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

[Goffredo Baroncelli <kreijack@gmail.com>]

On Saturday 30 January 2010, Goffredo Baroncelli wrote:
> 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.


Sorry, I forgot the reference:
[1] http://btrfs.wiki.kernel.org/index.php/Multiple_Device_Support

> 
> 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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it>
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