What is “btrfs quotas” and what is it for? Should it be enabled or not? Thank you !!!
The concept of quota has a long-standing tradition in the Unix world. Ever since computers allow multiple users to work simultaneously in one filesystem, there is the need to prevent one user from using up the entire space. Every user should get his fair share of the available resources.
In case of files, the solution is quite straightforward. Each file has an owner recorded along with it, and it has a size. Traditional quota just restricts the total size of all files that are owned by a user. The concept is quite flexible: if a user hits his quota limit, the administrator can raise it on the fly.
On the other hand, the traditional approach has only a poor solution to restrict directories. At installation time, the harddisk can be partitioned so that every directory (e.g. /usr, /var, …) that needs a limit gets its own partition. The obvious problem is that those limits cannot be changed without a reinstallation. The btrfs subvolume feature builds a bridge. Subvolumes correspond in many ways to partitions, as every subvolume looks like its own filesystem. With subvolume quota, it is now possible to restrict each subvolume like a partition, but keep the flexibility of quota. The space for each subvolume can be expanded or restricted on the fly.
As subvolumes are the basis for snapshots, interesting questions arise as to how to account used space in the presence of snapshots. If you have a file shared between a subvolume and a snapshot, whom to account the file to? The creator? Both? What if the file gets modified in the snapshot, should only these changes be accounted to it? But wait, both the snapshot and the subvolume belong to the same user home. I just want to limit the total space used by both! But somebody else might not want to charge the snapshots to the users.
Btrfs subvolume quota solves these problems by introducing groups of subvolumes and let the user put limits on them. It is even possible to have groups of groups. In the following, we refer to them as qgroups.
Each qgroup primarily tracks two numbers, the amount of total referenced space and the amount of exclusively referenced space.
referenced
space is the amount of data that can be reached from any of the subvolumes contained in the qgroup, while
exclusive
is the amount of data where all references to this data can be reached from within this qgroup.
Subvolume quota groups
The basic notion of the Subvolume Quota feature is the quota group, short qgroup. Qgroups are notated as level/id, e.g. the qgroup 3/2 is a qgroup of level 3. For level 0, the leading 0/ can be omitted. Qgroups of level 0 get created automatically when a subvolume/snapshot gets created. The ID of the qgroup corresponds to the ID of the subvolume, so 0/5 is the qgroup for the root subvolume. For the btrfs qgroup command, the path to the subvolume can also be used instead of 0/ID. For all higher levels, the ID can be chosen freely.
Each qgroup can contain a set of lower level qgroups, thus creating a hierarchy of qgroups.
5 Likes
This topic was automatically closed 14 days after the last reply. New replies are no longer allowed.