From: Li Zefan <lizf@cn.fujitsu.com>
Date: Thu, 1 Dec 2011 06:06:42 +0000 (+0800)
Subject: Btrfs: simplfy calculation of stripe length for discard operation
X-Git-Tag: v3.3-rc1~26^2~11^2~3
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ec9ef7a13be4dcce964c8503e8999087945e5b9e;p=pandora-kernel.git

Btrfs: simplfy calculation of stripe length for discard operation

For btrfs raid, while discarding a range of space, we'll need to know
the start offset and length to discard for each device, and it's done
in btrfs_map_block().

However the calculation is a bit complex for raid0 and raid10, so I
reimplement it based on a fact that:

        dev1          dev2           dev3    (raid0)
        -----------------------------------
        s0 s3 s6      s1 s4 s7       s2 s5

Each device has (total_stripes / nr_dev) stripes, or plus one.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
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