{"id":2460,"date":"2026-04-07T18:04:56","date_gmt":"2026-04-07T16:04:56","guid":{"rendered":"https:\/\/www.ludovicocaldara.net\/dba\/?p=2460"},"modified":"2026-04-07T18:04:56","modified_gmt":"2026-04-07T16:04:56","slug":"dg26ai-fsfo-lag-histogram","status":"publish","type":"post","link":"https:\/\/www.ludovicocaldara.net\/dba\/dg26ai-fsfo-lag-histogram\/","title":{"rendered":"Data Guard 26ai &#8211; #26: Fast-Start Failover Lag Histogram"},"content":{"rendered":"<p><a href=\"https:\/\/www.ludovicocaldara.net\/dba\/dg-26ai-series\/\">This post is part of a blog series.<\/a><\/p>\n<p>Oracle Database 26ai introduces <a href=\"https:\/\/docs.oracle.com\/en\/database\/oracle\/oracle-database\/26\/refrn\/V-FS_LAG_HISTOGRAM.html\">V$FS_LAG_HISTOGRAM<\/a>, a new view that exposes the distribution of Fast-Start Failover (FSFO) lag.<\/p>\n<p>When FSFO is configured (including observe-only mode), it samples the lag every minute, and puts it into buckets to count how often each range occurs. The buckets can be queried from this new view:<\/p>\n<pre class=\"lang:plsql decode:true\">SQL&gt; SELECT * FROM V$FS_LAG_HISTOGRAM;\r\n\r\nTHREAD# LAG_TYPE LAG_TIME LAG_COUNT     LAST_UPDATE_TIME\r\n      1 APPLY           5       122  01\/26\/2025 10:46:07\r\n      1 APPLY          10         5  01\/02\/2025 16:12:42\r\n      1 APPLY          15         2  12\/25\/2024 12:01:26\r\n      1 APPLY          30         0  \r\n      1 APPLY          60         0  \r\n      1 APPLY         120         0  \r\n      1 APPLY         180         0  \r\n      1 APPLY         300         0  \r\n      1 APPLY       65535         0  \r\n<\/pre>\n<p>Where:<\/p>\n<ul data-spread=\"false\">\n<li>LAG_TIME: upper bound of the bucket<\/li>\n<li>LAG_COUNT: number of occurrences<\/li>\n<li>LAST_UPDATE_TIME: most recent occurrence<\/li>\n<\/ul>\n<p>So:<\/p>\n<ul data-spread=\"false\">\n<li>5 \u2192 0\u20135 seconds<\/li>\n<li>10 \u2192 5\u201310 seconds<\/li>\n<li>15 \u2192 10\u201315 seconds<\/li>\n<\/ul>\n<p>This view is particularly useful to tune FastStartFailoverLagLimit in Maximum Performance mode (ASYNC replication).<\/p>\n<p>Why this matters:<\/p>\n<p>Instead of relying on point-in-time lag or implementing external monitoring (which is still a good idea), you can analyze the actual distribution and choose a value that reflects your RPO: failover will not occur if lag exceeds this limit.<\/p>\n<p><a href=\"https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-scaled.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-large wp-image-2461\" src=\"https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-1024x619.png\" alt=\"A slide with an example select from the view, redundant text with the blog post.\" width=\"584\" height=\"353\" srcset=\"https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-1024x619.png 1024w, https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-300x181.png 300w, https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-768x464.png 768w, https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-1536x928.png 1536w, https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-2048x1237.png 2048w, https:\/\/www.ludovicocaldara.net\/dba\/wp-content\/uploads\/2026\/04\/fsfo-lag-histogram-497x300.png 497w\" sizes=\"auto, (max-width: 584px) 100vw, 584px\" \/><\/a><\/p>\n<h2>How to set the FastStartFailoverLagLimit based on the buckets<\/h2>\n<p>Use the histogram to understand the typical and peak lag behavior, then set <strong>FastStartFailoverLagLimit<\/strong>:<\/p>\n<ul data-spread=\"false\">\n<li>Slightly above your normal\/daily peak lag (if any)<\/li>\n<li>In line with your acceptable data-loss window (RPO)<\/li>\n<li>Less than or equal to FastStartFailoverThreshold<\/li>\n<\/ul>\n<p>With proper setting, failovers are not unnecessarily prevented, while still respecting your data protection requirements.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>This post is part of a blog series. Oracle Database 26ai introduces V$FS_LAG_HISTOGRAM, a new view that exposes the distribution of Fast-Start Failover (FSFO) lag. When FSFO is configured (including observe-only mode), it samples the lag every minute, and puts &hellip; <a href=\"https:\/\/www.ludovicocaldara.net\/dba\/dg26ai-fsfo-lag-histogram\/\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[375,329],"tags":[],"class_list":["post-2460","post","type-post","status-publish","format-standard","hentry","category-oracle-ai-database-26ai","category-oracle-dg"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/posts\/2460","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/comments?post=2460"}],"version-history":[{"count":1,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/posts\/2460\/revisions"}],"predecessor-version":[{"id":2462,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/posts\/2460\/revisions\/2462"}],"wp:attachment":[{"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/media?parent=2460"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/categories?post=2460"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ludovicocaldara.net\/dba\/wp-json\/wp\/v2\/tags?post=2460"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}