{"id":864,"date":"2009-05-12T17:50:00","date_gmt":"2009-05-12T17:50:00","guid":{"rendered":"\/blogs\/paul\/post\/How-expensive-are-page-splits-in-terms-of-transaction-log.aspx"},"modified":"2017-04-13T09:54:03","modified_gmt":"2017-04-13T16:54:03","slug":"how-expensive-are-page-splits-in-terms-of-transaction-log","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/how-expensive-are-page-splits-in-terms-of-transaction-log\/","title":{"rendered":"How expensive are page splits in terms of transaction log?"},"content":{"rendered":"

\nPage splits are always thought of as expensive, but just how bad are they? In this post I want to create an example to show how much more transaction log is created when a page in an index has to split. I'm going to use the sys.dm_tran_database_transactions<\/font> DMV to show how much more transaction log is generated when a page has to split. You can find the list of columns and a small amount of explanation of each column in Books Online <\/font>here<\/font><\/a> – I was reminded of its existence by someone on <\/font>Twitter<\/font><\/a> (sorry, don't remember who it was and I couldn't find it in search).<\/font>\n<\/p>\n

\nIn the example, I'm going to create a table with approximately 1000-byte long rows:<\/font>\n<\/p>\n

\n

\n\tCREATE DATABASE PageSplitTest;
\n\tGO
\n\tUSE pagesplittest;
\n\tGO<\/font>\n\t<\/p>\n

\n\tCREATE TABLE BigRows (c1 INT, c2 CHAR (1000));
\n\tCREATE CLUSTERED INDEX BigRows_CL ON BigRows (c1);
\n\tGO<\/font>\n\t<\/p>\n

\n\tINSERT INTO BigRows VALUES (1, 'a');
\n\tINSERT INTO BigRows VALUES (2, 'a');
\n\tINSERT INTO BigRows VALUES (3, 'a');
\n\tINSERT INTO BigRows VALUES (4, 'a');
\n\tINSERT INTO BigRows VALUES (6, 'a');
\n\tINSERT INTO BigRows VALUES (7, 'a');
\n\tGO<\/font>\n\t<\/p>\n<\/blockquote>\n

\nI've engineered the case where the clustered index data page has space for one more row, and I've left a 'gap' at c1=5<\/font>. Let's add it as part of an explicit transaction and see how much transaction log is generated:<\/font>\n<\/p>\n

\n

\n\tBEGIN TRAN
\n\tINSERT INTO BigRows VALUES (8, 'a');
\n\tGO<\/font>\n\t<\/p>\n

\n\tSELECT [database_transaction_log_bytes_used] FROM sys.dm_tran_database_transactions
\n\tWHERE [database_id] = DB_ID ('PageSplitTest');
\n\tGO<\/font>\n\t<\/p>\n

\n\tdatabase_transaction_log_bytes_used
\n\t———————————–
\n\t1228<\/font>\n\t<\/p>\n<\/blockquote>\n

\nThat's about what I'd expect for that row. Now what about when I cause a page split by inserting the 'missing' c1=5<\/font> row into the full page?<\/font>\n<\/p>\n

\n

\n\t— commit previous transaction
\n\tCOMMIT TRAN
\n\tGO<\/font>\n\t<\/p>\n

\n\tBEGIN TRAN
\n\tINSERT INTO BigRows VALUES (5, 'a');
\n\tGO<\/font>\n\t<\/p>\n

\n\tSELECT [database_transaction_log_bytes_used] FROM sys.dm_tran_database_transactions
\n\tWHERE [database_id] = DB_ID ('PageSplitTest');
\n\tGO<\/font>\n\t<\/p>\n

\n\tdatabase_transaction_log_bytes_used
\n\t———————————–
\n\t6724<\/font>\n\t<\/p>\n<\/blockquote>\n

\nWow. 5.5x more bytes are written to the transaction log as part of the system transaction that does the split.<\/font>\n<\/p>\n

\nThe ratio gets worse as the row size gets smaller. For a row with an approximately 100-byte long row (use the same code as above, but change to a CHAR (100)<\/font>, insert 67 rows with a 'gap' somewhere then insert the 68th to cause the split), the two numbers are 328 and 5924 – the split cause 18 times more log to be generated! For a row with an approximately 10-byte long row, I got numbers of 240 and 10436<\/strong>, because I created skewed data (about 256 rows with the key value 8) and then inserted key value 5 which forced a (rare) non-middle page split. That's a ratio of more than 43<\/u><\/em><\/strong> times more log generated! You can try this yourself if you want: I changed the code to have a CHAR (10)<\/font>, inserted values 1, 2, 3, 4, 6, 7, then inserted 256 key values of 8 and then 2 of 5. The resulting page had only 6 rows – it split after the key value 5 – the Storage Engine doesn't always do a 50\/50 page split. And that's not even causing nasty cascading page-splits, or splits that have to split a page multiple times to fit a new (variable-sized) row in.<\/font>\n<\/p>\n

\nBottom line: page splits don't just cause extra IOs and index fragmentation, they generate a *lot* more transaction log. And all that log has to be (potentially) backed up, log shipped, mirrored….<\/font>\n<\/p>\n

\n<\/font>\n<\/p>\n

\n<\/font><\/p>\n","protected":false},"excerpt":{"rendered":"

Page splits are always thought of as expensive, but just how bad are they? In this post I want to create an example to show how much more transaction log is created when a page in an index has to split. I'm going to use the sys.dm_tran_database_transactions DMV to show how much more transaction log […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[42,47,48,62,66,98],"tags":[],"class_list":["post-864","post","type-post","status-publish","format-standard","hentry","category-fragmentation","category-indexes-from-every-angle","category-inside-the-storage-engine","category-on-disk-structures","category-performance-tuning","category-transaction-log"],"yoast_head":"\nHow expensive are page splits in terms of transaction log? - Paul S. 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