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Zach Brown
3c2b329675
Server commits use an allocator that has a limited number of available metadata blocks and entries in a list for freed blocks. The allocator is refilled between commits. Holders can't fully consume the allocator during the commit and that tended to work out because server commit holders commit before sending responses. We'd tend to commit frequently enough that we'd get a chance to refill the allocators before they were consumed. But there was no mechanism to ensure that this would be the case. Enough concurrent server holders were able to fully consume the allocators before committing. This causes scoutfs_meta_alloc and _free to return errors, leading the server to fail in the worst cases. This changes the server commit tracking to use more robust structures which limit the number of concurrent holders so that the allocators aren't exhausted. The commit_users struct stops holders from making progress once the allocators don't have room for more holders. It also lets us stop future holders from making progress once the commit work has been queued. The previous cute use of a rwsem didn't allow for either of these protections. We don't have precise tracking of each holder's allocation consumption so we don't try and reserve blocks for each holder. Instead we have a maxmimum consumption per holder and make sure that all the holders can't consume the allocators if they all use their full limit. All of this requires the holding code paths to be well behaved and not use more than the per-hold limit. We add some debugging code to print the stacks of holders that were active when the total holder limit was exceeded. This is the motivation for having state in the holders. We can record some data at the time their hold started that'll make it a little easier to track down which of the holders exceeded their limit. 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
Community Mailing List
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