Doris support choose medium when create table, and the cluster balance strategy is dependent
between different storage medium, and most use will not specify the storage medium when create table,
even they kown that they should choose a storage medium, they have no idea about the
cluster's storage medium, so, I think we should make storage_medium and storage_cooldown_time
configurable, and this should be the admin's responsibility.
For Example, if the cluster's storage medium is HDD, but we need to change part of machines to SSD,
if we change the machine, the tablets before change is stored in HDD and they can't find a dest path
to migrate, and user will create table as usual, it will make all tablets stored in old machines and
the new machines will only store a little tablets. Without this config the only way is admin need
to traverse all partitions in cluster and change the property of storage_medium, it will increase
operational and maintenance costs.
So I add a FE config default_storage_medium, so that user can set the default storage medium.
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