Re: What exactly is BTRFS Raid 10?

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 20.08.2022 14:28, Goffredo Baroncelli wrote:
> 
> RAID1:
> A new chunk is allocated to the two disks with more space available. Each new chunk has a size of 1GB x 2 = 2GB, but only 1GB is available for the data because the other one contains a copy of the data.
> A raid1 layout may have more than two disks. However the data is copied only two times, this means that you can tolerate only the lost of one device.
> For example the first chunk is allocated on the first two disks; the 2nd chunk is allocated on the first and the 3rd disk; the 3rd chunk is allocated on the 2nd and 3rd disk....
> 
...
> 
> RAID10:
> Is a mix of RAID0 and RAID1: the data is copied two times (so you can tolerate the lost of one device), but it is spread over near all the disks.
> If you have 7 disks, a new chunk is allocated over 6 disks (the greatest even number <= to the disk count) with more space available.
> If you write data to a disk, the first 64K are written on the 1st disk and and the 2nd disk (as 2nd copy). When you write the 2nd 64 k of data, these are written in the 3rd disk and 4th disk (as 2nd copy). And so on until you fill the chunk.
> When the chunk is filled, a new allocation occurred. Likely the 7th disk is used and one of the first 6 isn't for the new chunk.
> 

Is large IO processed in parallel? If I have 8 disks raid10 and issue
256K request - will btrfs submit 4 concurrent 64K requests to each disk?

And for raid1 - will there be single 256K physical disk request or 4 x
64K requests?

What about read requests - will all disks in raid1/raid10 be used
concurrently or btrfs always reads from the "primary" copy (and how it
is determined then)?