[linux-lvm] dmraid - where is the raid done?

[linux-lvm] dmraid - where is the raid done?

[Brian J. Murrell]

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I think this is the right place for dmraid questions.  Please redirect
me if it's not.  Thanx.

I understand that dmraid is just some kind of interface to the
_software_ raid that is provided on a number of SATA interface cards
such as the Promise and Adaptec cards.

Reading http://ata.wiki.kernel.org/index.php/SATA_RAID_FAQ I get the
message loud and clear that this is not real hardware RAID but rather
it's done in the BIOS on the card.

The message there seems to be, if you are going to do it in software
anyway, why not just skip the IO card RAID and just use MD?

The one reason I could think of for using the BIOS provided RAID would
be to reduce the data needing to traverse the PCI bus.  With host-raid
(i.e. MD) every write to disk needs to actually be written over the PCI
bus twice, once for each disk, right?

But do the BIOS RAID cards out there only need the write command and
data once and they do the work of writing it to the two disks on-board
the IO card?

b.


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Re: [linux-lvm] dmraid - where is the raid done?

[Drew]

2008/10/17 Brian J. Murrell <brian@interlinx.bc.ca>:
> I think this is the right place for dmraid questions.  Please redirect
> me if it's not.  Thanx.

Try the folks over at the software raid list. They'd be your best bet.

list: linux-raid@vger.kernel.org


-- 
Drew
Victoria, BC

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie

Re: [linux-lvm] dmraid - where is the raid done?

[Brian J. Murrell]

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On Mon, 2008-10-27 at 06:56 -0700, Drew wrote:
> 
> Try the folks over at the software raid list. They'd be your best bet.
> 
> list: linux-raid@vger.kernel.org

Thanx!  Subscribed and asked there (for the information of anyone
wanting to follow along).

b.


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Re: [linux-lvm] dmraid - where is the raid done?

[Bryn M. Reeves]

Drew wrote:
> 2008/10/17 Brian J. Murrell <brian@interlinx.bc.ca>:
>> I think this is the right place for dmraid questions.  Please redirect
>> me if it's not.  Thanx.
> 
> Try the folks over at the software raid list. They'd be your best bet.
> 
> list: linux-raid@vger.kernel.org
> 

Wrong list - that's for the Linux MD RAID subsystem (mdadm, /dev/md* and
friends). You're looking for ataraid-list:

http://www.redhat.com/mailman/listinfo/ataraid-list

Which is the place for discussion of dmraid development.

Regards,
Bryn.

Re: [linux-lvm] dmraid - where is the raid done?

[Drew]

> Wrong list - that's for the Linux MD RAID subsystem (mdadm, /dev/md* and
> friends). You're looking for ataraid-list:
>
> http://www.redhat.com/mailman/listinfo/ataraid-list
>
> Which is the place for discussion of dmraid development.

Evening Byrn,

I am aware that the dm-raid list was the best one if he had a dm-raid
specific question. However from what I read it sounded like he was
interested in knowing what the differences were between ata/dm-raid
and md (software) raid. Being a lurker on both lists I felt that the
folks over at md would be a better choice.


-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie

Re: [linux-lvm] dmraid - where is the raid done?

[Bryn M. Reeves]

Drew wrote:
>> Wrong list - that's for the Linux MD RAID subsystem (mdadm, /dev/md* and
>> friends). You're looking for ataraid-list:
>>
>> http://www.redhat.com/mailman/listinfo/ataraid-list
>>
>> Which is the place for discussion of dmraid development.
> 
> Evening Byrn,
> 
> I am aware that the dm-raid list was the best one if he had a dm-raid
> specific question. However from what I read it sounded like he was
> interested in knowing what the differences were between ata/dm-raid
> and md (software) raid. Being a lurker on both lists I felt that the
> folks over at md would be a better choice.

He was asking where the RAID is done in dmraid. I think the response of
the MD folks to that question based on past experience, would be either
very general or something like: "Huh? Go ask the device-mapper guys" ;)

If you're using dmraid to activate volumes on an ata RAID / fakeraid
adapter then all the RAID work is being done in the kernel by device-mapper.

When dmraid activates a volume, it reads the metadata from the disk that
describes the array and translates that into a device-mapper table,
usually using some combination of the linear, dm-raid1, and dm-raid45
targets. These are the kernel modules that implement the RAID algorithms
(dm-raid1 uses the kernel thread kcopyd to do the actual work - you'll
see this process consuming some resources on a system with a busy mirror).

For more information on the targets and the tables that are used to
configure them see Alasdairs talks on LVM/device-mapper here:

http://people.redhat.com/agk/talks/

Regards,
Bryn.