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The max_seq and active reader mechanisms in the item cache stop readers from reading old items and inserting them in the cache after newer items have been reclaimed by memory pressure. The max_seq field in the pages must reflect the greatest seq of the items in the page so that reclaim knows that the page contains items newer than old readers and must not be removed. We update the page max_seq as items are inserted or as they're dirtied in the page. There's an additional subtle effect that the max_seq can also protect items which have been erased. Deletion items are erased from the pages as a commit completes. The max_seq in that page will still protect it from being reclaimed even though no items have that seq value themselves. That protection fails if the range of keys containing the erased item is moved to another page with a lower max_seq. The item mover only updated the destination page's max_seq for each item that was moved. It missed that the empty space between the items might have a larger max_seq from an erased item. We don't know where the erased item is so we have to assume that a larger max_seq in the source page must be set on the destination page. This could explain very rare item cache corruption where nodes were seeing deleted directory entry items reappearing. It would take a specific sequence of events involving large directories with an isolated removal, a delayed item cache reader, a commit, and then enough insertions to split the page all happening in precisely the wrong sequence. Signed-off-by: Zach Brown <zab@versity.com>
Introduction
scoutfs is a clustered in-kernel Linux filesystem designed to support large archival systems. It features additional interfaces and metadata so that archive agents can perform their maintenance workflows without walking all the files in the namespace. Its cluster support lets deployments add nodes to satisfy archival tier bandwidth targets.
The design goal is to reach file populations in the trillions, with the archival bandwidth to match, while remaining operational and responsive.
Highlights of the design and implementation include:
- Fully consistent POSIX semantics between nodes
- Atomic transactions to maintain consistent persistent structures
- Integrated archival metadata replaces syncing to external databases
- Dynamic seperation of resources lets nodes write in parallel
- 64bit throughout; no limits on file or directory sizes or counts
- Open GPLv2 implementation
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