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https://issues.apache.org/jira/browse/HBASE-24152?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17103071#comment-17103071
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Zheng Wang commented on HBASE-24152:
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{quote}Was reviewing the PR but figured don't have enough understanding of what
is going on so let me ask here.
I don't follow why the PR is special casing SSD weight. Why it it not just
factored into the host general weight? Balancer works at the RS level, not at
type of storage? A local replica should be favored whether on ssd or not?
Help me out [~filtertip] Thank you.
{quote}
[~stack]
Consider this case when setting ONE_SSD as STORAGE_POLICY.
The region-1 opened by host-1, it includes one hdfs block which has three
replicas stored as below:
replica-1 on host-1(hdd)
replica-2 on host-2(hdd)
replica-3 on host-3(ssd)
Then the reader of hfile will read from hdd, because host-1 is local and has
high priority.
If we move the region-1 to host-3, the reader will read from ssd, and this cost
function could increase the possibility of the movement when making plans.
> Add ServerSsdLocalityCostFunction to StochasticLoadBalancer
> -----------------------------------------------------------
>
> Key: HBASE-24152
> URL: https://issues.apache.org/jira/browse/HBASE-24152
> Project: HBase
> Issue Type: New Feature
> Components: Balancer
> Reporter: Zheng Wang
> Assignee: Zheng Wang
> Priority: Major
>
> When use ONE_SSD storagy policy, or ALL_SSD but has not enough SSD, there
> will be some hdfs blocks on DISK and others on SSD,so it is reasonable to
> consider the locality of ssd for StochasticLoadBalancer.
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