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58 lines
2.5 KiB
HTML
58 lines
2.5 KiB
HTML
<header>Btrfs Subvolume Quotas</header>
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<h3>Introduction</h3>
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Btrfs quotas control disk usage for subvolumes through quota groups, usually
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called <tt>qgroups</tt>. Unlike traditional Unix quotas, they do not limit an
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individual user or group and do not provide soft limits, grace periods, or
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file-count limits. <p>
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Each Btrfs subvolume has a level-0 <tt>qgroup</tt>. The module displays the
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following usage and limit values for each <tt>qgroup</tt> :
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<dl>
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<dt><b>Referenced</b>
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<dd>All data reachable from the subvolume, including data shared with other
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subvolumes or snapshots.
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<dt><b>Exclusive</b>
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<dd>Data used only by the subvolume, which would be freed if it were deleted.
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<dt><b>Referenced limit</b>
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<dd>The maximum referenced space that the <tt>qgroup</tt> may use.
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<dt><b>Exclusive limit</b>
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<dd>The maximum exclusive space that the <tt>qgroup</tt> may use.
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</dl>
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<h3>Accounting Modes</h3>
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When Btrfs quotas are enabled, the accounting mode configured in the module
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settings is used :
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<dl>
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<dt><b>Full accounting</b>
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<dd>Tracks shared space between subvolumes and snapshots. This is the
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recommended mode when accurate referenced and exclusive usage is required.
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<dt><b>Simple accounting</b>
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<dd>Tracks original ownership with lower overhead, but does not fully track
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space shared between subvolumes and snapshots.
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</dl>
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In simple accounting mode, both values show space assigned to the subvolume
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that first wrote the data. <p>
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Changing the module setting does not convert an already-enabled filesystem.
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The selected mode is used the next time quotas are enabled. <p>
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<h3>Managing Btrfs Quotas</h3>
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The main module page shows each mounted Btrfs filesystem, its accounting mode,
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consistency state, and an action to enable or disable quotas. Click a filesystem
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path to view its <tt>qgroups</tt>, usage, and limits. Click a <tt>qgroup</tt> ID
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to edit its referenced and exclusive limits. The top-level <tt>qgroup</tt>
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<tt>0/5</tt> is shown for information only because limiting it could stop
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filesystem changes. <p>
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Because quota state and <tt>qgroup</tt> IDs apply to the whole underlying Btrfs
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filesystem, delegated Webmin users must be allowed to manage all filesystems to
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access these controls. A permission scoped to one mounted subvolume is not
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sufficient. <p>
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Full accounting also provides a rescan action for rebuilding <tt>qgroup</tt>
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accounting in the background. Disabling Btrfs quotas removes all
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<tt>qgroup</tt> configuration and limits on the filesystem, so the module
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always requests confirmation first. <p>
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<hr>
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