Dec 19, 2015

RAC storage option: Why Oracle ASM?

Long story in short - we have been planning for RAC. The database team is in favor of doing ASM for RAC where storage people are trying to push NFS/dNFS. NFS is the filesystem we have for single instance database at the moment, our storage system is NetApp. The DBA team understood the benefits and differentiating factors of ASM for RAC.


I compiled the list of reasons/benefits I had in mind.


Why Oracle ASM?


1. Recommended & predictable: It’s Oracle recommended and most commonly used storage architecture specially for RAC environment. This is mostly exercised option, possibility to hit a bug or any issue is less down this path. Oracle continuously working on improvements, every release comes with new ASM features. Training materials, community discussions, conference sessions, Oracle customer support and Oracle testing most familiar with this option.

2. Understands database file types: ASM is specially designed having database filesystem in mind. It understands the database file types and does striping by types or as instructed. For example, online redo could be striped 128KB where different datafiles could be striped as 1MB or as needed for performance. The redo striping could be vital for concurrency on 'log file sync' for a RAC environment. The other strip size could potentially decrease the database I/O and improve overall performance.

3. Wide range of I/O balancing: ASM rebalances and stripes I/O not only multiple disks but also multiple volume/aggregates removing potential I/O hotspots and ease of space management for large databases. Thus no need to think about I/O balancing and managing storage by volume/file system. Just create a single ASM disk group for data/index combining LUNs from different disk groups and storage controller of same type. Presently in volume based system, we need to worry about which datafiles would need to go to which volume and controller for better space and I/O utilization, which is getting difficult day by day as the database size increases, and will not be possible to do this correctly, even if I overlook the datafiles management overhead. ASM is the solution for this.

4. Convenient for big databases: Oracle Managed Files (OMF) and bigfile tablespaces are best suited for big databases and with ASM.

5. No I/O overhead: ASM does not perform any I/O by itself, so there is no translation layer for Oracle I/O to datafiles into disk block offsets. I/O from databases is directly applied to disk volumes without modification. This reduces overhead and improves performance (point collected from a doc). Something like dNFS I believe.

6. Not tied to underlying storage vendor: If we want to change storage technology (say ZS3) in future, having database on ASM will make things easy and feasible, changing underlying storage technology would be transparent, no visible database migration will be required. After 3 years, the database size would be 25TB+, migrating 25TB+ database to a new storage technology would be a mammoth task, will require tremendous effort and may require several weeks downtime for migration (unless we built another db in parallel). Using ASM won’t lock us down to any storage technology.

7. Optimized by default: 

When we use ASM for RAC, a lot of things are optimized by default and works pretty good without further time and resource investments thiking about I/O performance and all other config tweaking. We found I/O throughput gains about 4 times than NFS on the same storage!

8. Common references: We have not seen mentionable RAC production system without ASM, not from the few dozen people we know. Never seen any Database Architect or Database Administrator discussing RAC and they don’t have ASM on the storage side. It’s very conventional having ASM in storage layer with RAC - that’s the proven path and inlined with RAC.

From the studies of RAC customers (from publicly available sources), it’s clear that most people selected ASM for RAC implementations.

http://www.oracle.com/technetwork/database/features/ha-casestudies-098033.html
http://www.oracle.com/technetwork/documentation/ha-casestudies-083953.html
http://www.slideshare.net/MarkusMichalewicz/oracle-rac-customer-proven-scalalbility
http://www.oracle.com/us/products/middleware/data-integration/odi-customer-reference-booklet-2184014.pdf


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1 comment:

Foued said...

Thank you for sharing this post.