[jira] [Assigned] (KUDU-3427) Add a metric for delta of snapshot vs current timestamp for scan-at-snapshot scans

["Ashwani Raina (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KUDU-3427?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ashwani Raina reassigned KUDU-3427:
-----------------------------------

    Assignee: Ashwani Raina

> Add a metric for delta of snapshot vs current timestamp for scan-at-snapshot scans
> ----------------------------------------------------------------------------------
>
>                 Key: KUDU-3427
>                 URL: https://issues.apache.org/jira/browse/KUDU-3427
>             Project: Kudu
>          Issue Type: Improvement
>          Components: tserver
>            Reporter: Alexey Serbin
>            Assignee: Ashwani Raina
>            Priority: Major
>              Labels: operability, supportability
>
> Currently, the setting for the {{\-\-tablet_history_max_age_sec}} is set to quite arbitrary value of  60 * 60 * 24 * 7 seconds (7 days).  Keeping a lot of data in UNDO deltas for longer than necessary means using IO throughput, CPU cycles, and memory during various types of background maintenance jobs to process data which no longer needed.  However, as of Kudu 1.16.0 version, there isn't a simple way to tell whether the current setting of {{\-\-tablet_history_max_age_sec}} is appropriate for the workload running on a Kudu cluster.  An operator interested in optimizing the amount of tablet history stored has no visibility on what might be the optimal value for the {{\-\-tablet_history_max_age_sec}} based on the workloads run against the cluster.
> It would be great to add a per-tablet metric (a histogram?) to accumulate stats on the difference of snapshots used for scan operations in READ_AT_SNAPSHOT and READ_YOUR_WRITES mode vs current timestamp.



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