Re: btrfs raid assurance

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

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On Tue, Jul 25, 2017 at 09:55:37AM -0300, Hérikz Nawarro wrote:
> Hello everyone,
> 
> I'm migrating to btrfs and i would like to know, in a btrfs filesystem
> with 4 disks (multiple sizes) with -d raid0 & -m raid1, how many
> drives can i lost without losing the entire array?

   You can lose one device in the array, and the FS structure will be
OK -- it will still mount, and you'll be able to see all the filenames
and directory structures and so on.

   However, if you do lose one device, then you'll lose
(approximately) half of the bytes in all of your files, most likely in
alternating 64k slices in each file. Attempting to read the missing
parts will result in I/O errors being returned from the filesystem.

   So, while the FS is in theory still fine as a (probably read-only)
filesystem, it's actually going to be *completely* useless with a
missing device, because none of your file data will be usably intact.

   If you want the FS to behave well when you lose a device, you'll
need some kind of actual redundancy in the data storage part -- RAID-1
would be my recommendation (it stores two copies of each piece of
data, so you can lose up to one device and still be OK).

   Hugo.

-- 
Hugo Mills             | One of these days, I'll catch that man without a
hugo@... carfax.org.uk | quotation, and he'll look undressed.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                   Leto Atreides, Dune

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