$MDS_FWDIR/scripts/migrate_server export, as long as you don't add -l or -x to have logs or indexing data included, will normally be 250Mb - 1,5Gb in size from my experience. I've done a ton of advanced upgrades, and I have yet to see a compressed export of the database larger than 8Gb in size. And that 8Gb export was using the old $FWDIR/bin/upgrade_tools/migrate export, not $MDS_FWDIR/scripts/migrate_server export, which tends to be more efficient.
It all depends on your database, of course. The number of policy packages, IPS profiles, network objects etc, will affect the size, but you will most likely be somewhere between 250Mb to 2Gb. But during the export, it has to save all the files temporarily before compressing, so you need to have more than 2Gb free for the export to be successful.
Shouldn't be any need to involve lv_current/root. You can specify the location, and the recommendation is to use a path on lv_log as lv_log tends to have more space compared to lv_current, especially on management installations:
$MDS_FWDIR/scripts/migrate_server export -v R81.10 /var/log/cpmgmt-r8110-database.tgz.
A simple "df -hT" will show you how much space you have left on the partitions. If you don't have enough space on lv_log aka /var/log you can most likely remove some log files from $FWDIR/log/ (/var/log/CPsuite-R80.40/fw1/log). There should be tons of *.log, *.logaccount_ptr and *.loginitial_ptr you can safely remove to free up space.
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