Re: Understanding BTRFS storage

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

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On Wed, Aug 26, 2015 at 10:56:03AM +0200, George Duffield wrote:
> Hi
> 
> Is there a more comprehensive discussion/ documentation of Btrfs
> features than is referenced in
> https://btrfs.wiki.kernel.org/index.php/Main_Page...I'd love to learn
> more but it seems there's no readily available authoritative
> documentation out there?
> 
> I'm looking to switch from a 5x3TB mdadm raid5 array to a Btrfs based
> solution that will involve duplicating a data store on a second
> machine for backup purposes (the machine is only powered up for
> backups).
> 
> Two quick questions:
> - If I were simply to create a Btrfs volume using 5x3TB drives and not
> create a raid5/6/10 array I understand data would be striped across
> the 5 drives with no reduncancy ... i.e. if a drive fails all data is
> lost?  Is this correct?

   With RAID-1 metadata and single data, when you lose a device the FS
will continue to be usable. Any data that was stored on the missing
device will return an I/O error when you try to read it. With single
data, the data space is assigned to devices in 1 GiB chunks in
turn. Within that, files that are written once and not modified are
likely to be placed linearly within that sequence. Files that get
modified may have their modifications placed out of sequence on other
chunks and devices.

> - Is Btrfs RAID10 (for data) ready to be used reliably?

   I'd say yes, others may say no. I'd suggest using RAID-1 for now
anyway -- it uses the space better when you come to add new devices or
replace them (with different sizes).

   Hugo.

-- 
Hugo Mills             | Preventing talpidian orogenesis.
hugo@... carfax.org.uk |
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