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https://issues.apache.org/jira/browse/OAK-4014?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15513760#comment-15513760
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Michael Dürig edited comment on OAK-4014 at 9/22/16 4:35 PM:
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Histogram of the tar file sizes: !tar_sizes.png|width=400!
was (Author: mduerig):
Histogram of the tar file sizes: !tar_sizes.png!
> 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, tar_sizes.png
>
>
> 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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