Michael Dürig updated OAK-4014:
    Attachment: tar_sizes.m

[^tar_sizes.m] contains the sizes of the tar files of the x1, x4 and x8 
instances of our internal longevity test. I'm attaching them here as evidence 
for deciding what to do with this issue and with what priority. Some quick 
statistics over all sizes collected:

median = 269MB
mean = 246MB
Only 8.5% of the tar files are smaller than 100MB and 0.5% smaller then 1MB.

> The segment store should merge small TAR files into bigger ones
> ---------------------------------------------------------------
>                 Key: OAK-4014
>                 URL: https://issues.apache.org/jira/browse/OAK-4014
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segment-tar
>            Reporter: Francesco Mari
>            Assignee: Francesco Mari
>             Fix For: Segment Tar 0.0.16
>         Attachments: tar_sizes.m
> The cleanup process removes unused segments from TAR files and writes new 
> generations of those TAR files without the removed segments.
> In the long run, the size of some TAR file might be smaller than the maximum 
> size allowed for a TAR file. At the time this issue was created the default 
> maximum size of a TAR file is 256 MiB.
> If there are many small TAR files, it should be possible to merge them in 
> bigger files. This way, we can reduce the total number of TAR files in the 
> segment store, and thus the number of open file descriptors that Oak has to 
> maintain.
> A possible implementation for the merge operation is the following:
> # Sort the list of TAR files by size, ascending.
> # Pick TAR files for the sorted list until the sum of their sizes after the 
> merge is less than 256 MiB.
> # Merge the picked up files into a new TAR file and marked the picked up 
> files for deletion.
> # Continue picking up TAR files from the sorted list until the list is 
> exhausted or until it's only possible to pick a single TAR file.
> The merge process can run in a background thread but it is important that it 
> doesn't conflict with the cleanup operation, since merge and cleanup both 
> change the representation of TAR files on the file system. Two possible 
> solutions to avoid conflicts are:
> # Use a global lock for the whole set of TAR files.
> # Use a lock per TAR file. The cleanup and merge processes have to agree on 
> the order to use when acquiring the lock.

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