Re: Is it necessary to balance a btrfs raid1 array?

[Bob Williams <linux@barrowhillfarm.org.uk>]

On 10/09/14 19:43, Goffredo Baroncelli wrote:
> On 09/10/2014 02:27 PM, Bob Williams wrote:
>> I have two 2TB disks formatted as a btrfs raid1 array, mirroring both
>> data and metadata. Last night I started
>>
>> # btrfs filesystem balance <path>
> 
> 
> May be that I am missing something obvious, however I have to ask which 
> would be the purpose to balance a two disks RAID1 system.
> The balance command should move the data between the disks in order to
> avoid some disk full and other empty; but this assume that there is a
> not symmetrical uses of the disks. Which is not the case for a RAID1/two
> disks system.
> 
> If the disk were more than two the situation would be completely different.
> But Bob reports that the system is compose by two disks only.
> 
>>
>> and it is still running 18 hours later. This suggests that most stuff
>> only gets written to one physical device, which in turn suggests that
>> there is a risk of lost data if one physical device fails. Or is
>> there something clever about btrfs raid that I've missed? I've used
>> linux software raid (mdraid) before, and it appeared to write to both
>> devices simultaneously.
>>

So if a RAID1/two disk system uses the disks symmetrically, why did my
balance command take 22 hours? That's what puzzles me, as my
understanding of RAID1 is that the disk use *is* symmetrical.

Bob