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btrbk
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Merge branch 'resume_only'

pull/30/head
Axel Burri 2015-05-20 21:52:30 +02:00
parent 09e214acf4 b52f40d5b4
commit 725191583e
5 changed files with 234 additions and 114 deletions
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4
ChangeLog
View File

@ -4,6 +4,10 @@ btrbk-current
* Set PATH variable instead of using absolute "/sbin/btrfs" for
compatibility with all linux distros out there, which all install
'btrfs' in different locations (closes: #20).
* Added configuration option "snapshot_create", replacing option
"snapshot_create_always". This allows setups with multiple btrbk
instances on several hosts (closes: #18).
* Added command line option -r (resume only).
* Catch and display errors from "btrfs subvolume show".
* Include systemd service and timer unit for daily backups.

49
README.md
View File

@ -156,11 +156,10 @@ Retention policy:
- `/mnt/btr_backup/mylaptop/rootfs.YYYYMMDD`
- `/mnt/btr_backup/mylaptop/home.YYYYMMDD`
If you want the snapshots to be created even if the backup disk is not
attached (when you're on the road), simply add the following line to
the config:
If you want the snapshots to be created only if the backup disk is
attached, simply add the following line to the config:
snapshot_create_always yes
snapshot_create ondemand
Example: host-initiated backup on fileserver
@ -215,19 +214,51 @@ This will pull backups from alpha/beta.mydomain.com and locally create:
Example: local time-machine (daily snapshots)
---------------------------------------------
If all you want is a local time-machine of your home directory:
If all you want is creating snapshots of your home directory on a
regular basis:
/etc/btrbk/btrbk-timemachine.conf:
/etc/btrbk/btrbk.conf:
volume /mnt/btr_pool
snapshot_dir btrbk_snapshots
subvolume home
snapshot_dir btrbk_snapshots
snapshot_create_always yes
/etc/cron.daily/btrbk:
#!/bin/bash
/usr/sbin/btrbk -c /etc/btrbk/btrbk-timemachine.conf run
/usr/sbin/btrbk run
Note that you can run btrbk more than once a day, e.g. by creating the
above script in `/etc/cron.hourly/btrbk`, or by calling `sudo btrbk
run` from the command line.
Example: multiple btrbk instances
---------------------------------
Let's say we have a host (at 192.168.0.42) running btrbk with the
setup of the time-machine example above, and we need a backup server
to only fetch the snapshots.
/etc/btrbk/btrbk.conf (on backup server):
volume ssh://192.168.0.42/mnt/btr_pool
subvolume home
snapshot_dir btrbk_snapshots
snapshot_preserve_daily all
snapshot_create no
resume_missing yes
target_preserve_daily 0
target_preserve_weekly 10
target_preserve_monthly all
target send-receive /mnt/btr_backup/my-laptop.com
If the server runs btrbk with this config, the latest snapshot (which
is *always* transferred) as well as 10 weeklies and all monthlies are
received from 192.168.0.42. The source filesystem is never altered
because of `snapshot_preserve_daily all`.
Setting up SSH

255
btrbk
View File

@ -47,7 +47,7 @@ use Date::Calc qw(Today Delta_Days Day_of_Week);
use Getopt::Std;
use Data::Dumper;
our $VERSION = "0.17.2-dev";
our $VERSION = "0.18.0-dev";
our $AUTHOR = 'Axel Burri <axel@tty0.ch>';
our $PROJECT_HOME = '<http://www.digint.ch/btrbk/>';
@ -62,9 +62,8 @@ my %config_options = (
# NOTE: keys "volume", "subvolume" and "target" are hardcoded
snapshot_dir => { default => undef, accept_file => { relative => 1 } },
snapshot_name => { default => undef, accept_file => { name_only => 1 }, context => [ "subvolume" ] },
receive_log => { default => undef, accept => [ "sidecar", "no" ], accept_file => { absolute => 1 }, deprecated => "removed" },
snapshot_create => { default => "always", accept => [ "no", "always", "ondemand" ] },
incremental => { default => "yes", accept => [ "yes", "no", "strict" ] },
snapshot_create_always => { default => undef, accept => [ "yes", "no" ] },
resume_missing => { default => "yes", accept => [ "yes", "no" ] },
preserve_day_of_week => { default => "sunday", accept => [ (keys %day_of_week_map) ] },
snapshot_preserve_daily => { default => "all", accept => [ "all" ], accept_numeric => 1 },
@ -77,6 +76,22 @@ my %config_options = (
ssh_identity => { default => undef, accept_file => { absolute => 1 } },
ssh_user => { default => "root", accept_regexp => qr/^[a-z_][a-z0-9_-]*$/ },
btrfs_progs_compat => { default => undef, accept => [ "yes", "no" ] },
# deprecated options
snapshot_create_always => { default => undef, accept => [ "yes", "no" ],
deprecated => { yes => { warn => "Please use \"snapshot_create always\"",
replace_key => "snapshot_create",
replace_value => "always",
},
no => { warn => "Please use \"snapshot_create no\" or \"snapshot_create ondemand\"",
replace_key => "snapshot_create",
replace_value => "ondemand",
}
},
},
receive_log => { default => undef, accept => [ "sidecar", "no" ], accept_file => { absolute => 1 },
deprecated => { DEFAULT => { warn => "ignoring" } },
}
);
my @config_target_types = qw(send-receive);
@ -118,6 +133,7 @@ sub HELP_MESSAGE
print STDERR " --version display version information\n";
print STDERR " -c FILE specify configuration file\n";
print STDERR " -p preserve all backups (do not delete any old targets)\n";
print STDERR " -r resume only (no new snapshots, resume all missing backups)\n";
print STDERR " -v be verbose (set loglevel=info)\n";
print STDERR " -q be quiet (do not print summary at end of \"run\" command)\n";
print STDERR " -l LEVEL set loglevel (warn, info, debug, trace)\n";
@ -322,8 +338,8 @@ sub config_key($$)
my $key = shift || die;
TRACE "config_key: context=$node->{CONTEXT}, key=$key";
while(not exists($node->{$key})) {
return undef unless($node->{PARENT});
$node = $node->{PARENT};
# note: all config keys exist in root context (at least with default values)
$node = $node->{PARENT} || die;
}
TRACE "config_key: found value=" . ($node->{$key} // "<undef>");
return $node->{$key};
@ -394,6 +410,7 @@ sub parse_config(@)
my $cur = $root;
# set defaults
foreach (keys %config_options) {
next if $config_options{$_}->{deprecated}; # don't pollute hash with deprecated options
$root->{$_} = $config_options{$_}->{default};
}
@ -529,13 +546,21 @@ sub parse_config(@)
return undef;
}
if($config_options{$key}->{deprecated}) {
WARN "Found deprecated option \"$key $value\" in \"$file\" line $.: " .
($config_options{$key}->{deprecated}->{$value}->{warn} // $config_options{$key}->{deprecated}->{DEFAULT}->{warn});
my $replace_key = $config_options{$key}->{deprecated}->{$value}->{replace_key};
my $replace_value = $config_options{$key}->{deprecated}->{$value}->{replace_value};
if(defined($replace_key)) {
$key = $replace_key;
$value = $replace_value;
WARN "Using \"$key $value\"";
}
}
TRACE "config: adding option \"$key=$value\" to $cur->{CONTEXT} context";
$value = undef if($value eq "no"); # we don't want to check for "no" all the time
$cur->{$key} = $value;
if($config_options{$key}->{deprecated}) {
WARN "Found deprecated configuration option \"$key\" in \"$file\" line $.";
}
}
else
{
@ -1187,6 +1212,7 @@ sub schedule(@)
my $preserve_daily = $args{preserve_daily} // die;
my $preserve_weekly = $args{preserve_weekly} // die;
my $preserve_monthly = $args{preserve_monthly} // die;
my $preserve_latest = $args{preserve_latest} || 0;
my $log_verbose = $args{log_verbose};
if($log_verbose) {
@ -1222,6 +1248,11 @@ sub schedule(@)
}
}
if($preserve_latest && (scalar @sorted_schedule)) {
my $href = $sorted_schedule[-1];
$href->{preserve} ||= "preserve forced: latest in list";
}
# filter daily, weekly, monthly
my %first_in_delta_weeks;
my %last_weekly_in_delta_months;
@ -1277,7 +1308,7 @@ MAIN:
my @today = Today();
my %opts;
unless(getopts('hc:vql:p', \%opts)) {
unless(getopts('hc:prvql:', \%opts)) {
VERSION_MESSAGE();
HELP_MESSAGE(0);
exit 1;
@ -1297,6 +1328,7 @@ MAIN:
@config_src = ( $opts{c} ) if($opts{c});
my $quiet = $opts{q};
my $preserve_backups = $opts{p};
my $resume_only = $opts{r};
# check command line options
if($opts{h} || (not $command)) {
@ -1729,65 +1761,89 @@ MAIN:
if($action_run)
{
#
# create snapshots
#
my $timestamp = sprintf("%04d%02d%02d", @today);
foreach my $config_vol (@{$config->{VOLUME}})
if($resume_only) {
INFO "Skipping snapshot creation (option \"-r\" present)";
}
else
{
next if($config_vol->{ABORTED});
my $sroot = $config_vol->{sroot} || die;
foreach my $config_subvol (@{$config_vol->{SUBVOLUME}})
#
# create snapshots
#
my $timestamp = sprintf("%04d%02d%02d", @today);
foreach my $config_vol (@{$config->{VOLUME}})
{
next if($config_subvol->{ABORTED});
my $svol = $config_subvol->{svol} || die;
my $snapdir = config_key($config_subvol, "snapshot_dir") || "";
my $snapshot_basename = config_key($config_subvol, "snapshot_name") // die;
next if($config_vol->{ABORTED});
my $sroot = $config_vol->{sroot} || die;
foreach my $config_subvol (@{$config_vol->{SUBVOLUME}})
{
next if($config_subvol->{ABORTED});
my $svol = $config_subvol->{svol} || die;
my $snapdir = config_key($config_subvol, "snapshot_dir") || "";
my $snapshot_basename = config_key($config_subvol, "snapshot_name") // die;
# check if we need to create a snapshot
my $create_snapshot = config_key($config_subvol, "snapshot_create_always");
foreach my $config_target (@{$config_subvol->{TARGET}}) {
next if($config_target->{ABORTED});
$create_snapshot = 1 if($config_target->{target_type} eq "send-receive");
}
unless($create_snapshot) {
$config_subvol->{ABORTED} = "No targets defined for subvolume: $svol->{PRINT}";
WARN "Skipping subvolume section: $config_subvol->{ABORTED}";
next;
}
# find unique snapshot name
my @unconfirmed_target_name;
my @lookup = keys %{vinfo_subvol_list($sroot)};
@lookup = grep s/^\Q$snapdir\E\/// , @lookup;
foreach my $config_target (@{$config_subvol->{TARGET}}) {
if($config_target->{ABORTED}) {
push(@unconfirmed_target_name, vinfo($config_target->{url}, $config_target));
# check if we need to create a snapshot
my $snapshot_create = config_key($config_subvol, "snapshot_create") // "no";
if($snapshot_create eq "no") {
DEBUG "Snapshot creation disabled: snapshot_create=no";
next;
}
my $droot = $config_target->{droot} || die;
push(@lookup, keys %{vinfo_subvol_list($droot)});
}
@lookup = grep /^\Q$snapshot_basename.$timestamp\E(_[0-9]+)?$/ ,@lookup;
TRACE "Present snapshot names for \"$svol->{PRINT}\": " . join(', ', @lookup);
@lookup = map { /_([0-9]+)$/ ? $1 : 0 } @lookup;
@lookup = sort { $b <=> $a } @lookup;
my $postfix_counter = $lookup[0] // -1;
$postfix_counter++;
my $snapshot_name = $snapshot_basename . '.' . $timestamp . ($postfix_counter ? "_$postfix_counter" : "");
if($snapshot_create eq "always") {
DEBUG "Snapshot creation enabled: snapshot_create=always";
}
elsif($snapshot_create eq "ondemand") {
my $snapshot_needed = 0;
foreach my $config_target (@{$config_subvol->{TARGET}}) {
next if($config_target->{ABORTED});
if($config_target->{target_type} eq "send-receive") {
$snapshot_needed = 1;
last;
}
}
if($snapshot_needed) {
DEBUG "Snapshot creation enabled: snapshot_create=ondemand, and at least one send-receive target is present";
}
else {
DEBUG "Snapshot creation disabled: snapshot_create=ondemand, but no send-receive target is present";
next;
}
}
else {
die "illegal value for snapshot_create configuration option: $snapshot_create";
}
if(@unconfirmed_target_name) {
INFO "Failed to check all targets, assuming non-present subvolume \"$snapshot_name\" in: " . join(", ", map { "\"$_->{PRINT}\"" } @unconfirmed_target_name);
}
# find unique snapshot name
my @unconfirmed_target_name;
my @lookup = keys %{vinfo_subvol_list($sroot)};
@lookup = grep s/^\Q$snapdir\E\/// , @lookup;
foreach my $config_target (@{$config_subvol->{TARGET}}) {
if($config_target->{ABORTED}) {
push(@unconfirmed_target_name, vinfo($config_target->{url}, $config_target));
next;
}
my $droot = $config_target->{droot} || die;
push(@lookup, keys %{vinfo_subvol_list($droot)});
}
@lookup = grep /^\Q$snapshot_basename.$timestamp\E(_[0-9]+)?$/ ,@lookup;
TRACE "Present snapshot names for \"$svol->{PRINT}\": " . join(', ', @lookup);
@lookup = map { /_([0-9]+)$/ ? $1 : 0 } @lookup;
@lookup = sort { $b <=> $a } @lookup;
my $postfix_counter = $lookup[0] // -1;
$postfix_counter++;
my $snapshot_name = $snapshot_basename . '.' . $timestamp . ($postfix_counter ? "_$postfix_counter" : "");
# finally create the snapshot
INFO "Creating subvolume snapshot for: $svol->{PRINT}";
if(btrfs_subvolume_snapshot($svol, "$sroot->{PATH}/$snapdir/$snapshot_name")) {
$config_subvol->{SNAPSHOT} = vinfo_child($sroot, "$snapdir/$snapshot_name");
}
else {
$config_subvol->{ABORTED} = "Failed to create snapshot: $svol->{PRINT} -> $sroot->{PRINT}/$snapdir/$snapshot_name";
WARN "Skipping subvolume section: $config_subvol->{ABORTED}";
if(@unconfirmed_target_name) {
INFO "Failed to check all targets, assuming non-present subvolume \"$snapshot_name\" in: " . join(", ", map { "\"$_->{PRINT}\"" } @unconfirmed_target_name);
}
# finally create the snapshot
INFO "Creating subvolume snapshot for: $svol->{PRINT}";
if(btrfs_subvolume_snapshot($svol, "$sroot->{PATH}/$snapdir/$snapshot_name")) {
$config_subvol->{SNAPSHOT} = vinfo_child($sroot, "$snapdir/$snapshot_name");
}
else {
$config_subvol->{ABORTED} = "Failed to create snapshot: $svol->{PRINT} -> $sroot->{PRINT}/$snapdir/$snapshot_name";
WARN "Skipping subvolume section: $config_subvol->{ABORTED}";
}
}
}
}
@ -1805,6 +1861,7 @@ MAIN:
my $svol = $config_subvol->{svol} || die;
my $snapdir = config_key($config_subvol, "snapshot_dir") || "";
my $snapshot_basename = config_key($config_subvol, "snapshot_name") // die;
my $preserve_latest = $config_subvol->{SNAPSHOT} ? 0 : 1;
foreach my $config_target (@{$config_subvol->{TARGET}})
{
@ -1814,11 +1871,9 @@ MAIN:
if($target_type eq "send-receive")
{
if(config_key($config_target, "receive_log")) {
WARN "Ignoring deprecated option \"receive_log\" for target: $droot->{PRINT}"
}
#
# resume missing backups (resume_missing)
#
if(config_key($config_target, "resume_missing"))
{
INFO "Checking for missing backups of subvolume \"$svol->{PRINT}\" in: $droot->{PRINT}/";
@ -1863,6 +1918,7 @@ MAIN:
preserve_daily => config_key($config_target, "target_preserve_daily"),
preserve_weekly => config_key($config_target, "target_preserve_weekly"),
preserve_monthly => config_key($config_target, "target_preserve_monthly"),
preserve_latest => $preserve_latest,
);
my @resume = grep defined, @$preserve; # remove entries with no value from list (target subvolumes)
$resume_total = scalar @resume;
@ -1894,20 +1950,23 @@ MAIN:
} else {
INFO "No missing backups found";
}
} # /resume_missing
unless($resume_only)
{
# skip creation if resume_missing failed
next if($config_target->{ABORTED});
next unless($config_subvol->{SNAPSHOT});
# finally receive the previously created snapshot
INFO "Creating subvolume backup (send-receive) for: $svol->{PRINT}";
my ($latest_common_src, $latest_common_target) = get_latest_common($sroot, $svol, $droot);
macro_send_receive($config_target,
snapshot => $config_subvol->{SNAPSHOT},
target => $droot,
parent => $latest_common_src, # this is <undef> if no common found
);
}
# skip creation if resume_missing failed
next if($config_target->{ABORTED});
die unless($config_subvol->{SNAPSHOT});
# finally receive the previously created snapshot
INFO "Creating subvolume backup (send-receive) for: $svol->{PRINT}";
my ($latest_common_src, $latest_common_target) = get_latest_common($sroot, $svol, $droot);
macro_send_receive($config_target,
snapshot => $config_subvol->{SNAPSHOT},
target => $droot,
parent => $latest_common_src, # this is <undef> if no common found
);
}
else {
ERROR "Unknown target type \"$target_type\", skipping: $svol->{PRINT}";
@ -1921,8 +1980,8 @@ MAIN:
#
# remove backups following a preserve daily/weekly/monthly scheme
#
if($preserve_backups) {
INFO "Preserving all backups (option \"-p\" present)";
if($preserve_backups || $resume_only) {
INFO "Preserving all backups (option \"-p\" or \"-r\" present)";
}
else
{
@ -1936,7 +1995,9 @@ MAIN:
my $svol = $config_subvol->{svol} || die;
my $snapdir = config_key($config_subvol, "snapshot_dir") || "";
my $snapshot_basename = config_key($config_subvol, "snapshot_name") // die;
my $preserve_latest = $config_subvol->{SNAPSHOT} ? 0 : 1;
my $target_aborted = 0;
foreach my $config_target (@{$config_subvol->{TARGET}})
{
if($config_target->{ABORTED}) {
@ -1968,6 +2029,7 @@ MAIN:
preserve_daily => config_key($config_target, "target_preserve_daily"),
preserve_weekly => config_key($config_target, "target_preserve_weekly"),
preserve_monthly => config_key($config_target, "target_preserve_monthly"),
preserve_latest => $preserve_latest,
log_verbose => 1,
);
my $ret = btrfs_subvolume_delete($delete, commit => config_key($config_target, "btrfs_commit_delete"));
@ -2003,6 +2065,7 @@ MAIN:
preserve_daily => config_key($config_subvol, "snapshot_preserve_daily"),
preserve_weekly => config_key($config_subvol, "snapshot_preserve_weekly"),
preserve_monthly => config_key($config_subvol, "snapshot_preserve_monthly"),
preserve_latest => $preserve_latest,
log_verbose => 1,
);
my $ret = btrfs_subvolume_delete($delete, commit => config_key($config_subvol, "btrfs_commit_delete"));
@ -2033,19 +2096,19 @@ MAIN:
my $sroot = $config_vol->{sroot} || vinfo($config_vol->{url}, $config_vol);
foreach my $config_subvol (@{$config_vol->{SUBVOLUME}})
{
my @subvol_out;
my $svol = $config_subvol->{svol} || vinfo_child($sroot, $config_subvol->{rel_path});
push @out, "$svol->{PRINT}";
if($config_vol->{ABORTED}) {
push @out, "!!! $sroot->{PRINT}: ABORTED: $config_vol->{ABORTED}";
push @subvol_out, "!!! $sroot->{PRINT}: ABORTED: $config_vol->{ABORTED}";
$err_count++ unless($config_vol->{ABORTED_NOERR});
}
if($config_subvol->{ABORTED}) {
push @out, "!!! Subvolume \"$svol->{PRINT}\" aborted: $config_subvol->{ABORTED}";
push @subvol_out, "!!! Subvolume \"$svol->{PRINT}\" aborted: $config_subvol->{ABORTED}";
$err_count++ unless($config_subvol->{ABORTED_NOERR});
}
push @out, "+++ $config_subvol->{SNAPSHOT}->{PRINT}" if($config_subvol->{SNAPSHOT});
push @subvol_out, "+++ $config_subvol->{SNAPSHOT}->{PRINT}" if($config_subvol->{SNAPSHOT});
if($config_subvol->{SUBVOL_DELETED}) {
push @out, "--- $_->{PRINT}" foreach(sort { $b->{PATH} cmp $a->{PATH} } @{$config_subvol->{SUBVOL_DELETED}});
push @subvol_out, "--- $_->{PRINT}" foreach(sort { $a->{PATH} cmp $b->{PATH} } @{$config_subvol->{SUBVOL_DELETED}});
}
foreach my $config_target (@{$config_subvol->{TARGET}})
{
@ -2055,21 +2118,26 @@ MAIN:
$create_mode = ">>>" if($_->{parent});
# substr($create_mode, 0, 1, '%') if($_->{resume});
$create_mode = "!!!" if($_->{ERROR});
push @out, "$create_mode $_->{received_subvolume}->{PRINT}";
push @subvol_out, "$create_mode $_->{received_subvolume}->{PRINT}";
}
if($config_target->{SUBVOL_DELETED}) {
push @out, "--- $_->{PRINT}" foreach(sort { $b->{PATH} cmp $a->{PATH} } @{$config_target->{SUBVOL_DELETED}});
push @subvol_out, "--- $_->{PRINT}" foreach(sort { $a->{PATH} cmp $b->{PATH} } @{$config_target->{SUBVOL_DELETED}});
}
if($config_target->{ABORTED}) {
push @out, "!!! Target \"$droot->{PRINT}\" aborted: $config_target->{ABORTED}";
push @subvol_out, "!!! Target \"$droot->{PRINT}\" aborted: $config_target->{ABORTED}";
$err_count++ unless($config_target->{ABORTED_NOERR});
}
push(@unrecoverable, $config_target->{UNRECOVERABLE}) if($config_target->{UNRECOVERABLE});
}
push @out, "";
if(@subvol_out) {
push @out, "$svol->{PRINT}", @subvol_out, "";
}
else {
push @out, "$svol->{PRINT}", "<no_action>", "";
}
}
}
@ -2087,8 +2155,11 @@ MAIN:
print join("\n", @out);
if($preserve_backups) {
print "\nNOTE: Preserved all backups (option -p present)\n";
if($resume_only) {
print "\nNOTE: No snapshots created (option -r present)\n";
}
if($preserve_backups || $resume_only) {
print "\nNOTE: Preserved all backups (option -p or -r present)\n";
}
if($err_count) {
print "\nNOTE: Some errors occurred, which may result in missing backups!\n";

23
btrbk.conf.example
View File

@ -22,14 +22,15 @@
snapshot_dir _btrbk_snap
# Perform incremental backups (set to "strict" if you want to prevent
# creation of initial backups if no parent is found).
# creation of non-incremental backups if no parent is found).
incremental yes
# Always create snapshots, even if the target volume is unreachable.
snapshot_create_always yes
# Always create snapshots (set to "ondemand" to only create snapshots
# if the target volume is reachable).
snapshot_create always
# Resume missing backups if the target volume is reachable again.
# Useful in conjunction with "snapshot_create_always".
# Resume missing backups if the target volume is reachable again (set
# to "no" if you don't want to resume missing backups).
resume_missing yes
# ssh key for ssh volumes/targets
@ -115,3 +116,15 @@ volume ssh://my-remote-host.com/mnt/btr_pool
snapshot_dir snapshots/btrbk
snapshot_name data_main
target send-receive /mnt/btr_backup/_btrbk/my-remote-host.com
# Resume backups from remote host which runs its own btrbk instance
# creating snapshots for "home" in "/mnt/btr_pool/btrbk_snapshots".
volume ssh://my-remote-host.com/mnt/btr_pool
subvolume home
snapshot_dir btrbk_snapshots
snapshot_preserve_daily all
snapshot_create no
resume_missing yes
target send-receive /mnt/btr_backup/_btrbk/my-remote-host.com

17
doc/btrbk.conf.5
View File

@ -56,14 +56,15 @@ Base name of the created snapshot (and backup). Defaults to
\fI<subvolume-name>\fR. This option is only valid in the \fItarget\fR
section.
.TP
\fBsnapshot_create_always\fR yes|no
If set, the snapshots are always created, even if the backup subvolume
cannot be created (e.g. if the target subvolume cannot be
reached). Use in conjunction with the \fIresume_missing\fR option to
make sure that the backups are created as soon as the target subvolume
is reachable again. Useful for laptop filesystems in order to make
sure the snapshots are created even if you are on the road. Defaults
to \[lq]no\[rq].
\fBsnapshot_create\fR always|ondemand|no
If set to \[lq]ondemand\[rq], the snapshots are only created if the
target subvolume is reachable (useful if you are tight on disk space
and you only need btrbk for backups to an external disk which is not
always connected). If set to \[lq]always\[rq], the snapshots are
always created, regardless of the target reachability. If set to
\[lq]no\[rq], the snapshots are never created (useful in conjunction
with the \fIresume_missing\fR option if another instance of btrbk is
taking care of snapshot creation). Defaults to \[lq]always\[rq].
.TP
\fBincremental\fR yes|no|strict
Perform incremental backups. Defaults to \[lq]yes\[rq]. If set to