Re: Btrfs - distribute files equally across multiple devices

[Johannes Pfrang <johannespfrang@gmail.com>]

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Thank you. That's a very helpful explanation. I've just did balance
start -dconvert=single ;)
Fwiw, the best explanation about "single" I could find was in the
Glossary[1].
I don't have an account on the wiki, but your first paragraph would fit
great there!


[1] https://btrfs.wiki.kernel.org/index.php/Glossary


On 06.07.2015 19:53, Hugo Mills wrote:
> On Mon, Jul 06, 2015 at 06:22:52PM +0200, Johannes Pfrang wrote:
>    Not quite. In single mode, the FS will allocate linear chunks of
> space 1 GiB in size, and use those to write into (fitting many files
> into each chunk, potentially). The chunks are allocated as needed, and
> will go on the device with the most unallocated space.
>
>    So, with equal-sized devices, the first 1 GiB will go on the first
> device, the second 1 GiB on the second device, and so on.
>
>    With unequal devices, you'll put data on the largest device, until
> its free space reaches the size of the next largest, and then the
> chunks will be alternated between those two, until the free space on
> each of the two largest reaches the size of the third-largest, and so
> on.
>
>    (e.g. for devices sized 6 TB, 4 TB, 3 TB, the first 2 TB will go
> exclusively on the first device; the next 2 TB will go on the first
> two devices, alternating in 1 GB chunks; the rest goes across all
> three devices, again, alternating in 1 GB chunks.)
>
>    This is all very well for an append-only filesystem, but if you're
> changing the files on the FS at all, there's no guarantee as to where
> the changed extents will end up -- not even on the same device, let
> alone close to the rest of the file on the platter.
>
>    I did work out, some time ago, a prototype chunk allocator (the 1
> GiB-scale allocations) that would allow enough flexibility to control
> where the next chunk to be allocated would go. However, that still
> leaves the extent allocator to deal with, which is the second, and
> much harder, part of the problem.
>
>    Basically, don't assume any kind of structure to the location of
> your data on the devices you have, and keep good, tested, regular
> backups of anything you can't stand to lose and can't replace. There
> are no guarantees that would let you assume easily that any one file
> is on a single device, or that anything would survive the loss of a
> device.
I promise I won't assume that.

Two 4TB data disks:
- 3TiB+3TiB data=single,meta=raid1 replaceable/unimportant
- 654Gib|654Gib data/meta=raid1 important with regular backups

efficient + safe enough (for my use-case)
>
>    I'm sure this is an FAQ entry somewhere... It's come up enough
> times.
>
>    Hugo.
>
>


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