Zach BrownandZach Brown
9375b9d3b7
Dirty items in a client transaction are stored in OS pages. When the transaction is committed each item is stored in its position in a dirty btree block in the client's existing log btree. Allocators are refilled between transaction commits so a given commit must have sufficient meta allocator space (avail blocks and unused freed entries) for all the btree blocks that are dirtied. The number of btree blocks that are written, thus the number of cow allocations and frees, depends on the number of blocks in the log btree and the distribution of dirty items amongst those blocks. In a typical load items will be near each other and many dirty items in smaller kernel pages will be stored in fewer larger btree blocks. But with the right circumstances, the ratio of dirty pages to dirty blocks can be much smaller. With a very large directory and random entry renames you can easily have 1 btree block dirtied for every page of dirty items. Our existing allocator meta allocator fill targets and the number of dirty item cache pages we allowed did not properly take this in to account. It was possible (and, it turned out, relatively easy to test for with a hgue directory and random renames) to run out of meta avail blocks while storing dirty items in dirtied btree blocks. This rebalances our targets and thresholds to make it more likely that we'll have enough allocator resources to commit dirty items. Instead of having an arbitrary limit on the number of dirty item cache pages, we require that a given number of dirty item cache pages have a given number of allocator blocks available. We require a decent number of avialable blocks for each dirty page, so we increase the server's target number of blocks to give the client so that it can still build large transactions. This code is conservative and should not be a problem in practice, but it's theoretically possible to build a log btree and set of dirty items that would dirty more blocks that this code assumes. We will probably revisit this as we add proper support for ENOSPC. 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 two shared block devices
- 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 devices with the userspace utilities
- Mount the devices on all the nodes
In this example we run all of these commands on three nodes. The names of the block devices are 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=$PWD/scoutfs-utils-dev/src/scoutfs -
Make a New Filesystem (destroys contents, no questions asked)
We specify that two of our three nodes must be present to form a quorum for the system to function.
scoutfs mkfs -Q 2 /dev/meta_dev /dev/data_dev -
Mount the Filesystem
Each mounting node provides its local IP address on which it will run an internal server for the other mounts if it is elected the leader by the quorum.
mkdir /mnt/scoutfs mount -t scoutfs -o server_addr=$NODE_ADDR,metadev_path=/dev/meta_dev /dev/data_dev /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