Mar 14, 2015 List of blocks cached in DB smart flash cache are stored in buffer cache area on two dedicated flash “LRU lists” depending on object attribute FLASHCACHE: DEFAULT – standard last recently used algorithm decides how long such blocks are cached in flash cache.
Flashcache is a disk cache component for the Linux kernel, initially developed by Facebook since April 2010, and released as open source in 2011. Since January 2013, there is a fork of Flashcache, named EnhanceIO and developed by sTec, Inc.[1]
Flashcache works by using flash memory, a USB flash drive, SD card, CompactFlash or any kind of portable flash mass storage system as a write-back persistent cache. An internal SSD can also be used for increasing performance.
Overview[edit]Flash Cache And Auto-tuned Memory_target Free
Using flash memory (NAND memory devices) for caching allows Linux kernel to service random disk IO with better performance than without the cache. This caching applies to all disk content, not just the page file or system binaries. Flash memory based devices are usually a magnitude faster than spinning HDDs for random IO, but with less advantage or even slower in sequential read/writes. By default, flashcache caches all full blocksize IOs, but can be configured to only cache random IO whilst ignoring sequential IO.[2]
Similar technology exists in Microsoft Windows as ReadyBoost since Windows Vista.
Flash Cache And Auto-tuned Memory_target GameImplementation[edit]
Flashcache is built on top of the Linux kernel's device mapper. The data structure of the cache is a set-associativehash table, in which the cache is divided up into a number of fixed-size sets (buckets), using linear probing within a set to find blocks. The device mapper layer breaks up all I/O requests into blocksize chunks before passing the requests to the cache layer.[citation needed]
When a write request happens, the corresponding cache block is marked dirty; dirty cache blocks are written lazily to disk in the background. There are a few parameters to control the write-back policy: dirty-threshold, idleness and contiguity with other dirty blocks about to be written back.[citation needed]
Limitations[edit]
There are a few limitations, imposed by the implementation of flashcache:[citation needed]
See also[edit]
References[edit]
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Flashcache&oldid=899496344'
This section contains the following topics on configuring Database Smart Flash Cache (the flash cache):
See Also:
'Memory Architecture Overview' for a description of the flash cache
When to Configure the Flash Cache
Consider adding the flash cache when all of the following are true:
Note:
You cannot share the flash cache among multiple instances.
Sizing the Flash Cache![]()
As a general rule, size the flash cache to be between 2 times and 10 times the size of the buffer cache. Any multiplier less than two would not provide any benefit. If you are using automatic shared memory management, make the flash cache between 2 times and 10 times the size of
SGA_TARGET . Using 80% of the size of SGA_TARGET instead of the full size would also suffice for this calculation.
Tuning Memory for the Flash Cache![]()
For each database block moved from the buffer cache to the flash cache, a small amount of metadata about the block is kept in the buffer cache. For a single instance database, the metadata consumes approximately 100 bytes. For an Oracle Real Application Clusters (Oracle RAC) database, it is closer to 200 bytes. You must therefore take this extra memory requirement into account when adding the flash cache.
Note:
You can choose to not increase the buffer cache size to account for the flash cache. In this case, the effective size of the buffer cache is reduced. However, you can offset this loss by using a larger flash cache.
Flash Cache Initialization Parameters
Table 6-3 describes the initialization parameters that you use to configure the flash cache.
Table 6-3 Flash Cache Initialization Parameters
You can use
ALTER SYSTEM to set db_flash_cache_size to zero to disable the flash cache. You can also use ALTER SYSTEM to set the flash cache back to its original size to reenable it. However, dynamically changing the size of the flash cache is not supported. Purpose of boot camp mac.
Flash Cache in an Oracle Real Applications Clusters Environment
You must configure a flash cache on either all or none of the instances in an Oracle Real Application Clusters environment.
Comments are closed.
|
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
December 2020
Categories |