Hello, and welcome to this online, self-paced course module covering Exadata Smart Flash Log. My name is Peter Fusek. I am a curriculum developer at

Transcription

1 Hello, and welcome to this online, self-paced course module covering Exadata Smart Flash Log. My name is Peter Fusek. I am a curriculum developer at Oracle, and in various roles I have helped customers to get the most out of Oracle products since Today, I will be your tour guide for this session.

2 Before we begin, now might be a good time to take a look at some of the features of this Flashbased course player. Feel free to skip this slide and start the lecture if you ve attended similar Oracle online courses in the past. To your left, you will find a hierarchical course outline. This course enables and even encourages you to go at your own pace, which means you are free to skip over topics you already feel confident on, or jump right to a feature that really interests you, or go back and review topics that were already covered. Simply click on a course section to expand its contents and then select an individual slide. However, note that by default we will automatically walk you through the entire course without requiring you to use the outline. Standard Flash player controls are also found at the bottom of the player, including pause, previous, and next buttons. There is also an interactive progress bar to fast forward or rewind the current slide. Interactive slides may have additional controls and buttons along with instructions on how to use them. Also found at the bottom of the player is a panel containing any additional reference notes for the current slide. Feel free to read these reference notes at the conclusion of the course, in which case you can minimize this panel and restore it later. Or if you prefer you can pause and read them as we go along. Various handouts may be available from the Attachments button, including the audio narration scripts for this course. The course will now pause, so feel free to take some time and explore the interface. Then when you re ready to continue, click the next button below or alternatively click the Objectives slide in the course outline. Exadata Smart Flash Log - 2

3 Exadata Smart Flash Log - 3

4 Redo log files contain a record of the transactions processed by Oracle Database. Their primary function is to facilitate the recovery of all committed transactions if the database fails for any reason. Given their vital importance, database administrators configure redo log groups and mirrored log files to ensure high availability and to protect from the damaging effects of a corrupted log file. For intensive online transaction processing (OLTP) applications, fast response for database log writes is crucial since transactions are not committed until the log write is acknowledged. This is illustrated by the animation playing on the screen. Normally, log writes can be processed very quickly by modern storage systems because of the high-performance battery-protected caches that are built into the I/O subsystem. However, occasionally the I/O subsystem can suffer from performance hiccups. This is especially likely when the system is at peak load. These hiccups can have a significant impact on database performance. To compound matters, the availability requirements associated with redo log files dictate that a log write must wait for writes to complete on all the mirrored log files, so transactions can only be processed at the rate of the slowest disk. Placing the redo log files on flash disks is not a complete solution because flash technology can have similar performance hiccups due to erase cycles or wear levelling. The ideal solution requires a mechanism that provides consistently good log write performance in spite of the occasional performance hiccups that can occur in an I/O subsystem. Exadata Smart Flash Log - 4

5 Exadata Smart Flash Log provides a mechanism for improving the latency of database redo log write operations. Exadata Smart Flash Log uses a small portion of the high-performance flash memory on Exadata Storage Server as persistent temporary storage to provide low latency redo log writes. With Exadata Smart Flash Log enabled, log records are written simultaneously to disk and flash. The write is deemed completed when the fastest write completes, regardless of whether it is to disk or flash. Normally, write operations to Exadata Storage Server are very fast because the write is acknowledged as soon as it is written to the disk controller cache. However, when the disks containing the online redo log files experience temporary performance issues, such as during periods of heavy I/O load, the write operations to flash will complete faster and maintain consistently good processing performance. This is illustrated by the example in the slide where we see that the write to disk took 600 milliseconds (0.6 seconds), however the write to flash only took 2 milliseconds. Finally, Exadata Storage Server automatically manages Smart Flash Log and ensures that all log entries in flash are persisted to disk. This includes all crash recovery scenarios. So from a DBA perspective, the database behaves the same with or without Smart Flash Log. However end users benefit from consistently low latencies for redo log writes. Note that the best practices and configuration recommendations for redo log sizing and mirroring do not change when using Exadata Smart Flash Log. Exadata Smart Flash Log - 5

6 Exadata Smart Flash Log is a new feature introduced in Exadata Storage Server software version In addition, the minimum Oracle Database software version that can be used in conjunction with Exadata Smart Flash Log is version with Bundle Patch 11. Database Machines that are updated to software releases providing Exadata Smart Flash Log must be reconfigured to enable this feature. Prior to the advent of Smart Flash Log, all the flash memory on Exadata Storage Server would typically be allocated to Exadata Smart Flash Cache. To enable Smart Flash Log an administrator would need to temporarily drop Exadata Smart Flash Cache in order to provide the free space required for Smart Flash Log. After the Smart Flash Log area is defined, the Smart Flash Cache can be re-created to consume the remaining flash memory on the cell. The slide shows the command sequence for a single Exadata cell. The process can be executed without the requirement for any downtime; however, a short-term performance impact might be noticed due to temporarily removing the Exadata Smart Flash Cache. Exadata Smart Flash Log - 6

7 By default, Smart Flash Log is allocated 512 MB spread across all of the 16 flash-based cell disks in the Exadata Storage Server. That's 32 MB per flash-based cell disk. The default size is designed to be sufficient in all but the most extreme cases. Also, 512 MB represents approximately 0.13% of the 384 GB of flash memory on each Exadata cell. So the benefits derived from Smart Flash Log should outweigh any losses associated with having a slightly smaller Flash Cache. Optionally, the size of the Smart Flash Log can be set. If a size is specified in the CREATE FLASHLOG command, then that size is divided evenly across the cell disks hosting the Flash Log area. If a size is not specified, then a default size of 512 MB is used. Also, the Smart Flash Log can be configured on specific flash devices buy specifying a list of flash-based cell disks. The optional configuration examples at the bottom of the screen illustrate the available options. In the top example, a Flash Log area totaling 1 GB in size is created across all the flash disks, resulting in an allocation of 64 MB on each of the 16 flash disks in the cell. In the bottom example, the specified size of 256 MB is spread across the two flash disks specified in the command, resulting in an allocation of 128 MB on each flash disk. Exadata Smart Flash Log - 7

8 A set of 18 metrics associated with Exadata Smart Flash Log are recorded in each cell. This slide shows 10 of the metrics and the rest are introduced on the next slide. All the metrics can be queried using the LIST METRICCURRENT and LIST METRICHISTORY CellCLI commands. An example is displayed at the bottom of the slide. Here are some additional notes regarding some of the metrics on this slide: The first three metrics in the table relate to outliers. In the context of Smart Flash Log an outlier is a log write that takes longer than 0.5 seconds to complete. - The FL_ACTUAL_OUTLIERS metric records the total number of redo writes that exceeded 0.5 seconds on both flash and disk. The goal of Smart Flash Log is to keep this metric very low all the time; ideally at zero. - The FL_PREVENTED_OUTLIERS metric counts the number of times where a write to disk exceeded 0.5 seconds but the corresponding write to flash occurred within 0.5 seconds. This metric essentially shows Smart Flash Log doing its job. - The FL_FLASH_ONLY_OUTLIERS metric counts the times where a write to flash exceeds 0.5 seconds but the corresponding write to disk occurred within 0.5 seconds. This metric highlights the fact that keeping the entire redo log file solely on flash would not be a good idea. Large values associated with this metric could indicate a problem with flash on the cell. The efficiency metrics show the proportion of outliers saved by Smart Flash Log. The slide shows how FL_EFFICIENCY_PERCENTAGE is calculated. The other metrics are self-explanatory. Exadata Smart Flash Log - 8

9 This slide shows the rest of the metrics associated with Smart Flash Log. Here are some additional notes regarding some of the metrics on this slide: FL_IO_DB_BY_W and FL_IO_FL_BY_W both represent the total amount of data written to each type of storage. Because the same log records are written to both types of storage you might expect the value of these two metrics to be equal. However, as the example at the bottom of the slide clearly shows, this is not the case. The reason why the two values are not the same, and specifically why more data is written to flash, is because: 1. Each log entry written to flash contains an additional 512 bytes of control information. 2. Each flash I/O is padded to a 4K boundary for optimal flash I/O performance. FL_IO_W counts the total number of I/Os serviced by Smart Flash Log. So you should expect to see that FL_IO_W = FL_DISK_FIRST + FL_FLASH_FIRST. The final three metrics in the table (FL_IO_W_SKIP_BUSY, FL_IO_W_SKIP_BUSY_MIN and FL_IO_W_SKIP_LARGE) relate to situations where log writes to flash could not be conducted and were skipped. Large values for these metrics can be an indicator of a problem with flash, or that the size of the Flash Log area is too small. The other metrics are self-explanatory. Exadata Smart Flash Log - 9

10 The IORM (I/O Resource Management) plan can be used to specify if a database is allowed to use Exadata Smart Flash Log or not. This allows the Flash Log area to be reserved for the most important databases, which is especially useful in environments that are used to consolidate multiple databases. Set flashlog=on in the interdatabase plan directive to allow the associated databases to use Exadata Smart Flash Log, and set flashlog=off to prevent databases from using Exadata Smart Flash Log. If an interdatabase plan directive does not contain the flashlog attribute, then flashlog=on is assumed. Note that IORM only manages I/O queues for physical disks. IORM does not arbitrate requests to flash-based grid disks, requests serviced by Exadata Smart Flash Cache or writes to Exadata Smart Flash Log. So the flashlog directive in the IORM plan can only be used to control access to Smart Flash Log for a specific database. It cannot be used to prioritize access by different databases to Smart Flash Log. Exadata Smart Flash Log - 10

11 Exadata Smart Flash Log - 11

12 Now you can test what you've learnt using a series of quiz questions. Exadata Smart Flash Log - 12

13 Before concluding, let's summarize this course module covering Exadata Smart Flash Log. In this session, you should have learned how to: Describe the purpose of Exadata Smart Flash Log Describe the requirements for Exadata Smart Flash Log Describe how Exadata Smart Flash Log works Describe how to configure Exadata Smart Flash Log Describe the metrics associated with Exadata Smart Flash Log Thank-you for attending. Exadata Smart Flash Log - 13