Does bitcoind ERROR "Disk space is too low!" refer to RAM or actual harddrive? After having finished the "Initial Block Download" I needed to reindex:

[BTC_Core_Expmnt]# bitcoind -reindex -datadir=/mnt/BTC_Core_Expmnt


The debug.log showed everything starting up ok; and the process beginning:

[2020-01-20T07:43:41Z Loaded 119369 blocks from external file in 6005ms\
2020-01-20T07:43:41Z Reindexing block file blk00001.dat...]


Then a while later this:

[2020-01-20T09:05:59Z Reindexing finished]


But bitcoind continued grinding on... Well then what is all this? ::

[2020-01-20T09:05:59Z Pre-allocating up to position 0x100000 in\
[rev00000.dat 2020-01-20T09:05:59Z UpdateTip: new best=\ 
[00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048\
[height=1 version=0x00000001 log2_work=33.000022 tx=2\
[date='2009-01-09T02:54:25Z' progress=0.000000 cache=0.0MiB(1txo)]


In any case it got to this point ::

[2020-01-20T09:59:00Z UpdateTip: new best=\
[00000000000000001088ed0c0989828504a41d4b5b974c8c081b708cfdd3b7b0\
[height=359645 version=0x00000003 log2_work=82.893764 tx=71128141\
[date='2015-06-06T07:25:10Z' progress=0.143166\
[cache=669.7MiB(4972713txo)]


Then it fell over ::

[2020-01-20T09:59:00Z *** Disk space is too low!
[2020-01-20T09:59:00Z Error: Disk space is too low!\
[Error: Disk space is too low!
[2020-01-20T09:59:00Z Failed to connect best block\
[ (Disk space is too low! (code 0)) ]


Which is odd, because there should still be 20GB free since the size of
BTC_Core_Expmnt is supposed to be 300GB.

[BTC_Core_Expmnt]# du -h
102M    ./blocks/index
275G    ./blocks
56K     ./bitcoin-0.19.0.1/share/man/man1
60K     ./bitcoin-0.19.0.1/share/man
64K     ./bitcoin-0.19.0.1/share
73M     ./bitcoin-0.19.0.1/bin
8.0K    ./bitcoin-0.19.0.1/include
440K    ./bitcoin-0.19.0.1/lib
74M     ./bitcoin-0.19.0.1
16K     ./lost+found
3.4G    ./indexes/txindex
3.4G    ./indexes
914M    ./chainstate
280G    .
[BTC_Core_Expmnt]#


Is 300GB not enough in 2020 to run a full node?  I thought it'd do.  But, asking for any helpful tips because I cannot see anything else amiss from debug.log.
It refers to actual disk space. RAM is memory, not storage. The term "disk space" is very specific.


  Then a while later this:

[2020-01-20T09:05:59Z Reindexing finished]

  
  But bitcoind continued grinding on... Well then what is all this? ::


Your node was not fully synced, so after it finished reindexing the blocks on disk, it continued to download blocks from its peers and finish syncing.