Redesign DiskMRU to simplify it

[Janik Zikovsky]

DiskMRU got very complicated with a MRU, a write buffer, a free-space buffer and a small objects count. Possible problems with updating files are related to this? But I can't work it out with the complex structure of it.

I found that the MRU was too slow (in look-up) in practice, and the Small Objects buffer made loading a file super slow.

Write a new class. Include a "to-write" buffer (that will be handy for sorting the writes so that they are done sequentially) and a free-space manager. Forget about the MRU and the small objects.

[Janik Zikovsky]

See #4377: failures seem related to the DiskMRU, in particular, the SmallBuffer functionality.

Removing this will hopefully fix it. Will make a new class called "DiskWriteBuffer"

[Janik Zikovsky]

Refs #4138: Replaced DiskMRU with DiskBuffer, a simpler version

with no MRU or 'small object' buffer. This is now simple a to-write buffer that tracks free space. Hopefully fix issue in #4377.

Changeset: 786ff3bacdfb44a4d0807bc78fd097f107796943

[Janik Zikovsky]

Refs #4138: rename some methods and variables to match DiskBuffer

Changeset: 8710ed8a9b1ac80a0d3972d0077c5a6a7585efdb

[Janik Zikovsky]

Refs #4138: test fix

Changeset: f1ab1767f99b5e5e6c5873d08cc7cd01e770f967

[Janik Zikovsky]

Refs #4138: remove debugging output

Changeset: 28213176eb9851259c9cc06cee083238834b9864

[Janik Zikovsky]

Refs #4138: Avoid possible simultaneous read/write to file

by adding a mutex to DiskBuffer

Changeset: 75369a180342698363ad83d5ee71a95879c9b6f8

[Janik Zikovsky]

This has been completed and seems fully stable. The DiskMRU class has been taken out of the CMakeLists, replaced with DiskBuffer.

[Janik Zikovsky]

Refs #4138: clean up files

Changeset: 7dcdfa8a44f0f0567e44ce929e7a4263b7a9d7b9

[Stuart Campbell]

This ticket has been transferred to github issue ​4985