Re: [raidX vs single/dup]

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On Thu, Sep 26, 2013 at 12:22:49PM +0000, miaou sami wrote:
> Hi btrfs guys,
> 
> could someone explain to me the differences in mkfs.btrfs:
> 
> - between -d raid0 and -d single

   In RAID0, data is striped across all the devices, so the first 64k
of a file will go on device 1, the next 64k will go on device 2, and
so on. With single, files are allocated linearly on one device.

   (This is assuming smallish files, a filesystem with lots of space.
Even with single, files can still end up being scattered around over
multiple devices -- but with RAID0, even non-fragmented files are
striped)

> - between -m raid1 and -m dup

   In both cases, there are two copies of each metadata block. With
RAID1, it *requires* the two copies to live on different devices. With
DUP, it allows the two copies to live on the same device (e.g. if
there's only one device).

> - between -m raid0 and -m single

   As for -draid0 and -dsingle, but for metadata instead of data.

> My understanding is that raidX should be used in case of multi
> devices and single/dup should be used in case of single device to
> allow duplication, but it is not 100% clear to me...

> As btrfs raid concepts are quite different from traditionnal raid,
> shouldn't we use the words "stripped" and "mirrored" instead of
> raid0/raid1? or even "single" and "duplicated"?
> Then there would be no difference between single/raid0 and
> duplicated/raid1...

   But there _are_ differences between them, as explained above. :)

   I posted a patch a while ago to change the names to something more
logical and expressive, but it didn't get merged.

   Hugo.

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