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While reading this asm concepts i most of the time i came across stripping and mirroring.
But i am not getting clear idea about this stripping and mirroring.
Could you please give me clear idea about this to understand this
thanks for you help...
What are you reading?
Let's say you have a 30 MB file and 2 hard drives.
Mirroring duplicates the data to provide data redundancy. If one disk fails, the other one can be used to retrieve the data. This works transparently, without user intervention.
Disk 1: 30 MB
Disk 2: 30 MB
Stripping distributes the data accross storage media to increase the total storage capacity. There is no redundancy, if one disks fails, the file is lost and recover has to be performed from external backup. The storage is also not usable anymore if one disk member is lost.
Disk 1: 15 MB
Disk 2: 15 MB
Besides storage capacity and data redundancy there are also performance aspects since multiple disks are involved. A mirror will typically provide increased read performance since data can be read from 2 drives, but write performance will be the same than writing data to a single drive. Stripping will provide better read and write performance as the workload is split over multiple hardware. Stripping and Mirroring is often combined but an expensive storage solution since you need to have a least 4 disks.
Keep in mind that Oracle ASM provides data redundancy that seems similar to typical RAID 1 (mirroring) and 0 (stripping) solutions, but works fundamentally different. RAID works on the basis of disk data blocks. ASM woks on the basis of files and using the free disk space of disk failure groups. ASM can also provide a 2-way mirroring using 3 disks, in which case data is stored in a round robin fashion.
Follow an article that explain RAID concepts.
When using ASM, you can do the mirroring using NORMAL REDUNDANCY in ASM DISKGROUPS.
Oracle x RAID - Best Practices
ASM stripes files across all the disks within the disk group thus increasing performance, each stripe is called an ‘allocation unit’. ASM offers two types of stripping which is dependent on the type of database file
Coarse Stripping which is used for datafile, archive logs (1MB stripes)
Fine Stripping which is used for online redo logs, controlfile, flashback files(128KB stripes)
Disk mirroring provides data redundancy, this means that if a disk were to fail Oracle will use the other mirrored disk and would continue as normal. Oracle mirrors at the extent level, so you have a primary extent and a mirrored extent. When a disk fails, ASM rebuilds the failed disk using mirrored extents from the other disks within the group, this may have a slight impact on performance as the rebuild takes place.
All disks that share a common controller are in what is called a failure group, you can ensure redundancy by mirroring disks on separate failure groups which in turn are on different controllers, ASM will ensure that the primary extent and the mirrored extent are not in the same failure group. When mirroring you must define failure groups otherwise the mirroring will not take place.
There are three forms of Mirroring
External redundancy - doesn't have failure groups and thus is effectively a no-mirroring strategy
Normal redundancy - provides two-way mirroring of all extents in a disk group, which result in two failure groups
High redundancy - provides three-way mirroring of all extents in a disk group, which result in three failure groups