Re: Actual effect of mkfs.btrfs -m raid10 </dev/sdX> ... -d raid10 </dev/sdX> ...

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

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On Mon, Nov 18, 2013 at 11:12:03PM -0600, deadhorseconsulting wrote:
> In theory (going by the man page and available documentation, not 100%
> clear) does the following command indeed actually work as advertised
> and specify how metadata should be placed and kept only on the
> "devices" specified after the "-m" flag?
> 
> Thus given the following example:
> mkfs.btrfs -L foo -m raid10 <ssd> <ssd> <ssd> <ssd> -d raid10 <rust>
> <rust> <rust> <rust>
> 
> Would btrfs stripe/mirror and only keep metadata on the 4 specified SSD devices?
> Likewise then stripe/mirror and only keep data on the specified 4 spinning rust?

   No. The devices are general purpose. The -d and -m options only
specify the type of redundancy, not the devices to use. There's a
project[1] to look at this kind of more intelligent chunk allocator,
but it's not been updated in a while.

[1] https://btrfs.wiki.kernel.org/index.php/Project_ideas#Device_IO_Priorities

> In trying and creating this type of setup it appears that data is also
> being stored on the devices specified as "metadata devices". This is
> observed through via a "btrfs filesystem show". after committing a
> large amount of data to the filesystem The data devices have balanced
> data as expected with plenty of free space but the SSD device are
> reported as either nearly used or completely used.

   This will happen with RAID-10. The allocator will write stripes as
wide as it can: in this case, the first stripes will run across all 8
devices, until the SSDs are full, and then will write across the
remaining 4 devices.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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