Re: Better distribution of RAID1 data?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2019-02-15 10:40, Brian B wrote:
> It looks like the btrfs code currently uses the total space available on
> a disk to determine where it should place the two copies of a file in
> RAID1 mode.  Wouldn't it make more sense to use the _percentage_ of free
> space instead of the number of free bytes?
> 
> For example, I have two disks in my array that are 8 TB, plus an
> assortment of 3,4, and 1 TB disks.  With the current allocation code,
> btrfs will use my two 8 TB drives exclusively until I've written 4 TB of
> files, then it will start using the 4 TB disks, then eventually the 3,
> and finally the 1 TB disks.  If the code used a percentage figure
> instead, it would spread the allocations much more evenly across the
> drives, ideally spreading load and reducing drive wear.
> 
> Is there a reason this is done this way, or is it just something that
> hasn't had time for development?
It's simple to implement, easy to verify, runs fast, produces optimal or 
near optimal space usage in pretty much all cases, and is highly 
deterministic.

Using percentages reduces the simplicity, ease of verification, and 
speed (division is still slow on most CPU's, and you need division for 
percentages), and is likely to not be as deterministic (both because the 
choice of first devices is harder when all are 100% empty, and because 
of potential rounding errors), and probably won't produce optimal 
layouts quite as reliably (you either need to get into floating-point 
math (which is to be avoided in the kernel whenever possible), or you 
end up with much more quantized disk selection).

I could see an adapted percentage method that preferentially spreads 
across disks whenever possible _possibly_ making sense once we can 
properly parallelize disk access in BTRFS, but until then I see no 
reason to change something that is already working reasonably well.

In your particular case, I'd actually suggest using something under 
BTRFS to merge the smaller disks to get as many devices as close to 8TB 
as possible.  That should help spread load better.