Re: Safety of raid1 vs raid10

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Mon, Dec 09, 2024 at 06:44:14PM -0800, Scoopta wrote:
> Ok, this is what I thought but I have seen so many articles that imply
> raid10 groups drives and I have found no evidence for that. I was always
> under the impression that neither raid 1 nor 10 grouped drives together
> intentionally and any grouping was circumstantial.

In traditional, non-btrfs raid1/raid10 setups, this can easily be the
case, and people might be assuming this is also true on btrfs without
checking.

Traditional setups are much less flexible on space--you can't have
different sizes of drives--but you can organize the drives such that
each of them is entirely mirroring one other drive.  This can tolerate
up to N/2 failures, as long as each failure only removes one drive from
each pair--but it can also fail completely with only 2 drive failures,
if they're both mirrors of the same stripe.

> On 12/9/24 6:42 PM, Zygo Blaxell wrote:
> > On Mon, Dec 09, 2024 at 06:26:24PM -0800, Scoopta wrote:
> > > I've read online that btrfs raid10 is theoretically safer than raid1 because
> > > raid10 groups drives together into mirrored pairs making the filesystem more
> > > likely to successfully survive a multi-drive failure event. I can't find any
> > > documentation that says this to be the case. Is it true that btrfs pairs
> > > drives together for raid10 but not raid1, if this is the case what's the
> > > reasoning for it?
> > It is _possible_ for raid10 and raid1 to be arranged such that multiple drive
> > losses are possible.  e.g. if all odd numbered devices are paired with all
> > even numbered devices, then any odd numbered device can be lost and the
> > filesystem still survives.
> > 
> > This is not _guaranteed_, it is only _possible_.
> > 
> > In a filesystem with an odd number of drives, or with drives of varying
> > sizes, the block groups will only guarantee that one drive loss can be
> > tolerated in each block group, in order to have the flexibility needed
> > to fill all available space.  In such cases it is common that block
> > groups are arranged in such a way that loss of any two drives will break
> > the filesystem.
>