Nikita Amelchev created IGNITE-29175:
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Summary: Copy compressed snapshot partitions sparsely on restore
instead of punching holes
Key: IGNITE-29175
URL: https://issues.apache.org/jira/browse/IGNITE-29175
Project: Ignite
Issue Type: Improvement
Reporter: Nikita Amelchev
Assignee: Nikita Amelchev
Partition files of a compressed cache group are sparse: each page keeps only
its compressed bytes,
the rest of the page is a hole.
When such a group is restored from a snapshot, every partition goes through two
passes:
- the file is copied as a dense file, so the holes turn into zeros on disk;
- the copy is read again page by page, and a hole is punched after each page
with a separate
fallocate call.
On a store with many partitions this makes the restore much slower than
creating the same snapshot.
The copy can write only the compressed bytes of each page instead: the file
system leaves the rest
of the page unallocated, and the second pass is not needed.
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