Re: raid modes, balancing, and order in which data gets written

[Sean Bartell <wingedtachikoma@gmail.com>]

On Thu, Jul 15, 2010 at 10:29:07AM +0200, Mathijs Kwik wrote:
> Hi all,
> 
> I read that btrfs - in a raid mode - does not mimic the behavior of
> traditional (hw/sw) raid.
> After writing to a btrfs raid filesystem, data will only be
> distributed the way you expect after running a rebalance.

This is not the case. When you create a btrfs filesystem with RAID
enabled, stuff written from then on will be written just like with
traditional RAID.

The difference with traditional RAID is that different parts of the FS
can have different RAID settings. Btrfs reserves space in ~1GiB "block
groups" for data or metadata, each of which has its own RAID settings.
If you change the RAID mode for an existing filesystem (not yet
supported IIUC) or add/remove devices, the existing block groups will
keep their old RAID settings if at all possible.

Rebalancing essentially moves everything into new block groups, which
will use the new RAID settings and be more balanced between data and
metadata. It isn't useful unless you change RAID settings, add/remove
devices, or have too much space reserved for either data or metadata.

> [...]

> If you never rebalance manually, will the filesystem do this in the
> background (when idle)?
> Or will the fs never rebalance itself and only become "more balanced"
> again after writing/changing some files, which it will then place on
> the drive which has the lowest balance?

Rebalancing isn't done automatically, and nothing can become "more
balanced" until new block groups are created when you run out of space
in the old ones.

> Basically, I'm not sure I fully understood balancing, so any info on
> this would be great.
> In traditional raid0 and raid10 (block based), it is guaranteed that
> any big file will always be stiped between disks equally, so a certain
> performance can be assumed.
> With non-automatic balancing, I'm afraid some files might not be
> distributed as well as they could, resulting in lower performance.
> Is this an issue to be aware of, or can I safely assume that for most
> use cases the performance will roughly be the same as sw-raid?
> 2 cases I'm interested in:
> - big databases(lots of rewrites)
> - real-time video-capturing (sustained write to 1 or more big files,
> needing a guaranteed write throughput)

If you initially create the filesystem with the right RAID settings, it
will act just like normal software RAID. Balancing only comes into play
when you start changing your mind :).

> Any info on this or balancing in general will be greatly appreciated.