Zach BrownandZach Brown
dc47ec65e4
We were using a trailing owner offset to iterate over btree item values from the back of the block towards the front. We did this to reclaim fragmented free space in a block to satisfy an allocation instead of having to split the block, which is expensive mostly because it has to allocate and free metadata blocks. In the before times, we used to compact items by sorting items by their offset, moving them, and then sorting them by their keys again. The sorting by keys was expensive so we added these owner offsets to be able to compact without sorting. But the complexity of maintaining the owner metadata is not worth it. We can avoid the expensive sorting by keys by allocating a temporary array of item offsets and sorting only it by the value offset. That's nice and quick, it was the key comparisons that were expensive. Then we can remove the owner offset entirely, as well as the block header final free region that compaction needed. And we also don't compact as often in the modern era because we do the bulk of our work in the item cache instead of in the btree, and we've changed the split/merge/compaction heuristics to avoid constantly splitting/merging/comapcting and an item population happens to hover right around a shared threshold. Signed-off-by: Zach Brown <zab@versity.com>
Introduction
scoutfs is a clustered in-kernel Linux filesystem designed and built from the ground up to support large archival systems.
Its key differentiating features are:
- Integrated consistent indexing accelerates archival maintenance operations
- Log-structured commits allow nodes to write concurrently without contention
It meets best of breed expectations:
- Fully consistent POSIX semantics between nodes
- Rich metadata to ensure the integrity of metadata references
- Atomic transactions to maintain consistent persistent structures
- First class kernel implementation for high performance and low latency
- Open GPLv2 implementation
Learn more in the white paper.
Current Status
Alpha Open Source Development
scoutfs is under heavy active development. We're developing it in the open to give the community an opportunity to affect the design and implementation.
The core architectural design elements are in place. Much surrounding functionality hasn't been implemented. It's appropriate for early adopters and interested developers, not for production use.
In that vein, expect significant incompatible changes to both the format of network messages and persistent structures. To avoid mistakes the implementation currently calculates a hash of the format and ioctl header files in the source tree. The kernel module will refuse to mount a volume created by userspace utilities with a mismatched hash, and it will refuse to connect to a remote node with a mismatched hash. This means having to unmount, mkfs, and remount everything across many functional changes. Once the format is nailed down we'll wire up forward and back compat machinery and remove this temporary safety measure.
The current kernel module is developed against the RHEL/CentOS 7.x kernel to minimize the friction of developing and testing with partners' existing infrastructure. Once we're happy with the design we'll shift development to the upstream kernel while maintaining distro compatibility branches.
Community Mailing List
Please join us on the open scoutfs-devel@scoutfs.org mailing list hosted on Google Groups for all discussion of scoutfs.
Quick Start
This following a very rough example of the procedure to get up and running, experience will be needed to fill in the gaps. We're happy to help on the mailing list.
The requirements for running scoutfs on a small cluster are:
- One or more nodes running x86-64 CentOS/RHEL 7.4 (or 7.3)
- Access to a single shared block device
- IPv4 connectivity between the nodes
The steps for getting scoutfs mounted and operational are:
- Get the kernel module running on the nodes
- Make a new filesystem on the device with the userspace utilities
- Mount the device on all the nodes
In this example we run all of these commands on three nodes. The block device name is the same on all the nodes.
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Get the Kernel Module and Userspace Binaries
- Either use snapshot RPMs built from git by Versity:
rpm -i https://scoutfs.s3-us-west-2.amazonaws.com/scoutfs-repo-0.0.1-1.el7_4.noarch.rpm yum install scoutfs-utils kmod-scoutfs- Or use the binaries built from checked out git repositories:
yum install kernel-devel git clone git@github.com:versity/scoutfs-kmod-dev.git make -C scoutfs-kmod-dev module modprobe libcrc32c insmod scoutfs-kmod-dev/src/scoutfs.ko git clone git@github.com:versity/scoutfs-utils-dev.git make -C scoutfs-utils-dev alias scoutfs=$PWD/scoutfs-utils-dev/src/scoutfs -
Make a New Filesystem (destroys contents, no questions asked)
We specify that two of our three nodes must be present to form a quorum for the system to function.
scoutfs mkfs -Q 2 /dev/shared_block_device -
Mount the Filesystem
Each mounting node provides its local IP address on which it will run an internal server for the other mounts if it is elected the leader by the quorum.
mkdir /mnt/scoutfs mount -t scoutfs -o server_addr=$NODE_ADDR /dev/shared_block_device /mnt/scoutfs -
For Kicks, Observe the Metadata Change Index
The
meta_seqindex tracks the inodes that are changed in each transaction.scoutfs walk-inodes meta_seq 0 -1 /mnt/scoutfs touch /mnt/scoutfs/one; sync scoutfs walk-inodes meta_seq 0 -1 /mnt/scoutfs touch /mnt/scoutfs/two; sync scoutfs walk-inodes meta_seq 0 -1 /mnt/scoutfs touch /mnt/scoutfs/one; sync scoutfs walk-inodes meta_seq 0 -1 /mnt/scoutfs