Re: No space on empty, degraded raid10

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2014-09-11 02:40, Russell Coker wrote:
> On Mon, 8 Sep 2014, Austin S Hemmelgarn <ahferroin7@gmail.com> wrote:
>> Also, I've found out the hard way that system chunks really should be
>> RAID1, NOT RAID10, otherwise it's very likely that the filesystem
>> won't mount at all if you lose 2 disks.
> 
> Why would that be different?
> 
> In a RAID-1 you expect system problems if 2 disks fail, why would RAID-10 be 
> different?
That's still the case, but in a RAID1 with four disks, of the six
different pairs of two disks you could lose, only one will make the
filesystem un-mountable, whereas for a four disk RAID10, there are two
different pairs of two disks you could lose to make the filesystem
un-mountable.  In haven't run the numbers for higher numbers of disks,
but things are likely not better, because if you lose both copies of the
same stripe, things will fail.
> 
> Also it would be nice if there was a N-way mirror option for system data.  As 
> such data is tiny (32MB on the 120G filesystem in my workstation) the space 
> used by having a copy on every disk in the array shouldn't matter.
> 
N-way mirroring is in the queue for after RAID5/6 work; ideally, once it
is ready, mkfs should default to one copy per disk in the filesystem.


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