Thank you again for a great library.

We wanted to address one more merge issue in 
We need to merge a large number of small files.  We use mixed mode, memory and 
disk for cache.  Initially, we would often get "Maximum allowed scratch file 
memory exceeded.", unless we turned off the check by passing "-1" to 
org.apache.pdfbox.io.MemoryUsageSetting#MemoryUsageSetting.  I believe, this is 
what the users that opened https://issues.apache.org/jira/browse/PDFBOX-3721 
where running into.

Our research indicates that the core issue with the memory model is that 
instead of sharing a single cache, it breaks it up into equal sized fixed 
partitions based on the number of input + output files being merged.  This 
means that each partition must be big enough to hold the final output file.  
When 400 1-page files are merged, this creates 401 partitions, but each of 
which needs to be big enough to hold the final 400 pages.  Even worse, the 
merge algorithm needs to keep all files open until the end.

Given this, near the end of the merge, we're actually caching 400 x 1-page 
input files, and 1 x 400-page output file, or 801 pages.
However, with the partitioned cache, we need to declare room for 401  x 
400-pages, or 160,400 pages in total when specifying "maxStorageBytes".  This 
would be a very high number, usually in GIGs.

Given the current limitation that we need to keep all the input files open 
until the output file is written (HUGE), we came up with 2 options.

  1.  Good: Split the cache in ½, give ½ to the output file, and segment the 
other ½ across the input files. (Still keeping them open until then end).
  2.  Better: Dynamically allocate in 16 page (64K) chunks from memory or disk 
on demand, release cache as documents are closed after merge.  This is our 
current implementation till PDFBOX-3999 is addressed.

We would like to submit our current implementation as a Patch to 2.0.10, unless 
this is already addressed.

Thank you

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