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

[Wols Lists <antlists@youngman.org.uk>]

On 03/05/20 06:44, Caveman Al Toraboran wrote:
> 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?

I'd recommend having a spare in the array. That way, a single failure
would not affect redundancy at all. You can then replace the spare at
your leisure.

If you want to grow the array, I'd also suggest "raid 5 + spare". That's
probably better than 6 for writing. but 6 is better than 5 for
redundancy. Look at having a journal - that could speed up write speed
for raid 6.
> 
> Q2: how would the total number of disks in a
>     RAID10 setup affect the tolerance towards
>     the failing disks?
> 
Sadly, it doesn't. If you have two copies, losing two disks COULD take
out your raid.

>     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.

That's a gamble ...
> 
>     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.

iirc far is good for speed.

>     2. tolerance guarantee towards failing
>        disks.

Guarantees? If you have two mirrors. the guarantee is just ONE disk. Yes
you can gamble on losing more.

>     3. total available space.

iirc you can NOT grow the far layout.
> 
> rgrds,
> cm.
> 
You have looked at the wiki - yes I know I push it regularly :-)

https://raid.wiki.kernel.org/index.php/Linux_Raid

Cheers,
Wol