Support blk reservation

[yuwmao]

Application has an option to reserve some blks in a chunk when allocation.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.02%. Comparing base (1a0cef8) to head (8f08335).
Report is 156 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/blkalloc/append_blk_allocator.cpp	20.00%	2 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #680      +/-   ##
==========================================
+ Coverage   56.51%   66.02%   +9.51%     
==========================================
  Files         108      109       +1     
  Lines       10300    11348    +1048     
  Branches     1402     1545     +143     
==========================================
+ Hits         5821     7493    +1672     
+ Misses       3894     3124     -770     
- Partials      585      731     +146     

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[Besroy]

Sorry, I have a small question. If space is reserved at the HS level, does this mean that followers also reserve this space? In that case, when a follower has garbage, would it still trigger a NO_SPACE_LEFT condition, causing the PG state to become unclean? Or will there be some modifications at the upper layer to prevent this issue?

[xiaoxichen]

It has to be done in HS as from HO's point of view, if it get a multiBlk it has no way to know how much space still available on the chunk --- this is specific to allocator.

But what you said makes sense, the reservation is done through allocation_hint CB. We aware we should set no reservation for seal shard, thanks for reminding we need to consider the role here