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

[Frank Steinmetzger]

Am Wed, Dec 21, 2022 at 08:03:36PM +0000 schrieb Wol:
> 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.)

Yes? In a mirror setup, all member drives of a mirror have the same content
(at least in ZFS).

Raid 10 distributes its content across several mirrors. This is the cause
for its increased performance. So when one of the mirrors (not single drive,
but a whole set of mirrored drives) fails, the pool is gone.

> Linux will happily give you a 2-copy mirror across 3 drives - 3x6TB drives
> will give you 9TB useful storage ...

I admit, I’ve never head of that. (Though it sounds like raid-5 to me.)

> > 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 ...

Strange that I didn’t think of that, even though it’s a perfectly clear
concept. In ZFS there is also a replace function which would do just that.
Currently I plan on keeping my old drives (who would want to buy them off of
me anyways) and just reorganise them in Z1 over Z2. I’ll just have to move
all data off to temprary external drives.

> > 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 ...

Yeah, I bought my drives from different shops back then for that reason.

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