Improve fsimage load time by writing sub-sections to the fsimage index

Loading an fsimage is basically a single threaded process. The current fsimage is written out in sections, eg iNode, iNode_Directory, Snapshots, Snapshot_Diff etc. Then at the end of the file, an index is written that contains the offset and length of each section. The image loader code uses this index to initialize an input stream to read and process each section. It is important that one section is fully loaded before another is started, as the next section depends on the results of the previous one.

What I would like to propose is the following:

1. When writing the image, we can optionally output sub_sections to the index. That way, a given section would effectively be split into several sections, eg:

   inode_section offset 10 length 1000
     inode_sub_section offset 10 length 500
     inode_sub_section offset 510 length 500
     
   inode_dir_section offset 1010 length 1000
     inode_dir_sub_section offset 1010 length 500
     inode_dir_sub_section offset 1010 length 500

Here you can see we still have the original section index, but then we also have sub-section entries that cover the entire section. Then a processor can either read the full section in serial, or read each sub-section in parallel.

2. In the Image Writer code, we should set a target number of sub-sections, and then based on the total inodes in memory, it will create that many sub-sections per major image section. I think the only sections worth doing this for are inode, inode_reference, inode_dir and snapshot_diff. All others tend to be fairly small in practice.

3. If there are under some threshold of inodes (eg 10M) then don't bother with the sub-sections as a serial load only takes a few seconds at that scale.

4. The image loading code can then have a switch to enable 'parallel loading' and a 'number of threads' where it uses the sub-sections, or if not enabled falls back to the existing logic to read the entire section in serial.

Working with a large image of 316M inodes and 35GB on disk, I have a proof of concept of this change working, allowing just inode and inode_dir to be loaded in parallel, but I believe inode_reference and snapshot_diff can be make parallel with the same technique.

Some benchmarks I have are as follows:

Threads   1     2     3     4 
--------------------------------
inodes    448   290   226   189 
inode_dir 326   211   170   161 
Total     927   651   535   488 (MD5 calculation about 100 seconds)

The above table shows the time in seconds to load the inode section and the inode_directory section, and then the total load time of the image.

With 4 threads using the above technique, we are able to better than half the load time of the two sections. With the patch in HDFS-13694 it would take a further 100 seconds off the run time, going from 927 seconds to 388, which is a significant improvement. Adding more threads beyond 4 has diminishing returns as there are some synchronized points in the loading code to protect the in memory structures.