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The default, according to zmcontrol, is 100Mb. Looking at the code is checks for the config variable zimbra_disk_threshold so you should be able to set it to whatever you like using
Code:
su - zimbra
zmlocalconfig -e zimbra_disk_threshold = 100
Also, check /opt/zimbra/libexec/zmdisklog as they are hardcoded in there
Code:
my $DISK_CRIT_THRESHOLD = 90;
my $DISK_WARN_THRESHOLD = 80;