Bluestore: tens of milliseconds latency in prepare stage

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Hi, everyone:

I'm currently encountering a performance issue in Nautilus Bluestore.
With the perf dump stats by mgr prometheus module, I found Bluestore
will produce tens of milliseconds in prepare stage. The latency is
calculated with l_bluestore_state_prepare_lat logger. The max latency
I have seen is about 70ms, and those abnormal latency only occur in
high IO load sometimes. When the abnormal latency occurs, the backend
nvme disk is also suffering a high await.

As far as I know, prepare stage in Bluestore does not contain disk
operations, how that huge latency be possible?

The `bluestore alloctor fragmentation block` shows about 0.5, and with
re-constructing osd, it indeed reduced latency in the whole IO link to
an acceptable level, including prepare stage.

Could anyone give some tips on this? Thanks!
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