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https://issues.apache.org/jira/browse/KUDU-1703?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Alves updated KUDU-1703:
------------------------------
    Description: 
When we fix safe time advancement, replicas will start to block on snapshot 
scans for arbitrary amounts of time, waiting to have a consistent view of the 
world at that timestamp before serving the scan. This will be a serious problem 
for lagging replicas, which might be several seconds or even minutes behind.

Moreover in the absence of writes, the same will happen even for non-lagging 
replicas, which will have their safe times updated only when the leader 
heartbeats.

We need to at least make sure that:
- Blocked scanner threads are not starving other work.
- If the replica's safe time is lagging by a lot, the replica refuses to do the 
scan.

We might also consider other optimizations (like pinging the leader).


  was:
When we fix safe time advancement, replicas will start to block on snapshot 
scans for arbitrary amounts of time, waiting to have a consistent view of the 
world at that timestamp before serving the scan.

This will be a serious problem for lagging replicas, which might be several 
seconds or even minutes behind. Moreover in the absence of writes, the same 
will happen even for non-lagging replicas, which will have their safe times 
updated only when the leader heartbeats.

We need to at least make sure that:
- Blocked scanner threads are not starving other work.
- If the replica's safe time is lagging by a lot, the replica refuses to do the 
scan.

We might also consider other optimizations (like pinging the leader).


        Summary: Handle snapshot reads that might block indefinitely  (was: 
Handle lagging replicas for snapshot reads)

> Handle snapshot reads that might block indefinitely
> ---------------------------------------------------
>
>                 Key: KUDU-1703
>                 URL: https://issues.apache.org/jira/browse/KUDU-1703
>             Project: Kudu
>          Issue Type: Sub-task
>    Affects Versions: 1.1.0
>            Reporter: David Alves
>            Assignee: David Alves
>
> When we fix safe time advancement, replicas will start to block on snapshot 
> scans for arbitrary amounts of time, waiting to have a consistent view of the 
> world at that timestamp before serving the scan. This will be a serious 
> problem for lagging replicas, which might be several seconds or even minutes 
> behind.
> Moreover in the absence of writes, the same will happen even for non-lagging 
> replicas, which will have their safe times updated only when the leader 
> heartbeats.
> We need to at least make sure that:
> - Blocked scanner threads are not starving other work.
> - If the replica's safe time is lagging by a lot, the replica refuses to do 
> the scan.
> We might also consider other optimizations (like pinging the leader).



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