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.
