At 08/15/2016 10:10 PM, Austin S. Hemmelgarn wrote:
On 2016-08-15 10:08, Anand Jain wrote:
IMHO it's better to warn user about 2 devices RAID5 or 3 devices RAID6.
Any comment is welcomed.
Based on looking at the code, we do in fact support 2/3 devices for
raid5/6 respectively.
Personally, I agree that we should warn when trying to do this, but I
absolutely don't think we should stop it from happening.
How does 2 disks RAID5 work ?
One disk is your data, the other is your parity. In essence, it works
like a really computationally expensive version of RAID1 with 2 disks,
which is why it's considered a degenerate configuration.
I totally agree with the fact that 2 disk raid5 is just a slow raid1.
Three disks in
RAID6 is similar, but has a slight advantage at the moment in BTRFS
because it's the only way to configure three disks so you can lose two
and not lose any data as we have no support for higher order replication
than 2 copies yet.
It's true that btrfs doesn't support any other raid level which can
provide 2 parities.
But the use case to gain the ability to lose 2 disks in a 3 disk raid6
setup seems more like a trick other than normal use case.
Either in mkfs man page, or warning at mkfs time (but still allowing to
do it), IMHO it's better to tell user "yes, you can do it, but it's not
a really good idea"
Thanks,
Qu
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