dmu_prefetch: zfs_prefetch_disable should not disable metaslab_prefetch
activate zfs_prefetch_disable disable all dmu_prefetch.
For metaslab_prefetch is a very bad idea:
When zfs decide to use a new metaslab, it's not loaded.
With 2To Hard Drive and a lot of spacemap entries, activate a new metaslab can take so long time (more than ten of seconds for my case) and hang IO.
I propose a patch for this issue.
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