[gentoo-user] which linux RAID setup to choose?

[Caveman Al Toraboran <toraboracaveman@protonmail.com>]

hi - i'm to setup my 1st RAID, and i'd appreciate
if any of you volunteers some time to share your
valuable experience on this subject.

my scenario
-----------

    0. i don't boot from the RAID.

    1. read is as important as write.  i don't
       have any application-specific scenario that
       makes me somehow favor one over another.
       so RAIDs that speed up the read (or write)
       while significantly harming the write (or
       read) is not welcome.

    2. replacing failed disks may take a week or
       two.  so, i guess that i may have several
       disks fail one after another in the 1-2
       weeks (specially if they were bought
       about the same time).

    3. i would like to be able to grow the RAID's
       total space (as needed), and increase its
       reliability (i.e. duplicates/partities) as
       needed.

       e.g. suppose that i got a 2TB RAID that
       tolerates 1 disk failure.  i'd like to, at
       some point, to have the following options:

         * only increase the total space (e.g.
           make it 3TB), without increasing
           failure toleration (so 2 disk failure
           would result in data loss).

         * or, only increase the failure tolerance
           (e.g. such that 2 disks failure would
           not lead to data loss), without
           increasing the total space (e.g. space
           remains 2TB).

         * or, increase, both, the space and the
           failure tolerance at the same time.

    4. only interested in software RAID.

my thought
----------

i think these are not suitable:

    * RAID 0: fails to satisfy point (3).

    * RAID 1: fails to satisfy points (1) and (3).

    * RAIDs 4 to 6: fails to satisfy point (3)
      since they are stuck with a fixed tolerance
      towards failing disks (i.e. RAIDs 4 and 5
      tolerate only 1 disk failure, and RAID 6
      tolerates only 2).


this leaves me with RAID 10, with the "far"
layout.  e.g. --layout=n2 would tolerate the
failure of two disks, --layout=n3 three, etc.  or
is it?  (i'm not sure).

my questions
------------

Q1: which RAID setup would you recommend?

Q2: how would the total number of disks in a
    RAID10 setup affect the tolerance towards
    the failing disks?

    if the total number of disks is even, then
    it is easy to see how this is equivalent
    to the classical RAID 1+0 as shown in
    md(4), where any disk failure is tolerated
    for as long as each RAID1 group has 1 disk
    failure only.

    so, we get the following combinations of
    disk failures that, if happen, we won't
    lose any data:

          RAID0
      ------^------
    RAID1       RAID1
    --^--       --^--
    F   .       .   .       < cases with
    .   F       .   .       < single disk
    .   .       F   .       < failures
    .   .       .   F       <

    F   .       .   F       < cases with
    .   F       F   .       < two disk
    .   F       .   F       < failures
    F   .       F   .       <
    .   F       F   .       <

    this gives us 4+5=9 possible disk failure
    scenarious where we can survive it without
    any data loss.

    but, when the number of disks is odd, then
    written bytes and their duplicates will
    start wrap around, and it is difficult for
    me to intuitively see how would this
    affect the total number of scenarious
    where i will survive a disk failure.

Q3: what are the future growth/shrinkage
    options for a RAID10 setup?  e.g. with
    respect to these:

    1. read/write speed.
    2. tolerance guarantee towards failing
       disks.
    3. total available space.

rgrds,
cm.