So something lost and something kept instead of all lost.
1. Do I understand the way how it works right?
2. Can the FDE partition become undecryptable and unbootable?
3. Is it possible to split the HDD into a few encrypted partitions?
4. Will it help against corruption?
5. If yes, how to split?
Esp. given that for large disks one probably wants GPT, not MBR.
6. What are the recommendations to lessen PO impact of a FDE
except UPS and backups?
I am not deeply familiar with the way softraid manages itself, but
generally speaking, it is very hard for an encrypted block device to
become suddenly unreadable all of a sudden due to a power outage.
The way most encrypted block devices work is there is a header which, in
combination with your passphrase, is used to decrypt the contents of the
block device. The block device (let's call it the encrypted filesystem)
is itself encrypted in blocks of a given size.
The header is unlikely to get many write operations done on it so it is
unlikely it will end up in an inconsistent state in the event of a
sudden power loss. That said, if something happens to the header, all
the data is lost. Forever. In all honesty, you are much more likely to
lose your header to random drive corruption due to old age than to power
outages.
The encrypted contents might get corrupted due to a number of reasons,
but damage is usually limited. Say, if a given partition is encrypted
with an algorithm that uses blocks of N size, a single bit getting
corrupted on that set makes the whole block unrecoverable. In practical
terms it means a single bit getting corrupted causes an amplified level
of corruption but that usually means a corrupted bit lays waste to a
single file.
Having more than one encrypted block device means you have more headers,
which means you have more things that can break.
The recommended way to survive power outages is to use UPS and backups.
Everything else is a poorman's mitigation. That said, using filesystem
options that reduce write operations is a good start (because it reduces
the chance that the filesystem will become offline while there are
pending write operations on it). If your NAS is mainly read-only, a good
idea is to have your system filesystems ( such as /, /tmp etc) separated
and your actual storage filesystem (let's say /storage) on a softraid
volume of its own. This is good hygiene anyway.
