[CentOS] question about software Raid 1

Robert Arkiletian robark at gmail.com
Sun Sep 21 21:34:38 UTC 2008


On Sun, Sep 21, 2008 at 10:26 AM, Nataraj <incoming-centos at rjl.com> wrote:
> Does software raid 1 compare checksums or otherwise verify that the same
> bits are coming from both disks during reads?  What I'm interested in,

No. Reads are distributed over disks to increase performance.

> is whether bit errors that were somehow undetected by the hardware would
> be detected by the raid 1 software.

Depends on the type of error. However, the sad thing is, if you use 3
disks for raid 1 the kernel does not do the right thing. Let me
explain.

Say you have 3 disks in a raid 1 array. If there is a mismatch then
the smart thing to do would be to take a vote of the 3 disks. 2 out of
3 wins (assuming they are not all different). The odd man out should
be corrected (if possible). But what actually happens is the highest
numbered disk is copied to the others.

I haven't looked at the latest kernel code but if this
http://linas.org/linux/raid.html
is correct then I think the kernel maintainers should address this
issue. I don't think it would be hard to implement.


-- 
Robert Arkiletian
Eric Hamber Secondary, Vancouver, Canada
Fl_TeacherTool http://www3.telus.net/public/robark/Fl_TeacherTool/
C++ GUI tutorial http://www3.telus.net/public/robark/


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