RAID1 Mirroring question.

RAID1 Mirroring question.

[Timo.Bolse]

Hi List,

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?


Timo

Re: RAID1 Mirroring question.

[Mark Hahn]

> 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?

afaikt, MD raid1 assumes that any data-corrupting errors are reported.
if the device corrupts data and lies to the driver/kernel/md, then you
are in trouble.  this applies to both reads and writes.

it's easy to imagine a ckraid1 tool that somehow forces reads of all
copies of each block. but with two mirrors, there would presumably
be no way to decide which to use...

all that said, I've never seen this happen.

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!

regards, mark hahn.

Re: RAID1 Mirroring question.

[Daniel Pittman]

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
blunt ax. It is equally vain to try to do it with ten blunt axes instead.
        -- Edsger Dijkstra