Re: RAID-10 arrays built with btrfs & md report 2x difference in available size?

[Goffredo Baroncelli <kreijack@gmail.com>]

On Saturday 30 January 2010, 0bo0 wrote:

> Is the goal NOT to accurately represent the actual available space?
> Seems rather odd that users are simply to know/accept that "available
> space" in btrfs RAID-10 != "available space" in md RIAD-10 ...

As reported more time in this ML, btrfs is able to store the data in 
striping/raid1 mode per-file-basis. 

The space on the disk is grouped in chunk. The raid mode is set per-chunk-
basis [1]. So a file stored in a chunk may be written two times (in one or two 
different disk), and another file stored in another chunk may be written with 
a different policy.


In fact the btrfs store the data in "raid0" mode, and the metadata in raid1 
mode, even with only one disk. Even tough the words "raid1/0" are incorrect 
with only one disk.

So key points are:
- it is incorrect to say that the btrfs filesystem is configured in raidX mode
- it is correct that the file xyz is stored in raidX mode
- is quite simple to evaluate the space available. It is more complex to 
evaluate before the file creation how many of the space available a file of a 
certain size consumes.
- unfortunately, today are not available tools that permits to manage the raid 
mode of a file

BR
G.Baroncelli



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