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https://github.com/kdave/btrfsmaintenance

Scripts for btrfs maintenance tasks like periodic scrub, balance, trim or defrag on selected mountpoints or directories.
https://github.com/kdave/btrfsmaintenance

administration btrfs

Last synced: 11 days ago
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Scripts for btrfs maintenance tasks like periodic scrub, balance, trim or defrag on selected mountpoints or directories.

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README 

        
.nh

.TH Btrfs maintenance toolbox

.PP

Table of contents:



.RS

.IP \(bu 2

Quick start

\[la]#quick-start\[ra]

.IP \(bu 2

Distro integration

\[la]#distro-integration\[ra]

.IP \(bu 2

Tuning periodic snapshotting

\[la]#tuning-periodic-snapshotting\[ra]



.RE



.PP

This is a set of scripts supplementing the btrfs filesystem and aims to automate

a few maintenance tasks. This means the \fIscrub\fP, \fIbalance\fP, \fItrim\fP or

\fIdefragmentation\fP\&.



.PP

Each of the tasks can be turned on/off and configured independently. The

default config values were selected to fit the default installation profile

with btrfs on the root filesystem.



.PP

Overall tuning of the default values should give a good balance between effects

of the tasks and low impact of other work on the system. If this does not fit

your needs, please adjust the settings.



.SH Tasks

.PP

The following sections will describe the tasks in detail. There's one config

option that affects the task concurrency, \fB\fCBTRFS\_ALLOW\_CONCURRENCY\fR\&. This is

to avoid extra high resource consumption or unexpected interaction among the

tasks and will serialize them in the order they're started by timers.



.SS scrub

.PP

\fBDescription:\fP Scrub operation reads all data and metadata from the devices

and verifies the checksums. It's not mandatory, but may point out problems with

faulty hardware early as it touches data that might not be in use and bit rot.



.PP

If there's a redundancy of data/metadata, ie. the \fIDUP\fP or \fIRAID1/5/6\fP profiles, scrub

is able to repair the data automatically if there's a good copy available.



.PP

\fBImpact when active:\fP Intense read operations take place and may slow down or

block other filesystem activies, possibly only for short periods.



.PP

\fBTuning:\fP



.RS

.IP \(bu 2

the recommended period is once in a month but a weekly period is also acceptable

.IP \(bu 2

you can turn off the automatic repair (\fB\fCBTRFS\_SCRUB\_READ\_ONLY\fR)

.IP \(bu 2

the default IO priority is set to \fIidle\fP but scrub may take long to finish,

you can change priority to \fInormal\fP (\fB\fCBTRFS\_SCRUB\_PRIORITY\fR)



.RE



.PP

\fBRelated commands:\fP



.RS

.IP \(bu 2

you can check status of last scrub run (either manual or through the cron

job) by \fB\fCbtrfs scrub status /path\fR

.IP \(bu 2

you can cancel a running scrub anytime if you find it inconvenient (\fB\fCbtrfs

scrub cancel /path\fR), the progress state is saved each 5 seconds and next

time scrub will start from that point



.RE



.SS balance

.PP

\fBDescription:\fP The balance command can do a lot of things, in general moves

data around in big chunks. Here we use it to reclaim back the space of the

underused chunks so it can be allocated again according to current needs.



.PP

The point is to prevent some corner cases where it's not possible to eg.

allocate new metadata chunks because the whole device space is reserved for all

the chunks, although the total space occupied is smaller and the allocation

should succeed.



.PP

The balance operation needs enough workspace so it can shuffle data around. By

workspace we mean device space that has no filesystem chunks on it, not to be

confused by free space as reported eg. by \fB\fCdf\fR\&.



.PP

\fBImpact when active:\fP Possibly big. There's a mix of read and write operations, is

seek\-heavy on rotational devices. This can interfere with other work in case

the same set of blocks is affected.



.PP

The balance command uses filters to do the work in smaller batches.



.PP

Before kernel version 5.2, the impact with quota groups enabled can be extreme.

The balance operation performs quota group accounting for every extent being

relocated, which can have the impact of stalling the file system for an

extended period of time.



.PP

\fBExpected result:\fP If possible all the underused chunks are removed, the

value of \fB\fCtotal\fR in output of \fB\fCbtrfs fi df /path\fR should be lower than before.

Check the logs.



.PP

The balance command may fail with \fIno space\fP reason but this is considered a

minor fault as the internal filesystem layout may prevent the command to find

enough workspace. This might be a time for manual inspection of space.



.PP

\fBTuning:\fP



.RS

.IP \(bu 2

you can make the space reclaim more aggressive by adding higher percentage to

\fB\fCBTRFS\_BALANCE\_DUSAGE\fR or \fB\fCBTRFS\_BALANCE\_MUSAGE\fR\&. Higher value means bigger

impact on your system and becomes very noticeable.

.IP \(bu 2

the metadata chunks usage pattern is different from data and it's not

necessary to reclaim metadata block groups that are more than 30 full. The

default maximum is 10 which should not degrade performance too much but may

be suboptimal if the metadata usage varies wildly over time. The assumption

is that underused metadata chunks will get used at some point so it's not

absolutely required to do the reclaim.

.IP \(bu 2

the useful period highly depends on the overall data change pattern on the

filesystem



.RE



.PP

\fBChanged defaults since 0.5:\fP



.PP

Versions up to 0.4.2 had usage filter set up to 50% for data and up to 30% for

metadata.  Based on user feedback, the numbers have been reduced to 10% (data)

and 5% (metadata). The system load during the balance service will be smaller

and the result of space compaction still reasonable. Multiple data chunks filled

to less than 10% can be merged into fewer chunks. The file data can change in

large volumes, eg. deleting a big file can free a lot of space. If the space is

left unused for the given period, it's desirable to make it more compact.

Metadata consumption follows a different pattern and reclaiming only the almost

unused chunks makes more sense, otherwise there's enough reserved metadata

space for operations like reflink or snapshotting.



.PP

A convenience script is provided to update the unchanged defaults,

\fB\fC/usr/share/btrfsmaintenance/update\-balance\-usage\-defaults.sh\fR .



.SS trim

.PP

\fBDescription:\fP The TRIM operation (aka. \fIdiscard\fP) can instruct the underlying device to

optimize blocks that are not used by the filesystem. This task is performed

on\-demand by the \fIfstrim\fP utility.



.PP

This makes sense for SSD devices or other type of storage that can translate

the TRIM action to something useful (eg. thin\-provisioned storage).



.PP

\fBImpact when active:\fP Should be low, but depends on the amount of blocks

being trimmed.



.PP

\fBTuning:\fP



.RS

.IP \(bu 2

the recommended period is weekly, but monthly is also fine

.IP \(bu 2

the trim commands might not have an effect and are up to the device, eg. a

block range too small or other constraints that may differ by device

type/vendor/firmware

.IP \(bu 2

the default configuration is \fIoff\fP because of the the system fstrim.timer



.RE



.SS defrag

.PP

\fBDescription:\fP Run defragmentation on configured directories. This is for

convenience and not necessary as defragmentation needs are usually different

for various types of data.



.PP

Please note that the defragmentation process does not descend to other mount

points and nested subvolumes or snapshots. All nested paths would need to be

enumerated in the respective config variable. The command utilizes \fB\fCfind

\-xdev\fR, you can use that to verify in advance which paths will the

defragmentation affect.



.PP

\fBSpecial case:\fP



.PP

There's a separate defragmentation task that happens automatically and

defragments only the RPM database files. This is done via a \fIzypper\fP plugin

and the defrag pass triggers at the end of the installation.



.PP

This improves reading the RPM databases later, but the installation process

fragments the files very quickly so it's not likely to bring a significant

speedup here.



.SH Periodic scheduling

.PP

There are now two ways how to schedule and run the periodic tasks: cron and

systemd timers. Only one can be active on a system and this should be decided

at the installation time.



.SS Cron

.PP

Cron takes care of periodic execution of the scripts, but they can be run any

time directly from \fB\fC/usr/share/btrfsmaintenance/\fR, respecting the configured

values in \fB\fC/etc/sysconfig/btrfsmaintenance\fR\&.



.PP

The changes to configuration file need to be reflected in the \fB\fC/etc/cron\fR

directories where the scripts are linked for the given period.



.PP

If the period is changed, the cron symlinks have to be refreshed:



.RS

.IP \(bu 2

manually \-\- use \fB\fCsystemctl restart btrfsmaintenance\-refresh\fR (or the \fB\fCrcbtrfsmaintenance\-refresh\fR shortcut)

.IP \(bu 2

in \fIyast2\fP \-\- sysconfig editor triggers the refresh automatically

.IP \(bu 2

using a file watcher \-\- if you install \fB\fCbtrfsmaintenance\-refresh.path\fR, this will utilize the file monitor to detect changes and will run the refresh



.RE



.SS Systemd timers

.PP

There's a set of timer units that run the respective task script. The periods

are configured in the \fB\fC/etc/sysconfig/btrfsmaintenance\fR file as well. The

timers have to be installed using a similar way as cron.  Please note that the

'\fI\&.timer' and respective '\fP\&.service' files have to be installed so the timers

work properly.



.PP

Some package managers (eg. \fB\fCapt\fR) will configure the timers automatically at

install time \- you can check with \fB\fCls /usr/lib/systemd/system/btrfs*\fR\&.



.PP

To install the timers manually, run \fB\fCbtrfsmaintenance\-refresh\-cron.sh timer\fR\&.



.SH Quick start

.PP

The tasks' periods and other parameters should fit most use cases and do not

need to be touched. Review the mount points (variables ending with

\fB\fC\_MOUNTPOINTS\fR) whether you want to run the tasks there or not.



.SH Distro integration

.PP

Currently the support for widely used distros is present.  More distros can be

added. This section describes how the pieces are put together and should give

some overview.



.SS Installation

.PP

For debian based systems, run \fB\fCdist\-install.sh\fR as root.



.PP

For non\-debian based systems, check for distro provided package or

do manual installation of files as described below.



.RS

.IP \(bu 2

\fB\fCbtrfs\-*.sh\fR task scripts are expected at \fB\fC/usr/share/btrfsmaintenance\fR

.IP \(bu 2

\fB\fCsysconfig.btrfsmaintenance\fR configuration template is put to:

.RS

.IP \(bu 2

\fB\fC/etc/sysconfig/btrfsmaintenance\fR on SUSE and RedHat based systems or derivatives

.IP \(bu 2

\fB\fC/etc/default/btrfsmaintenance\fR on Debian and derivatives



.RE



.IP \(bu 2

\fB\fC/usr/lib/zypp/plugins/commit/btrfs\-defrag\-plugin.sh\fR or

\fB\fC/usr/lib/zypp/plugins/commit/btrfs\-defrag\-plugin.py\fR post\-update script for

zypper (the package manager), applies to SUSE\-based distros for now

.IP \(bu 2

cron refresh scripts are installed (see bellow)



.RE



.PP

The defrag plugin has a shell and python implementation, choose what suits the

installation better.



.SS cron jobs

.PP

The periodic execution of the tasks is done by the 'cron' service.  Symlinks to

the task scripts are located in the respective directories in

\fB\fC/etc/cron.\fR\&.



.PP

The script \fB\fCbtrfsmaintenance\-refresh\-cron.sh\fR will synchronize the symlinks

according to the configuration files. This can be called automatically by a GUI

configuration tool if it's capable of running post\-change scripts or services.

In that case there's \fB\fCbtrfsmaintenance\-refresh.service\fR systemd service.



.PP

This service can also be automatically started upon any modification of the

configuration file in \fB\fC/etc/sysconfig/btrfsmaintenance\fR by installing the

\fB\fCbtrfsmaintenance\-refresh.path\fR systemd watcher.



.SS Post\-update defragmentation

.PP

The package database files tend to be updated in a random way and get

fragmented, which particularly hurts on btrfs. For rpm\-based distros this means files

in \fB\fC/var/lib/rpm\fR\&. The script or plugin simply runs a defragmentation on the affected files.

See \fB\fCbtrfs\-defrag\-plugin.sh\fR or \fB\fCbtrfs\-defrag\-plugin.py\fR for more details.



.PP

At the moment the 'zypper' package manager plugin exists. As the package

managers differ significantly, there's no single plugin/script to do that.



.SS Settings

.PP

The settings are copied to the expected system location from the template

(\fB\fCsysconfig.btrfsmaintenance\fR). This is a shell script and can be sourced to obtain

values of the variables.



.PP

The template contains descriptions of the variables, default and possible

values and can be deployed without changes (expecting the root filesystem to be

btrfs).



.SH Tuning periodic snapshotting

.PP

There are various tools and handwritten scripts to manage periodic snapshots

and cleaning. The common problem is tuning the retention policy constrained by

the filesystem size and not running out of space.



.PP

This section will describe factors that affect that, using snapper

\[la]https://snapper.io\[ra]

as an example, but adapting to other tools should be straightforward.



.SS Intro

.PP

Snapper is a tool to manage snapshots of btrfs subvolumes. It can create

snapshots of given subvolume manually, periodically or in a pre/post way for

a given command. It can be configured to retain existing snapshots according

to time\-based settings. As the retention policy can be very different for

various use cases, we need to be able to find matching settings.



.PP

The settings should satisfy user's expectation about storing previous copies of

the subvolume but not taking too much space. In an extreme, consuming the whole

filesystem space and preventing some operations to finish.



.PP

In order to avoid such situations, the snapper settings should be tuned according

to the expected use case and filesystem size.



.SS Sample problem

.PP

Default settings of snapper on default root partition size can easily lead to

no\-space conditions (all TIMELINE values set to 10). Frequent system updates

make it happen earlier, but this also affects long\-term use.



.SS Factors affecting space consumption

.RS

.IP "  1." 5

frequency of snapshotting

.IP "  2." 5

amount of data changes between snapshots (delta)

.IP "  3." 5

snapshot retention settings

.IP "  4." 5

size of the filesystem



.RE



.PP

Each will be explained below.



.PP

The way how the files are changed affects the space consumption. When a new

data overwrite existing, the new data will be pinned by the following snapshot,

while the original data will belong to previous snapshot.  This means that the

allocated file blocks are freed after the last snapshot pointing to them is

gone.



.SS Tuning

.PP

The administrator/user is supposed to know the approximate use of the partition

with snapshots enabled.



.PP

The decision criteria for tuning is space consumption and we're optimizing to

maximize retention without running out of space.



.PP

All the factors are intertwined and we cannot give definite answers but rather

describe the tendencies.



.SS Snapshotting frequency

.RS

.IP \(bu 2

\fBautomatic\fP: if turned on with the \fB\fCTIMELINE\fR config option, the periodic

snapshots are taken hourly. The daily/weekly/monthly/yearly periods will keep

the first hourly snapshot in the given period.

.IP \(bu 2

\fBat package update\fP: package manager with snapper support will create

pre/post snapshots before/after an update happens.

.IP \(bu 2

\fBmanual\fP: the user can create a snapshot manually with \fB\fCsnapper create\fR,

with a given snapshot type (ie. single, pre, post).



.RE



.SS Amount of data change

.PP

This is a parameter hard to predict and calculate. We work with rough

estimates, eg. megabytes, gigabytes etc.



.SS Retention settings

.PP

The user is supposed to know possible needs of recovery or examination of

previous file copies stored in snapshots.



.PP

It's not recommended to keep too old snapshots, eg. monthly or even yearly if

there's no apparent need for that. The yearly snapshots should not substitute

backups, as they reside on the same partition and cannot be used for recovery.



.SS Filesystem size

.PP

Bigger filesystem allows for longer retention, higher frequency updates and

amount of data changes.



.PP

As an example of a system root partition, the recommended size is 30 GiB, but

50 GiB is selected by the installer if the snapshots are turned on.



.PP

For non\-system partition it is recommended to watch remaining free space.

Although getting an accurate value on btrfs is tricky, due to shared extents

and snapshots, the output of \fB\fCdf\fR gives a rough idea. Low space, like under a

few gigabytes is more likely to lead to no\-space conditions, so it's a good

time to delete old snapshots or review the snapper settings.



.SS Typical use cases

.SS A rolling distro

.RS

.IP \(bu 2

frequency of updates: high, multiple times per week

.IP \(bu 2

amount of data changed between updates: high



.RE



.PP

Suggested values:



.PP

.RS



.nf

TIMELINE\_LIMIT\_HOURLY="12"

TIMELINE\_LIMIT\_DAILY="5"

TIMELINE\_LIMIT\_WEEKLY="2"

TIMELINE\_LIMIT\_MONTHLY="1"

TIMELINE\_LIMIT\_YEARLY="0"



.fi

.RE



.PP

The size of root partition should be at least 30GiB, but more is better.



.SS Regular/enterprise distro

.RS

.IP \(bu 2

frequency of updates: low, a few times per month

.IP \(bu 2

amount of data changed between updates: low to moderate



.RE



.PP

Most data changes come probably from the package updates, in the range of

hundreds of megabytes per update.



.PP

Suggested values:



.PP

.RS



.nf

TIMELINE\_LIMIT\_HOURLY="12"

TIMELINE\_LIMIT\_DAILY="7"

TIMELINE\_LIMIT\_WEEKLY="4"

TIMELINE\_LIMIT\_MONTHLY="6"

TIMELINE\_LIMIT\_YEARLY="1"



.fi

.RE



.SS Big file storage

.RS

.IP \(bu 2

frequency of updates: moderate to high

.IP \(bu 2

amount of data changed between updates: no changes in files, new files added, old deleted



.RE



.PP

Suggested values:



.PP

.RS



.nf

TIMELINE\_LIMIT\_HOURLY="12"

TIMELINE\_LIMIT\_DAILY="7"

TIMELINE\_LIMIT\_WEEKLY="4"

TIMELINE\_LIMIT\_MONTHLY="6"

TIMELINE\_LIMIT\_YEARLY="0"



.fi

.RE



.PP

Note, that deleting a big file that has been snapshotted will not free the space

until all relevant snapshots are deleted.



.SS Mixed

.RS

.IP \(bu 2

frequency of updates: unpredictable

.IP \(bu 2

amount of data changed between updates: unpredictable



.RE



.PP

Examples:



.RS

.IP \(bu 2

home directory with small files (in range of kilobytes to megabytes), large files (hundreds of megabytes to gigabytes).

.IP \(bu 2

git trees, bare and checked out repositories



.RE



.PP

Not possible to suggest config numbers as it really depends on user

expectations. Keeping a few hourly snapshots should not consume too much space

and provides a copy of files, eg. to restore after accidental deletion.



.PP

Starting point:



.PP

.RS



.nf

TIMELINE\_LIMIT\_HOURLY="12"

TIMELINE\_LIMIT\_DAILY="7"

TIMELINE\_LIMIT\_WEEKLY="1"

TIMELINE\_LIMIT\_MONTHLY="0"

TIMELINE\_LIMIT\_YEARLY="0"



.fi

.RE



.SS Summary

.TS

allbox;

l l l l l l 

l l l l l l .

\fB\fCType\fR	\fB\fCHourly\fR	\fB\fCDaily\fR	\fB\fCWeekly\fR	\fB\fCMonthly\fR	\fB\fCYearly\fR

Rolling	12	5	2	1	0

Regular	12	7	4	6	1

Big files	12	7	4	6	0

Mixed	12	7	1	0	0

.TE



.SH About

.PP

The goal of this project is to help administering btrfs filesystems. It is not

supposed to be distribution specific. Common scripts/configs are preferred but

per\-distro exceptions will be added when necessary.



.PP

License: GPL 2

\[la]https://www.gnu.org/licenses/gpl-2.0.html\[ra]



.PP

Contributing guide

\[la]CONTRIBUTING.md\[ra]\&.