Re: How file store when using Btrfs on multi-devices? What happen when a device fail?

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

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On Thu, May 03, 2012 at 01:56:10PM +0700, Fajar A. Nugraha wrote:
> On Thu, May 3, 2012 at 1:46 PM, Chu Duc Minh <chu.ducminh@gmail.com> wrote:
> > Hi, i have some questions when using Btrfs on multi-devices:
> > 1. a large file will always be stored wholely on a device or it may
> > spread on some devices/partitions?
> 
> IIRC:
> - in raid1 mode, it will be written on all disks (or was it TWO disks,
> regarless how many device in a mirror? can't remember which).

   Any given block of the file will appear on two disks. There's no
guarantee that all the blocks of a file will be stored on the same two
disks, though.

> - in raid10 and raid0, it will always be spread, on a minimum of two devices

   Again, any given block of a file will appear on precisely two disks
(for RAID-10), or precisely one disk (for RAID-0). However, for a
sufficiently large file (i.e. multiples of the stripe size, which is
64KiB, I think), the blocks will be spread across all disks. If the
file is highly fragmented, this is a statistical statement; if the
file is contiguous, it is a guarantee.

   Hugo.

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