Re: RAID1 Mirroring question.

[Daniel Pittman <daniel@rimspace.net>]

On Mon, 5 Apr 2004, Mark Hahn wrote:
>> We have a Linux Software RAID1 (mirroring). If there is a little
>> error on one of the disks (such a little error that the kernel dosn't
>> recognize it). There is a read request on the raid for a specific
>> file. The output of one of the disks differ from the output of the
>> other disk. (But there are no errors recognized by the kernel / fs /
>> raid-driver. Only one inverted bit for example) What is RAID/MD
>> doing? Are there checksums for the original file?

[...]

> then again, raid1 is a sort of ugly niche feature, IMO.  how many 
> systems can afford two but not three disks?  raid5 is not scary!

*blink*  Quite a few systems don't have the capacity for three-disk
rather than two-disk RAID.  My laptop, for example, could not add a
third disk at all.

Also, RAID-5 does not have *any* improvement over RAID-1 in terms of
detecting this sort of single-disk unreported error.

RAID-5 also has a higher cost in terms of CPU use - enough that it
presents problems in a number of embedded system scenarios where RAID-1
is fine.

Three disk RAID-1, on the other hand, does allow you to detect a single
device failure by a "two to one vote" detection system...

       Daniel

-- 
About the use of language: it is impossible to sharpen a pencil with a
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