why md/raid5 dont have bad block remap

why md/raid5 dont have bad block remap

[jin zhencheng]

hi ,

I think it isn't  complicated for raid5 to add bad block remap.

"bad block remap" is means that if  a bio is written error, we not
fauty the disk, and try to write this bio  to other place which
alloced  for bad block remap.

i dont  know why MD/raid5 add this function ?


THS

Re: why md/raid5 dont have bad block remap

[Mikael Abrahamsson]

On Mon, 28 Dec 2009, jin zhencheng wrote:

> hi ,
>
> I think it isn't  complicated for raid5 to add bad block remap.
>
> "bad block remap" is means that if  a bio is written error, we not
> fauty the disk, and try to write this bio  to other place which
> alloced  for bad block remap.
>
> i dont  know why MD/raid5 add this function ?

What is done is upon read error the parity is read and the block that had 
a read error is calculated and written, and then the unit in question has 
the responsibility of mapping the block/sector as faulty and write it 
someplace else.

Drives which has used up that space and thus can't remap should be 
replaced with a working drive, it's obviously faulty.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: why md/raid5 dont have bad block remap

[jin zhencheng]

I think maybe the disk deal with one write comnd with errror,but  the
disk maybe is also ok,and rewrite to other place or  rewrite again
will be good.

why one write error ,set the whole disk faulty.

Disk if very expensive!



On Mon, Dec 28, 2009 at 7:07 PM, Mikael Abrahamsson <swmike@swm.pp.se> wrote:
> On Mon, 28 Dec 2009, jin zhencheng wrote:
>
>> hi ,
>>
>> I think it isn't  complicated for raid5 to add bad block remap.
>>
>> "bad block remap" is means that if  a bio is written error, we not
>> fauty the disk, and try to write this bio  to other place which
>> alloced  for bad block remap.
>>
>> i dont  know why MD/raid5 add this function ?
>
> What is done is upon read error the parity is read and the block that had a
> read error is calculated and written, and then the unit in question has the
> responsibility of mapping the block/sector as faulty and write it someplace
> else.
>
> Drives which has used up that space and thus can't remap should be replaced
> with a working drive, it's obviously faulty.
>
> --
> Mikael Abrahamsson    email: swmike@swm.pp.se
>
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Re: why md/raid5 dont have bad block remap

[Mikael Abrahamsson]

On Mon, 28 Dec 2009, jin zhencheng wrote:

> I think maybe the disk deal with one write comnd with errror,but  the
> disk maybe is also ok,and rewrite to other place or  rewrite again
> will be good.
>
> why one write error ,set the whole disk faulty.

If the disk has *WRITE* errors, something is seriously wrong. Read errors 
is one thing, write errors is completely different.

> Disk if very expensive!

Most drives have at least 2-3 years warranty and that should be used. If 
the drive has write errors, then it's really time to replace it, it won't 
be reliable.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: Re: why md/raid5 dont have bad block remap

[spren.gm]

> from:Mikael Abrahamsson
>On Mon, 28 Dec 2009, jin zhencheng wrote:
>
>> I think maybe the disk deal with one write comnd with errror,but  the
>> disk maybe is also ok,and rewrite to other place or  rewrite again
>> will be good.
>>
>> why one write error ,set the whole disk faulty.
>
>If the disk has *WRITE* errors, something is seriously wrong. Read errors 
>is one thing, write errors is completely different.
>
>> Disk if very expensive!
>
>Most drives have at least 2-3 years warranty and that should be used. If 
>the drive has write errors, then it's really time to replace it, it won't 
>be reliable.

I agree, the capacity and safety  of data is the most important issue for raid layer, 
 if a write error happens on one drive, more errors may soonly follow, 
where and how much will be remapped?  it would involve complicated algorithms and may cause serious performance decline.
------------------				 
spren.gm@gmail.com
2009-12-28