Home > Articles > Data > Oracle

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Exadata Smart Flash Logging

Exadata Storage Software 11.2.2.4 introduced the Smart Flash Logging feature. The intent of this feature is to reduce overall redo log sync times by allowing the Exadata Flash storage to serve as a secondary destination for redo log writes. During a redo log sync, Oracle writes to the disk and Flash simultaneously and allows the redo log sync operation to complete as soon as either device completes its write.

In the event that the Flash Cache wins the race to write, the data need be held for only a short time until the storage server is certain that all writes have made it to the redo log. Since the Smart Flash Log is only a temporary store, only a small amount of Flash storage is required—512MB per cell (out of 3.2TB on an X4, 1.6TB on an X3, or 365GB on an X2 system).

Figure 15.14 illustrates the essential flow of control. Oracle processes performing DML generate redo entries which are written to the redo buffer (1). Periodically or upon COMMIT the LGWR flushes the buffer (2), resulting in an I/O request to the CELLSRV process (3). CELLSRV writes to Flash and grid disk simultaneously (4), and when either I/O completes, it returns control to the LGWR (5).

Figure 15.14

Figure 15.14 Exadata Smart Flash Logging

The use of Flash SSD to optimize redo log operations has been a somewhat contentious topic. Many—including this author—have argued that Flash SSD is a poor choice for redo log workloads. The nature of sequential redo log I/O tends to favor the spinning magnetic disk since sequential I/O minimizes seek latency, while penalizing Flash-based SSD, since the continual overwriting of existing blocks makes the probability of a block erase very high.

However, the Exadata Smart Flash Logging feature is not predicated on some theoretical write I/O advantage for Flash SSD. Rather it aims to “smooth out” redo log writes by running redo log writes out through two channels (grid disk and Flash SSD) and allowing the redo log write to complete when either of the two completes.

Redo log sync waits—which occur whenever a COMMIT occurs—generally involve only a couple of milliseconds of wait time since they involve only a small sequential write operation on an (ideally) relatively lightly loaded disk subsystem. Keeping redo logs on separate ASM disk groups from data files helps ensure that heavy data file I/O loads do not affect the time taken for redo operations.

However, it’s inevitable that from time to time a redo log sync operation will conflict with some other I/O—an archive read or Data Guard operation, for instance. In these circumstances some redo log sync operations may take a very long time indeed.

Following is some Oracle trace log data that shows some redo log sync waits:

WAIT #4..648: nam='log file sync’ ela= 710
WAIT #4..648: nam='log file sync’ ela= 733
WAIT #4...648: nam='log file sync’ ela= 621
WAIT #4...648: nam='log file sync’ ela= 507
WAIT #4...648: nam='log file sync’ ela= 683
WAIT #4...648: nam='log file sync’ ela= 2084
WAIT #4...648: nam='log file sync’ ela= 798
WAIT #4...648: nam='log file sync’ ela= 1043
WAIT #4...648: nam='log file sync’ ela= 2394
WAIT #4...648: nam='log file sync’ ela= 932
WAIT #4...648: nam='log file sync’ ela= 291780
WAIT #4...648: nam='log file sync’ ela= 671
WAIT #4...648: nam='log file sync’ ela= 957
WAIT #4...648: nam='log file sync’ ela= 852
WAIT #4...648: nam='log file sync’ ela= 639
WAIT #4...648: nam='log file sync’ ela= 699
WAIT #4...648: nam='log file sync’ ela= 819

The ela entry shows the elapsed time in microseconds. Most of the waits are less than 1 millisecond (1000 microseconds), but in the middle we see an anomalous wait of 291,780 microseconds (about one-third of a second!).

Occasional very high redo log sync waits like the one just shown might not seem too disturbing until you remember that redo log sync waits are frequently included in the most critical application transactions. Online operations such as saving a shopping cart, confirming an order, and saving a profile change all generally involve some sort of commit operation, and it’s well known that today’s online consumers rapidly lose patience when operations delay even by fractions of a second. So even occasional high redo log wait times are cause for concern. It’s the intent of Exadata Smart Flash Logging to smooth out these disturbing outliers.

Controlling and Monitoring Smart Flash Logging

Exadata Smart Flash Logging is enabled by default and you don’t have to do anything specifically to enable it—other than to make sure your Storage Cells are running at least Exadata Storage Software 11.2.2.4.

You can confirm your Flash Log status by issuing a LIST FLASHLOG command at a CellCLI prompt:

CellCLI> list flashlog  detail
         name:                   exa1cel01_FLASHLOG
         cellDisk:               FD_09_exa1cel01,FD_02_exa1cel01,
         creationTime:           2012-07-07T06:56:23-07:00
         degradedCelldisks:
         effectiveSize:          512M
         efficiency:             100.0
         id:                     3c08cfe1-ea43-4fde-85c2-0bbd5cbd11ec
         size:                   512M
         status:                 normal

You can control the behavior of Exadata Smart Flash Logging by using a resource management plan. This allows you to turn Exadata Smart Flash Logging on or off for individual databases.

So, for instance, this command will turn Exadata Smart Flash Logging off for database GUY and leave it on for all other databases:

ALTER IORMPLAN dbplan=((name=’GUY’,flashLog=false),
                       (name=other,flashlog=on))’

You can monitor the behavior of Exadata Smart Flash Logging by using the following CellCLI command:

CellCLI> list metriccurrent where objectType=’FLASHLOG’;
   FL_ACTUAL_OUTLIERS              FLASHLOG        1 IO requests
   FL_BY_KEEP                      FLASHLOG        0
   FL_DISK_FIRST                   FLASHLOG        253,540,190 IO requests
      ...... ......
   FL_FLASH_FIRST                  FLASHLOG        11,881,503 IO requests
      ...... ......
   FL_PREVENTED_OUTLIERS           FLASHLOG        275,125 IO requests

These are probably the most interesting CellCLI metrics generated by this command:

  • FL_DISK_FIRST—the grid disk log write completed first during the redo log write operation
  • FL_FLASH_FIRST—the Flash SSD completed first during the redo log write operation
  • FL_PREVENTED_OUTLIERS—the number of redo log writes that were optimized by the Flash Logging that would otherwise have taken longer than 500 milliseconds to complete

Testing Exadata Smart Flash Logging

Let’s look at an example. Say we test Exadata Smart Flash Logging by running 20 concurrent processes, each of which performs 200,000 updates and commits—a total of 4 million redo log sync operations. Now, Exadata Smart Flash Logging is disabled using a resource plan (see the ALTER IORMPLAN statement in the previous section) and the tests are repeated. We capture every redo log sync wait in a DBMS_MONITOR trace file for analysis using the R statistical package.

With Exadata Smart Flash Logging disabled, our key CellCLI metrics look like this:

FL_DISK_FIRST            32,669,310 IO requests
FL_FLASH_FIRST            7,318,741 IO requests
FL_PREVENTED_OUTLIERS       774,146 IO requests

With Exadata Smart Flash Logging enabled, the metrics look like this:

FL_DISK_FIRST            33,201,462 IO requests
FL_FLASH_FIRST            7,337,931 IO requests
FL_PREVENTED_OUTLIERS       774,146 IO requests

So for this particular cell the Flash disk “won” only 3.8% of the time (the ratio of FL_FLASH_FIRST and FL_DISK_FIRST) and prevented no outliers. (Outliers are redo log syncs that take longer than 500 milliseconds to complete.)So on the surface, it would seem that very little has been achieved.

However, statistical analysis of the redo log sync times provides a somewhat different interpretation. Table 15.1 summarizes the key statistics for the two tests.

Table 15.1 Effect of Exadata Smart Flash Logging on Redo Log Sync Waits

Redo Log Sync Time (microseconds)

Smart Flash Logging

Min

Median

Mean

99%

Max

On

1.0

650

723

1656

75,740

Off

1.0

627

878

4662

291,800

Exadata Smart Flash Logging reduced the mean log file sync wait time by over 15%—and this difference was statistically significant. There was also a significant reduction in the 99th percentile—the minimum wait time for the top 1% of waits was reduced from about 4.6 seconds to 1.6 seconds.

Figure 15.15 shows the distribution of log file sync waits with the Exadata Smart Flash Logging feature enabled and disabled. Turning Exadata Smart Flash Logging on created a strange hump on the high side of what otherwise looks like a normal bell curve distribution. Understanding that hump requires that we take a look at the distribution of very high outlier redo log waits.

Figure 15.15

Figure 15.15 Distribution of log file sync waits with Exadata Smart Flash Logging

Figure 15.16 shows the distribution of the top 10,000 waits. This shows far more clearly how Exadata Smart Flash Logging worked to reduce high outlier log file sync waits. These waits have been pulled back, but to a point that is still above the average wait time for other log file sync waits. This creates the hump in Figure 15.15 and represents a significant reduction in the time taken for outlying redo log waits.

Figure 15.16

Figure 15.16 Distribution of top 10,000 log file sync waits with Exadata Smart Flash Logging

While Flash SSD is not necessarily an ideal storage medium for redo write I/O, Exadata Smart Flash Logging does reduce the impact of very high outlier redo log writes.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020