Possibility for a parallel relaxed RAID?

Possibility for a parallel relaxed RAID?

[Berkey B Walker]

There maybe many folks out there who want to use RAID on their personal, 
non-production systems.  Assuming access and thru-put values are not 
critical a problem might be "You can't use Desktop drives for RAID".  
Which, I think, most of us know is not really true, but - - If the 
timing issues were to be relaxed, allowing the drive to fix itself, 
before being kicked by the md process, might not the average Joe be 
better served?  There is a big difference between going a mile and 
buying a commodity drive, and being "up" in an hour vs. finding a 
working system, going online, paying price+, and getting/paying fast 
shipping. Which might resolve the issue in days instead of hours.

Any possibility of a parallel, less critical to drive response, release 
of md?  Or a patch to allow same?

Thanks
Berk Walker berkatpanixperiodcom

Re: Possibility for a parallel relaxed RAID?

[Neil Brown]

On Thu, 18 Mar 2010 15:01:44 -0400
Berkey B Walker <berk@panix.com> wrote:

> There maybe many folks out there who want to use RAID on their personal, 
> non-production systems.  Assuming access and thru-put values are not 
> critical a problem might be "You can't use Desktop drives for RAID".  
> Which, I think, most of us know is not really true, but - - If the 
> timing issues were to be relaxed, allowing the drive to fix itself, 
> before being kicked by the md process, might not the average Joe be 
> better served?  There is a big difference between going a mile and 
> buying a commodity drive, and being "up" in an hour vs. finding a 
> working system, going online, paying price+, and getting/paying fast 
> shipping. Which might resolve the issue in days instead of hours.
> 
> Any possibility of a parallel, less critical to drive response, release 
> of md?  Or a patch to allow same?

This is not a function of 'md'.  md has no timeouts for drives responding.
It just submits a request and waits for a success/fail reply.

It may be a function of the lower level SATA/SCSI/FC/whatever driver.  You
would do better to ask the developers of those drivers, maybe start with the
maintainer of libata.

NeilBrown

Re: Possibility for a parallel relaxed RAID?

[Randy Terbush]

Neil,

How does md make the decision to drop a drive from the array? Is it a
fail message back from the ATA layer? Quickly getting over my head
here, but I would like to think there are different types of failures
and a failure to complete error correction on the drive should not be
a fatal error to md.

This request is the same as the post I have made earlier this week
regarding "RAID class" drives which I would love to get your response
to.

Tks

On Thu, Mar 18, 2010 at 3:54 PM, Neil Brown <neilb@suse.de> wrote:
> On Thu, 18 Mar 2010 15:01:44 -0400
> Berkey B Walker <berk@panix.com> wrote:
>
>> There maybe many folks out there who want to use RAID on their personal,
>> non-production systems.  Assuming access and thru-put values are not
>> critical a problem might be "You can't use Desktop drives for RAID".
>> Which, I think, most of us know is not really true, but - - If the
>> timing issues were to be relaxed, allowing the drive to fix itself,
>> before being kicked by the md process, might not the average Joe be
>> better served?  There is a big difference between going a mile and
>> buying a commodity drive, and being "up" in an hour vs. finding a
>> working system, going online, paying price+, and getting/paying fast
>> shipping. Which might resolve the issue in days instead of hours.
>>
>> Any possibility of a parallel, less critical to drive response, release
>> of md?  Or a patch to allow same?
>
> This is not a function of 'md'.  md has no timeouts for drives responding.
> It just submits a request and waits for a success/fail reply.
>
> It may be a function of the lower level SATA/SCSI/FC/whatever driver.  You
> would do better to ask the developers of those drivers, maybe start with the
> maintainer of libata.
>
> NeilBrown
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Re: Possibility for a parallel relaxed RAID?

[Neil Brown]

On Thu, 18 Mar 2010 17:00:04 -0600
Randy Terbush <randy@terbush.org> wrote:

> Neil,
> 
> How does md make the decision to drop a drive from the array? Is it a
> fail message back from the ATA layer? Quickly getting over my head
> here, but I would like to think there are different types of failures
> and a failure to complete error correction on the drive should not be
> a fatal error to md.

md gets an error from the block layer, which gets it from the ata layer.
There is a facility to transfer back a status describing the type of error
however
  - I don't believe it is used consistently, nor is there a list of 
    usable errors
  - md currently makes a minimal assumption about an error, so the
    only use in md for this would be to be less gentle, not more gentle.

If a drive has "a failure to complete error correction", then I suspect they
presents as a read error.  md will try to get the data from elsewhere and
re-write it.  If that fails it becomes a write error, and a device that
cannot be written to is useless, so md fails it.

There are plans to add support for a bad-block-list to md so that when
we get a write failure or a read failure on a degraded array we can mark just
that block as faulty.  However these plans are currently languishing due to
lack of time.

> 
> This request is the same as the post I have made earlier this week
> regarding "RAID class" drives which I would love to get your response
> to.

I think the only part of the which is relevant to me is:

>  However, I do not understand why the RAID system
> cannot detect the type of drive it is dealing with and either disable
> the behavior on the drive or allow more time for the drive to respond
> before kicking it out of the array.

It doesn't disable anything because no-one has written code to do so.
I know virtually nothing about ata controller and protocols and such so I am
not really the person to do it.
This would either be something that mdadm did when it added a drive to an
array, or something that the kernel did when md asked it some how.
I suggest you talk to someone who knows about sata - or submit a patch
yourself.

The "allow more time for the drive to respond" is not an issue for md.  It
has no timeouts.   It might be an issue for the sata controller.

NeilBrown


> 
> Tks
> 
> On Thu, Mar 18, 2010 at 3:54 PM, Neil Brown <neilb@suse.de> wrote:
> > On Thu, 18 Mar 2010 15:01:44 -0400
> > Berkey B Walker <berk@panix.com> wrote:
> >
> >> There maybe many folks out there who want to use RAID on their personal,
> >> non-production systems.  Assuming access and thru-put values are not
> >> critical a problem might be "You can't use Desktop drives for RAID".
> >> Which, I think, most of us know is not really true, but - - If the
> >> timing issues were to be relaxed, allowing the drive to fix itself,
> >> before being kicked by the md process, might not the average Joe be
> >> better served?  There is a big difference between going a mile and
> >> buying a commodity drive, and being "up" in an hour vs. finding a
> >> working system, going online, paying price+, and getting/paying fast
> >> shipping. Which might resolve the issue in days instead of hours.
> >>
> >> Any possibility of a parallel, less critical to drive response, release
> >> of md?  Or a patch to allow same?
> >
> > This is not a function of 'md'.  md has no timeouts for drives responding.
> > It just submits a request and waits for a success/fail reply.
> >
> > It may be a function of the lower level SATA/SCSI/FC/whatever driver.  You
> > would do better to ask the developers of those drivers, maybe start with the
> > maintainer of libata.
> >
> > NeilBrown
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >

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