieve redundancy and/or performance.  Often people talk about RAID levelsfor disk.  Below is a short description of common RAID levels.  Many of these are not available through LVMand most, other than levels 0 and 1, are typically implemented in hardware in external storage devices rather than in software solutions like LVM.

RAID Level Description Redundancy Space Available
RAID 0 Striping - Data is divided evenly between two or more devices.  Same as described above. No Sum of all devices combined
RAID 1 Mirroring - Data is duplicated between two or more devices for redundancy.  Same as described above. Yes The size of a single device
RAID 4 Not typically used.  Data is divided on three or more devices, one of which contains a parity so if any single device fails, its contents can be recreated from the other devices and the parity. Yes Sum of all devices minus one  (e.g., If four 10g disks are used, 30g will be available for use)
RAID 5 Same as RAID 4, but the parity is distributed evenly among all disks in the group, thereby preventing the single parity disk from becoming a bottleneck. Yes Sum of all devices minus one  (e.g., If four 10g disks are used, 30g will be available for use)


RAID 6 Though loosely defined, typically RAID 6 is similar to RAID 5 but two parities are kept to protect against up to two simultaneous disk failures.  Four or more devices are typically combined. Yes Sum of all devices minus two  (e.g., If seven 10g disks are used, 50g will be available for use)
Table 12.2:   Common RAID Levels

The parity used in RAID levels 4 and 5 is a value that indicates if bits from each disk which data is written to add up to odd or even.  Since this data is binary, meaning it can only have a value of 1 or 0, use this single parity bit to determine if any missing bit should be a 1 or a 0 by adding all the known bits and comparing it to the parity.

In many modern applications, RAID levels can also be combined.  A common disk configuration for Oracle databases may be RAID 10 or RAID 1+0, which is a combination of devices that have been mirrored for redundancy, then striped together for improved performance.  Similarly, RAID 0+1 can refer to a configuration in which disks have been striped together, and then data is mirrored between two stripes.

Other RAID levelsand combinations exist, but the most common ones have been described here.  RAID levels more complicated than mirroring and striping can most efficiently be implemented in hardware.  Some server systems include hardware to implement RAID on internal disks and most external enclosures and SAN (Storage Area Network) solutions support it independent of the server.



This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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