Re: Better distribution of RAID1 data?

[Hugo Mills <hugo@carfax.org.uk>]

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On Fri, Feb 15, 2019 at 10:40:56AM -0500, 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?

   I don't think it'll make much difference. I spent a long time a
couple of years ago trying to prove (mathematically) that the current
strategy always produces an optimal usage of the available space -- I
wasn't able to complete the theorem, but a lot of playing around with
it convinced me that at least if there are cases where it's
non-optimal, they're bizarre corner cases.

> 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?

   I'd guess it's the easiest algorithm to use, plus it seems to
provide optimal space usage (almost?) all of the time. 

   Hugo.

-- 
Hugo Mills             | Be pure.
hugo@... carfax.org.uk | Be vigilant.
http://carfax.org.uk/  | Behave.
PGP: E2AB1DE4          |                                   Torquemada, Nemesis

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