Add IOhint in CephFS¶

Summary
In Infernalis, I propose a BP: osd: rados io hints improvements. In that BP, I only added iohint in osd and librbd. But don't add iohint for cephfs. This BP will add iohint for cephfs
Owners
Jianpeng Ma<Jianpeng.ma@intel.com>
Jian Zhang<jian.zhang@intel.com>

Interested Parties
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Current Status
Please describe the current status of Ceph as it relates to this blueprint. Is there something that this replaces? Are there current features that are related?

Detailed Description
1:Add mount fadvise option. Because cephfs can mount subidr, so different subdir can have different usage, like some subdir don't need cache(don¡¯t use in the near future)
2:pass posix_fadvise flags from cephfs to osd. Now in kernel for posix_fadvise don't transfer fadvise flag to osd. We can try do this in order to osd know the behavior of IO to get better performance.

Work items
This section should contain a list of work tasks created by this blueprint. Please include engineering tasks as well as related build/release and documentation work. If this blueprint requires cleanup of deprecated features, please list those tasks as well.

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Build / release tasks
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Documentation tasks
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Deprecation tasks
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