Re: [gentoo-user] NAS and replacing with larger drives

[Wol <antlists@youngman.org.uk>]

On 21/12/2022 06:19, Frank Steinmetzger wrote:
> Am Wed, Dec 21, 2022 at 05:53:03AM +0000 schrieb Wols Lists:
> 
>> On 21/12/2022 02:47, Dale wrote:
>>> I think if I can hold out a little while, something really nice is going
>>> to come along.  It seems there is a good bit of interest in having a
>>> Raspberry Pi NAS that gives really good performance.  I'm talking a NAS
>>> that is about the same speed as a internal drive.  Plus the ability to
>>> use RAID and such.  I'd like to have a 6 bay with 6 drives setup in
>>> pairs for redundancy.  I can't recall what number RAID that is.
>>> Basically, if one drive fails, another copy still exists.  Of course,
>>> two independent NASs would be better in my opinion.  Still, any of this
>>> is progress.
>>
>> That's called either Raid-10 (linux), or Raid-1+0 (elsewhere). Note that 1+0
>> is often called 10, but linux-10 is slightly different.
> 
> In layman’s term, a stripe of mirrors. Raid-1 is the mirror, Raid-0 a (JBOD)
> pool. So mirror + pool = mirrorpool, hence the 1+0 → 10.

Except raid-10 is not a stripe of mirrors. It's each block is saved to 
two different drives. (Or 3, or more, so long as you have more drives 
than mirrors.)

Linux will happily give you a 2-copy mirror across 3 drives - 3x6TB 
drives will give you 9TB useful storage ...
> 
>> I'd personally be inclined to go for raid-6. That's 4 data drives, 2 parity
>> (so you could have an "any two" drive failure and still recover).
>> A two-copy 10 or 1+0 is vulnerable to a two-drive failure. A three-copy is
>> vulnerable to a three-drive failure.
> 
> At first, I had only two drives in my 4-bay NAS, which were of course set up
> as a mirror. After a year, when it became full, I bought the second pair of
> drives and had long deliberations by then, what to choose. I went for raid-6
> (or RaidZ2 in ZFS parlance). With only four disks, it has the same net
> capacity as a pair of mirrors, but at the advantage that *any* two drives
> may fail, not just two particular ones. A raid of mirrors has performance
> benefits over a parity raid, but who cares for a simple Gbit storage device.
> 
> With increasing number of disks, a mirror setup is at a disadvantage with
> storage efficiency – it’s always 50 % or less, if you mirror over more than
> two disks. But with only four disks, that was irrelevant in my case. On the
> plus-side, each mirror can have a different physical disk size, so you can
> more easily mix’n’match what you got lying around, or do upgrades in smaller
> increments.
> 
> If I wanted to increase my capacity, I’d have to replace *all* drives with
> bigger ones. With a mirror, only the drives in one of the mirrors need
> replacing. And the rebuild process would be quicker and less painful, as
> each drive will only be read once to rebuild its partner, and there is no
> parity calculation involved. In a RAID, each drive is replaced one by one,
> and each replacement requires a full read of all drives’ payload.

If you've got a spare SATA connection or whatever, each replacement does 
not need a full read of all drives. "mdadm --add /dev/sdx --replace 
/dev/sdy". That'll stream sdy on to sdx, and only hammer the other 
drives if sdy complains ...

> With older
> drives, this is cause for some concern whether the disks may survive that.
> That’s why, with increasing disk capacities, raid-5 is said to be obsolete.
> Because if another drive fails during rebuild, you are officially screwed.
> 
> Fun, innit?
> 
They've always said that. Just make sure you don't have multiple drives 
from the same batch, then they're less likely statistically to fail at 
the same time. I'm running raid-5 over 3TB partitions ...

Cheers,
Wol