{"id":20281,"date":"2013-05-10T12:52:51","date_gmt":"2013-05-10T07:22:51","guid":{"rendered":"http:\/\/vskills.in\/certification\/tutorial\/?p=20281"},"modified":"2024-04-12T14:16:39","modified_gmt":"2024-04-12T08:46:39","slug":"compaction","status":"publish","type":"page","link":"https:\/\/www.vskills.in\/certification\/tutorial\/compaction\/","title":{"rendered":"Compaction"},"content":{"rendered":"

Certify and Increase Opportunity.<\/span>
\nBe <\/span>
\nGovt. Certified Apache Cassandra Professional
\n<\/span><\/a><\/p>\n

Compaction in Apache Cassandra
\n<\/strong><\/span><\/p>\n

Compaction<\/strong><\/p>\n

Cassandra\u2019s log-structured storage engine enables Cassandra\u2019s amazing performance and features like application-transparent compression by turning all updates into data files called sstables that are written sequentially to disk. No update-in-place is done (because that would require doing random i\/o); instead, a new version of the columns being inserted or updated is written in the next sstable.<\/p>\n

Thus, over time, many versions of a row may exist in different sstables. Each of these versions may have different sets of columns. If the sstables were allowed to accumulate, reading a row could require many seeks in different files to return a result.<\/p>\n

To prevent read speed from deteriorating, compaction runs in the background, merging sstables together. (This can also be done performantly, without random i\/o, because rows are sorted by primary key within in each sstable.)
\nTiered Compaction<\/p>\n

Cassandra\u2019s size-tiered compaction stragety is very similar to the one described in Google\u2019s Bigtable paper: when enough similar-sized sstables are present (four by default), Cassandra will merge them.<\/p>\n

There are three problems with size-tiered compaction in update-heavy workloads:<\/p>\n

Performance can be inconsistent because there are no guarantees as to how many sstables a row may be spread across: in the worst case, we could have columns from a given row in each sstable.
\nA substantial amount of space can be wasted since there is no guarantee as to how quickly obsolete columns will be merged out of existance; this is particularly noticeable when there is a high ratio of deletes.
\nSpace can also be a problem as sstables grow larger from repeated compactions, since an obsolete sstable cannot be removed until the merged sstable is completely written.  In the worst case of a single set of large sstable with no obsolete rows to remove, Cassandra would need 100% as much free space as is used by the sstables being compacted, into which to write the merged one.<\/p>\n

Leveled Compaction
\nCassandra 1.0 introduces the Leveled Compaction Strategy, based on LevelDB from the Chromium team at Google.<\/p>\n

Leveled compaction creates sstables of a fixed, relatively small size (5MB by default in Cassandra\u2019s implementation), that are grouped into \u201clevels.\u201d Within each level, sstables are guaranteed to be non-overlapping. Each level is ten times as large as the previous.<\/p>\n

This solves the above problems with tiered compaction:<\/p>\n

Leveled compaction guarantees that 90% of all reads will be satisfied from a single sstable (assuming nearly-uniform row size). Worst case is bounded at the total number of levels \u2014 e.g., 7 for 10TB of data.
\nAt most 10% of space will be wasted by obsolete rows.
\nOnly enough space for 10x the sstable size needs to be reserved for temporary use by compaction.<\/p>\n

Leveled compaction can be enabled by creating (or updating) a ColumnFamily with the compaction_strategy option set to LeveledCompactionStrategy. When updating an existing column family, reads and writes can continue as usual while leveling of existing sstables is performed in the background.
\nConsiderations<\/p>\n

Because leveled compaction makes the above guarantees, it does roughly twice as much i\/o as size-tiered compaction. For primarily insert-oriented workloads, this extra i\/o does not pay off in terms of the above benefits, since there are few obsolete row versions involved.<\/p>\n

Leveled compaction ignores the concurrent_compactors setting. Concurrent compaction is designed to avoid tiered compaction\u2019s problem of a backlog of small compaction sets becoming blocked temporarily while the compaction system is busy with a large set. Leveled compaction does not have this problem, since all compaction sets are roughly the same size. Leveled compaction does honor the multithreaded_compaction setting, which allows using one thread per sstable to speed up compaction. However, most compaction tuning will still involve using compaction_throughput_mb_per_sec (default: 16) to throttle compaction back.<\/p>\n

\n

Apply for Apache Cassandra Certification Now!!<\/h3>\n

http:\/\/www.vskills.in\/certification\/Certified-Apache-Cassandra-Professional<\/a><\/p>\n

Go To Tutorial- Certified Apache Cassandra Professional Tutorial<\/strong><\/a><\/h5>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Certify and Increase Opportunity. Be Govt. Certified Apache Cassandra Professional Compaction in Apache Cassandra Compaction Cassandra\u2019s log-structured storage engine enables Cassandra\u2019s amazing performance and features like application-transparent compression by turning all updates into data files called sstables that are written sequentially to disk. No update-in-place is done (because that would require doing random i\/o); instead,…<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"categories":[63],"tags":[],"class_list":["post-20281","page","type-page","status-publish","hentry","category-apache-cassandra"],"acf":[],"yoast_head":"\nCompaction - Tutorial<\/title>\n<meta name=\"description\" content=\"Compaction Vskills Government Certification in Apache Cassandra is very popular in India amongst developers working in the IT Bid data industry.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.vskills.in\/certification\/tutorial\/compaction\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Compaction - 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