Re: Help with space

Chris Murphy
2014-05-02T22:33:11Z

On May 2, 2014, at 3:08 PM, Hugo Mills <hugo@carfax.org.uk> wrote:

> On Fri, May 02, 2014 at 01:21:50PM -0600, Chris Murphy wrote:
>>
>> On May 2, 2014, at 2:23 AM, Duncan <1i5t5.duncan@cox.net> wrote:
>>>
>>> Something tells me btrfs replace (not device replace, simply replace)
>>> should be moved to btrfs device replace…
>>
>> The syntax for "btrfs device" is different though; replace is like balance: btrfs balance start and btrfs replace start. And you can also get a status on it. We don't (yet) have options to stop, start, resume, which could maybe come in handy for long rebuilds and a reboot is required (?) although maybe that just gets handled automatically: set it to pause, then unmount, then reboot, then mount and resume.
>>
>>> Well, I'd say two copies if it's only two devices in the raid1... would
>>> be true raid1. But if it's say four devices in the raid1, as is
>>> certainly possible with btrfs raid1, that if it's not mirrored 4-way
>>> across all devices, it's not true raid1, but rather some sort of hybrid
>>> raid, raid10 (or raid01) if the devices are so arranged, raid1+linear if
>>> arranged that way, or some form that doesn't nicely fall into a well
>>> defined raid level categorization.
>>
>> Well, md raid1 is always n-way. So if you use -n 3 and specify three devices, you'll get 3-way mirroring (3 mirrors). But I don't know any hardware raid that works this way. They all seem to be raid 1 is strictly two devices. At 4 devices it's raid10, and only in pairs.
>>
>> Btrfs raid1 with 3+ devices is unique as far as I can tell. It is something like raid1 (2 copies) + linear/concat. But that allocation is round robin. I don't read code but based on how a 3 disk raid1 volume grows VDI files as it's filled it looks like 1GB chunks are copied like this
>>
>> Disk1 Disk2 Disk3
>> 134 124 235
>> 679 578 689
>>
>> So 1 through 9 each represent a 1GB chunk. Disk 1 and 2 each have a chunk 1; disk 2 and 3 each have a chunk 2, and so on. Total of 9GB of data taking up 18GB of space, 6GB on each drive. You can't do this with any other raid1 as far as I know. You do definitely run out of space on one disk first though because of uneven metadata to data chunk allocation.
>
> The algorithm is that when the chunk allocator is asked for a block
> group (in pairs of chunks for RAID-1), it picks the number of chunks
> it needs, from different devices, in order of the device with the most
> free space. So, with disks of size 8, 4, 4, you get:
>
> Disk 1: 12345678
> Disk 2: 1357
> Disk 3: 2468
>
> and with 8, 8, 4, you get:
>
> Disk 1: 1234568A
> Disk 2: 1234579A
> Disk 3: 6789

Sure in my example I was assuming equal size disks. But it's a good example to have uneven disks also, because it exemplifies all the more the flexibility btrfs replication has, over alternatives, with odd numbered *and* uneven size disks.

Chris Murphy