Boost the remove operation

[RoyWFHuang]

Inspired by Ext4's rec_len, add nr_blk in struct simplefs_file to record how many contiguous blocks a file occupies. This method quickly locate the next available block for storing new file data.

Additionally, it improves the removal process in directories — by setting the inode to 0 and merging block information with the previous entry

there is the test data:

remove 30600 files

• legacy:
  24855.97 msec task-clock # 0.965 CPUs utilized
  6809 context-switches # 273.938 /sec
  9 cpu-migrations # 0.362 /sec
  2287 page-faults # 92.010 /sec
  58198042110 cycles # 2.341 GHz
  45389686010 instructions # 0.78 insn per cycle
  10407138113 branches # 418.698 M/sec
  18144553 branch-misses # 0.17% of all branches

  25.746234000 seconds time elapsed
   0.143492000 seconds user
  24.560907000 seconds sys
  

• like ext4
  13845.94 msec task-clock # 1.000 CPUs utilized
  68 context-switches # 4.911 /sec
  0 cpu-migrations # 0.000 /sec
  2285 page-faults # 165.030 /sec
  34152461876 cycles # 2.467 GHz
  41149033388 instructions # 1.20 insn per cycle
  9555245488 branches # 690.112 M/sec
  5952012 branch-misses # 0.06% of all branches

  13.852185875 seconds time elapsed
   0.074034000 seconds user
  13.774539000 seconds sys
  

close #63

[jserv]

This patch looks great. Can you write down the known limitations as well?

[RoyWFHuang]

ok, give me some time to summarize

[RoyWFHuang]

I don't think there are any additional restrictions. Same as old version.
Here are some conclusions about this change:

1. The maximum number of files allowed per directory is reduced to 30,600 (for 200 M image file)
2. Listing operation is slower compared to the legacy method
3. Creating operation is slower compared to the legacy method

Here is the performance test result:
https://hackmd.io/@royhuang/Boost_ext_blk_access#Test-the-performance

[jserv]

Thank @RoyWFHuang for contributing!