feat: Use buf write to reduce system call on index write

[zuston]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

How are these changes tested?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.31%. Comparing base (f09f8af) to head (cee9186).
⚠️ Report is 610 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2579      +/-   ##
============================================
+ Coverage     56.12%   59.31%   +3.18%     
- Complexity      976     1444     +468     
============================================
  Files           119      146      +27     
  Lines         11743    13743    +2000     
  Branches       2251     2353     +102     
============================================
+ Hits           6591     8151    +1560     
- Misses         4012     4370     +358     
- Partials       1140     1222      +82     

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