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  Oracle Database Tips by Donald Burleson

Finding the source of bandwidth bottlenecks

The first step in applying SSD to RAC is to locate the root cause of the I/O contention. The most common cause of I/O bandwidth saturation in Oracle RAC is a poorly performing I/O subsystem. However, more detailed exploration is warranted to see what specific data files contribute to the saturation of the storage. The source of Oracle I/O saturation may be due to one of these causes:

  • Non-database processes read from the same device as Oracle data files
  • Another database sharing the same file systems (Oracle RAC)
  • A poorly tuned I/O subsystem (e.g. RAID5 for high-update data files).

There are two approaches to I/O monitoring Oracle RAC for disk bandwidth bottlenecks:

Monitor enqueues at the disk level - Use OS tools, such as iostat of vendor-based disk monitors

Monitor at the Oracle instance level - Use an AWR or STATSPACK report to monitor buffer busy waits and high access times for each instance.

As a review, the hallmark feature of RAC is the ability for many Oracle instances to simultaneously read the Oracle files. This complicates the process of locating high concurrent access data files because the file I/O on each of the RAC instances must be interrogated.

Because Oracle file I/O is measured at the instance level, finding bandwidth bottlenecks may mean reading AWR reports on dozens of RAC nodes. To simplify the process of identification, most Oracle professionals will measure I/O at the disk level using native I/O monitors such as SAR and iostat. They would then look for disk enqueues, a condition where read/write requests are waiting for access to the disk.

In the example below, a SAR command was issued to locate the disk enqueues:

root > sar –d –f /var/adm/sa/sa16

In the SAR output, review the avque column seeking high device backlogs:

SunOS prod1 5.6 Generic_105181-23 sun4u 05/16/01

01:00:00 device %busy avque r+w/s blks/s avwait avserv

sd22 100 72.4 2100 2971 0.0 87.0
sd23 0 0.0 0 0 0.0 0.0
sd24 0 0.0 0 0 0.0 0.0
sd25 100 72.4 2100 2971 0.0 87.0


Because of the transient nature of disk enqueues, many Oracle professionals also use disk vendor specific time-series tools (e.g. the EMC Symmetrics console) to track disk enqueues over time.


The above book excerpt is from:

Oracle RAC & Grid Tuning with Solid State Disk
Expert Secrets for High Performance Clustered Grid Computing

ISBN: 0-9761573-5-7
Mike Ault, Donald K. Burleson

http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm  


Market Survey of SSD vendors for Oracle:

There are many vendors who offer rack-mount solid-state disk that work with Oracle databases, and the competitive market ensures that product offerings will continuously improve while prices fall.  SearchStorage notes that SSD is will soon replace platter disks and that hundreds of SSD vendors may enter the market:

"The number of vendors in this category could rise to several hundred in the next 3 years as enterprise users become more familiar with the benefits of this type of storage."

As of January 2015, many of the major hardware vendors (including Sun and EMC) are replacing slow disks with RAM-based disks, and Sun announced that all of their large servers will offer SSD.

Here are the major SSD vendors for Oracle databases (vendors are listed alphabetically):

2008 rack mount SSD Performance Statistics

SearchStorage has done a comprehensive survey of rack mount SSD vendors, and lists these SSD rack mount vendors, with this showing the fastest rack-mount SSD devices:

manufacturer model technology interface performance metrics and notes
IBM RamSan-400 RAM SSD

Fibre Channel
InfiniBand

3,000MB/s random sustained external throughput, 400,000 random IOPS
Violin Memory Violin 1010 RAM SSD

PCIe

1,400MB/s read, 1,00MB/s write with ×4 PCIe, 3 microseconds latency
Solid Access Technologies USSD 200FC RAM SSD

Fibre Channel
SAS
SCSI

391MB/s random sustained read or write per port (full duplex is 719MB/s), with 8 x 4Gbps FC ports aggregated throughput is approx 2,000MB/s, 320,000 IOPS
Curtis HyperXCLR R1000 RAM SSD

Fibre Channel

197MB/s sustained R/W transfer rate, 35,000 IOPS

Choosing the right SSD for Oracle

When evaluating SSD for Oracle databases you need to consider performance (throughput and response time), reliability (Mean Time Between failures) and TCO (total cost of ownership).  Most SSD vendors will provide a test RAM disk array for benchmark testing so that you can choose the vendor who offers the best price/performance ratio.

Burleson Consulting does not partner with any SSD vendors and we provide independent advice in this constantly-changing market.  BC was one of the earliest adopters of SSD for Oracle and we have been deploying SSD on Oracle database since 2005 and we have experienced SSD experts to help any Oracle shop evaluate whether SSD is right for your application.  BC experts can also help you choose the SSD that is best for your database.  Just  call 800-766-1884 or e-mail.:  for SSD support details.

DRAM SSD vs. Flash SSD

With all the talk about the Oracle “flash cache”, it is important to note that there are two types of SSD, and only DRAM SSD is suitable for Oracle database storage.  The flash type SSD suffers from serious shortcomings, namely a degradation of access speed over time.  At first, Flash SSD is 5 times faster than a platter disk, but after some usage the average read time becomes far slower than a hard drive.  For Oracle, only rack-mounted DRAM SSD is acceptable for good performance:

Avg. Read speed

Avg. write speed

Platter disk

10.0 ms.

  7.0 ms.

DRAM SSD

 0.4 ms.

  0.4 ms.

Flash SSD    

 1.7 ms.

 94.5 ms.

 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

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