avoid buffer-size larger than memory limit

[zwwhdls]

close #730

[codecov-commenter]

Codecov Report

Patch coverage: 77.77% and project coverage change: +0.01% 🎉

Comparison is base (864d9db) 38.16% compared to head (26bc113) 38.18%.
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
+ Coverage   38.16%   38.18%   +0.01%     
==========================================
  Files          33       33              
  Lines        4087     4106      +19     
==========================================
+ Hits         1560     1568       +8     
- Misses       2380     2388       +8     
- Partials      147      150       +3     

Files Changed	Coverage Δ
pkg/config/config.go	0.00% <ø> (ø)
pkg/juicefs/juicefs.go	49.27% <60.00%> (+0.20%)	⬆️
pkg/config/setting.go	75.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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