Re: Help me understand what is going on with my RAID1 FS

[Andrei Borzenkov <arvidjaar@gmail.com>]

10.09.2017 23:17, Dmitrii Tcvetkov пишет:
>>> Drive1      Drive2    Drive3
>>> X           X
>>> X                         X
>>>             X             X
>>>
>>> Where X is a chunk of raid1 block group.  
>>
>> But this table clearly shows that adding third drive increases free
>> space by 50%. You need to reallocate data to actually make use of it,
>> but it was done in this case.
> 
> It increases it but I don't see how this space is in any way useful
> unless data is in single profile. After full balance chunks will be
> spread over 3 devices, how it helps in raid1 data profile case?
> 
A1 A2  => A1 A2 - => A1 A2 B1 => A1 A2 B1
B1 B2     B1 B2 -    -  B2  -    C1 B2 C2

It is raid1 profile on three disks fully utilizing them (assuming equal
sizes of course). Where "raid1" means - each data block has two copies
on different devices.