Compare Oracle High Availability to Microsoft SQL Server 2008

一份全面对比Oracle与SQL Server高可用性的文档，虽然是Oracle自己搞的，不过也列出了SQL Server 2008一些高亮特性:

SQL Server 2008 B&R Highlights

• New Backup Compression
  • Oracle has offered backup compression since Oracle Database 10g
• Restore Read-only Database Filegroups without Transaction Logs
  • Oracle has supported this with read-only tablespaces for years
• Restore Individual Data Pages
  • Oracle has offered Block Media Recovery since Oracle9i
• Piecemeal & Online Restore, Mirrored Backups, Data Page & Backup Checksum
  • Oracle has offered these features for years..

Agenda

• Oracle High Availability Overview
• Better Data Protection, Availability and ROI
  • Database and Site Failures
  • Human Error
  • Data Failures
• Less Downtime for Planned Maintenance
  • Rolling Database Upgrades
  • Online Database Maintenance
• Resources

11g Enhancements include:
Fast Mirror Re-sync
Changes tracked when disk offline and re-sync’d when back online
Automatic repair
Using good blocks from mirror copy
Preferred Mirror Read
Read from the closest disk, even if it is not the primary mirror side
Fast VLDB Start
Larger extent sizes automatic for large files (16x reduction in memory)
ASM Rolling Upgrades
Instances in an ASM cluster can be upgraded independently.

Problem:
In extended clusters, disk locality affects read performance
In 10g ASM always reads from the primary mirror side

Solution:
Perform reads from the closest disk, even if it is not the primary mirror side
Controlled by ASM_PREFERRED_READ_FAILURE_GROUP

Problem:
Very large databases take a long time to startup as the instance loads a huge extent map
VLDB extent maps consume large chunks of SGA
10g ASM files consume 8 bytes of SGA per 1MB extent of file space
1MB extent file size limits:
35 TB external, 5.8 TB normal, 3.9 TB high redundancy
Solution:
For large files, automatically use larger extent sizes
Memory consumption reduced by factor of 16
64MB extent file size limits:
140 PB external, 32PB nornam, 15PB high redundancy
No manual admin required

When a normal read from an ASM disk group fails with an I/O error ASM satisfies the write from a mirrored extent in Normal and High redundancy mode. ASM attempts to remap the extent where the read failed by relocating it to another allocation unit on the same disk with the contents from the good mirror. If the remap succeeds on the same disk, the old allocation unit is marked as unusable. This process happens automatically only on blocks that are read. It is possible that some blocks and extents on an ASM disk group are seldom read; e.g. the secondary extents. The ASMCMD remap invokes bad block remapping for the specified blocks. One can use the ASMCMD remap command if the storage array returns an error on a physical block, then the ASMCMD repair can initiate a read on that block to trigger the repair.
ASMCMD> asmcmd remap DATA DATA_00015000-7500

———————
Problem:
Currently, when ASM offlines a disk due to I/O errors, the disk is immediately dropped from the disk group
In the case of a temporary failure, the disk must be added back to the disk group which requires a full rebalance
Solution:
Allow disks to be in an offline state during which missed writes are tracked
Only the relevant changes will be synchronized from the mirror copies when the disk is brought back online

ASM Fast Disk Resync Overview
ASM fast disk resync significantly reduces the time required to resynchronize a transient failure of a disk. When a disk goes offline following a transient failure, ASM tracks the extents that are modified during the outage. When the transient failure is repaired, ASM can quickly resynchronize only the ASM disk extents that have been affected during the outage.
This feature assumes that the content of the affected ASM disks has not been damaged or modified.
When an ASM disk path fails, the ASM disk is taken offline but not dropped if you have set the DISK_REPAIR_TIME attribute for the corresponding disk group. The setting for this attribute determines the duration of a disk outage that ASM tolerates while still being able to resynchronize after you complete the repair.
Note: The tracking mechanism uses one bit for each modified allocation unit. This ensures that the tracking mechanism is very efficient.

———————————–
Rolling upgrades:The ASM rolling upgrade feature enables you to independently upgrade or patch ASM nodes in a cluster without affecting database availability, thus providing greater flexibility and uptime. This feature is available starting from Oracle Database 11g forward.
Before patching or upgrading ASM, you must place the ASM cluster in a ‘rolling migration’ mode. This enables the ASM instance to operate in a multi-version environment.
ALTER SYSTEM START ROLLING MIGRATION TO number;
Now you can shut down each ASM instance and perform the software upgrade. The ASM instance can rejoin the cluster after the upgrade because it is in rolling migration mode. After all the nodes are successfully upgraded, you can end the rolling migration mode to return to full functionality normal operation.
ALTER SYSTEM STOP ROLLING MIGRATION;
You can also use the same procedure to roll back (down grade) nodes if you encounter problems with the migration. The migration fails if there are rebalancing operations occurring anywhere in the cluster. You must wait until the rebalance completes before attempting to start a rolling migration.

© 2010, www.oracledatabase12g.com. 版权所有.文章允许转载,但必须以链接方式注明源地址,否则追究法律责任.

相关文章 | Related posts:

1. Oracle Database 11g: High Availability Student Guide
2. Compare and Converge in an Oracle Streams Replication Environment
3. SQL Server vs Oracle
4. Know about Oracle High Water Mark
5. Resolving High CPU Usage on Oracle Servers
6. OCP-8i DBA:Getting Started with the Oracle Server
7. Oracle Database Client / Server / Interoperability Support Between Different Oracle Versions
8. Design for Higher Availability and Faster Recovery Lawrence To Center of Expertise Worldwide Customer Support Oracle Corporation October 18th,
9. Step By Step guide for installing Oracle RAC 10gR2 on Windows (2003 and 2008)
10. Step By Step guide for installing Oracle RAC 11gR2 on Windows (2003 and 2008)