1BTRFS-QUOTA(8) Btrfs Manual BTRFS-QUOTA(8)
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6 btrfs-quota - control the global quota status of a btrfs filesystem
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9 btrfs quota <subcommand> <args>
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12 The commands under btrfs quota are used to affect the global status of
13 quotas of a btrfs filesystem. The quota groups (qgroups) are managed by
14 the subcommand btrfs qgroup(8).
15 Note
17 Qgroups are different than the traditional user quotas and designed
18 to track shared and exclusive data per-subvolume. Please refer to
19 the section HIERARCHICAL QUOTA GROUP CONCEPTS for a detailed
20 description.
21 PERFORMANCE IMPLICATIONS
23 When quotas are activated, they affect all extent processing, which
24 takes a performance hit. Activation of qgroups is not recommended
25 unless the user intends to actually use them.
26 STABILITY STATUS
28 The qgroup implementation has turned out to be quite difficult as it
29 affects the core of the filesystem operation. Qgroup users have hit
30 various corner cases over time, such as incorrect accounting or system
31 instability. The situation is gradually improving and issues found and
32 fixed.
33
35 The concept of quota has a long-standing tradition in the Unix world.
36 Ever since computers allow multiple users to work simultaneously in one
37 filesystem, there is the need to prevent one user from using up the
38 entire space. Every user should get his fair share of the available
39 resources.
40 In case of files, the solution is quite straightforward. Each file has
42 an owner recorded along with it, and it has a size. Traditional quota
43 just restricts the total size of all files that are owned by a user.
44 The concept is quite flexible: if a user hits his quota limit, the
45 administrator can raise it on the fly.
46 On the other hand, the traditional approach has only a poor solution to
48 restrict directories. At installation time, the harddisk can be
49 partitioned so that every directory (eg. /usr, /var/, ...) that needs a
50 limit gets its own partition. The obvious problem is that those limits
51 cannot be changed without a reinstallation. The btrfs subvolume feature
52 builds a bridge. Subvolumes correspond in many ways to partitions, as
53 every subvolume looks like its own filesystem. With subvolume quota, it
54 is now possible to restrict each subvolume like a partition, but keep
55 the flexibility of quota. The space for each subvolume can be expanded
56 or restricted on the fly.
57 As subvolumes are the basis for snapshots, interesting questions arise
59 as to how to account used space in the presence of snapshots. If you
60 have a file shared between a subvolume and a snapshot, whom to account
61 the file to? The creator? Both? What if the file gets modified in the
62 snapshot, should only these changes be accounted to it? But wait, both
63 the snapshot and the subvolume belong to the same user home. I just
64 want to limit the total space used by both! But somebody else might not
65 want to charge the snapshots to the users.
66 Btrfs subvolume quota solves these problems by introducing groups of
68 subvolumes and let the user put limits on them. It is even possible to
69 have groups of groups. In the following, we refer to them as qgroups.
70 Each qgroup primarily tracks two numbers, the amount of total
72 referenced space and the amount of exclusively referenced space.
73 referenced
75 space is the amount of data that can be reached from any of the
76 subvolumes contained in the qgroup, while
77 exclusive
79 is the amount of data where all references to this data can be
80 reached from within this qgroup.
81 SUBVOLUME QUOTA GROUPS
83 The basic notion of the Subvolume Quota feature is the quota group,
84 short qgroup. Qgroups are notated as level/id, eg. the qgroup 3/2 is a
85 qgroup of level 3. For level 0, the leading 0/ can be omitted. Qgroups
86 of level 0 get created automatically when a subvolume/snapshot gets
87 created. The ID of the qgroup corresponds to the ID of the subvolume,
88 so 0/5 is the qgroup for the root subvolume. For the btrfs qgroup
89 command, the path to the subvolume can also be used instead of 0/ID.
90 For all higher levels, the ID can be chosen freely.
91 Each qgroup can contain a set of lower level qgroups, thus creating a
93 hierarchy of qgroups. Figure 1 shows an example qgroup tree.
94 +---+
96 |2/1|
97 +---+
98 / \
99 +---+/ \+---+
100 |1/1| |1/2|
101 +---+ +---+
102 / \ / \
103 +---+/ \+---+/ \+---+
104 qgroups |0/1| |0/2| |0/3|
105 +-+-+ +---+ +---+
106 | / \ / \
107 | / \ / \
108 | / \ / \
109 extents 1 2 3 4
110 Figure1: Sample qgroup hierarchy
112 At the bottom, some extents are depicted showing which qgroups
114 reference which extents. It is important to understand the notion of
115 referenced vs exclusive. In the example, qgroup 0/2 references extents
116 2 and 3, while 1/2 references extents 2-4, 2/1 references all extents.
117 On the other hand, extent 1 is exclusive to 0/1, extent 2 is exclusive
119 to 0/2, while extent 3 is neither exclusive to 0/2 nor to 0/3. But
120 because both references can be reached from 1/2, extent 3 is exclusive
121 to 1/2. All extents are exclusive to 2/1.
122 So exclusive does not mean there is no other way to reach the extent,
124 but it does mean that if you delete all subvolumes contained in a
125 qgroup, the extent will get deleted.
126 Exclusive of a qgroup conveys the useful information how much space
128 will be freed in case all subvolumes of the qgroup get deleted.
129 All data extents are accounted this way. Metadata that belongs to a
131 specific subvolume (i.e. its filesystem tree) is also accounted.
132 Checksums and extent allocation information are not accounted.
133 In turn, the referenced count of a qgroup can be limited. All writes
135 beyond this limit will lead to a Quota Exceeded error.
136 INHERITANCE
138 Things get a bit more complicated when new subvolumes or snapshots are
139 created. The case of (empty) subvolumes is still quite easy. If a
140 subvolume should be part of a qgroup, it has to be added to the qgroup
141 at creation time. To add it at a later time, it would be necessary to
142 at least rescan the full subvolume for a proper accounting.
143 Creation of a snapshot is the hard case. Obviously, the snapshot will
145 reference the exact amount of space as its source, and both source and
146 destination now have an exclusive count of 0 (the filesystem nodesize
147 to be precise, as the roots of the trees are not shared). But what
148 about qgroups of higher levels? If the qgroup contains both the source
149 and the destination, nothing changes. If the qgroup contains only the
150 source, it might lose some exclusive.
151 But how much? The tempting answer is, subtract all exclusive of the
153 source from the qgroup, but that is wrong, or at least not enough.
154 There could have been an extent that is referenced from the source and
155 another subvolume from that qgroup. This extent would have been
156 exclusive to the qgroup, but not to the source subvolume. With the
157 creation of the snapshot, the qgroup would also lose this extent from
158 its exclusive set.
159 So how can this problem be solved? In the instant the snapshot gets
161 created, we already have to know the correct exclusive count. We need
162 to have a second qgroup that contains all the subvolumes as the first
163 qgroup, except the subvolume we want to snapshot. The moment we create
164 the snapshot, the exclusive count from the second qgroup needs to be
165 copied to the first qgroup, as it represents the correct value. The
166 second qgroup is called a tracking qgroup. It is only there in case a
167 snapshot is needed.
168 USE CASES
170 Below are some usecases that do not mean to be extensive. You can find
171 your own way how to integrate qgroups.
172 SINGLE-USER MACHINE
174 Replacement for partitions
175 The simplest use case is to use qgroups as simple replacement for
177 partitions. Btrfs takes the disk as a whole, and /, /usr, /var,
178 etc. are created as subvolumes. As each subvolume gets it own
179 qgroup automatically, they can simply be restricted. No hierarchy
180 is needed for that.
181 Track usage of snapshots
183 When a snapshot is taken, a qgroup for it will automatically be
185 created with the correct values. Referenced will show how much is
186 in it, possibly shared with other subvolumes. Exclusive will be the
187 amount of space that gets freed when the subvolume is deleted.
188 MULTI-USER MACHINE
190 Restricting homes
191 When you have several users on a machine, with home directories
193 probably under /home, you might want to restrict /home as a whole,
194 while restricting every user to an individual limit as well. This
195 is easily accomplished by creating a qgroup for /home , eg. 1/1,
196 and assigning all user subvolumes to it. Restricting this qgroup
197 will limit /home, while every user subvolume can get its own
198 (lower) limit.
199 Accounting snapshots to the user
201 Let’s say the user is allowed to create snapshots via some
203 mechanism. It would only be fair to account space used by the
204 snapshots to the user. This does not mean the user doubles his
205 usage as soon as he takes a snapshot. Of course, files that are
206 present in his home and the snapshot should only be accounted once.
207 This can be accomplished by creating a qgroup for each user, say
208 1/UID. The user home and all snapshots are assigned to this qgroup.
209 Limiting it will extend the limit to all snapshots, counting files
210 only once. To limit /home as a whole, a higher level group 2/1
211 replacing 1/1 from the previous example is needed, with all user
212 qgroups assigned to it.
213 Do not account snapshots
215 On the other hand, when the snapshots get created automatically,
217 the user has no chance to control them, so the space used by them
218 should not be accounted to him. This is already the case when
219 creating snapshots in the example from the previous section.
220 Snapshots for backup purposes
222 This scenario is a mixture of the previous two. The user can create
224 snapshots, but some snapshots for backup purposes are being created
225 by the system. The user’s snapshots should be accounted to the
226 user, not the system. The solution is similar to the one from
227 section Accounting snapshots to the user, but do not assign system
228 snapshots to user’s qgroup.
229
231 disable <path>
232 Disable subvolume quota support for a filesystem.
233 enable <path>
235 Enable subvolume quota support for a filesystem.
236 rescan [-s] <path>
238 Trash all qgroup numbers and scan the metadata again with the
239 current config.
240 Options
242 -s
244 show status of a running rescan operation.
245 -w
247 wait for rescan operation to finish(can be already in
248 progress).
249
251 btrfs quota returns a zero exit status if it succeeds. Non zero is
252 returned in case of failure.
253
255 btrfs is part of btrfs-progs. Please refer to the btrfs wiki
256 http://btrfs.wiki.kernel.org for further details.
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259 mkfs.btrfs(8), btrfs-subvolume(8), btrfs-qgroup(8)
260Btrfs v5.15.1 11/22/2021 BTRFS-QUOTA(8)