dm-raid vs md-raid

[Anthony Wright]

Does that mean dm-raid can only be used in conjunction with a fake-raid 
controller, or can it be used with any hard disk? When you say its not 
well supported, do you mean it's not as mature, it doesn't support many 
controllers or not many people can help you if you have a problem?

 From Ted's comment, am I to understand that with md-raid the format of 
data stored on the disk is affected by the motherboard, so after an 
upgrade I risk being unable to access data on an md-raid array?

I'm really interested in the whole area of networked disks, particularly 
AoE (ATA over Ethernet). I understand that with md-raid if I disconnect 
the network cable between the machine doing the raid, and the networked 
disk the driver has only the option of pausing all IO which stops all IO 
on the array, or telling md-raid that the disk has failed in which case 
it will be removed from the array, and require a full rebuild when it's 
added back into the array. Does dm-raid's log functionality offer a 
third option, allowing the driver to say the disk has gone, but it'll 
probably be back shortly, so work as a degraded array for a while replay 
the writes when the disk is reconnected?

The reason I'm interested in is for use with Xen virtual servers which 
can be moved from one physical machine to another while they are still 
running (10ms-100ms interruption). However disks add a whole new level 
of complexity to that, particularly if you want RAID disks. What I was 
thinking about was having AoE networked disks on both physical machines 
with dm-raid running on the virtual server. If the virtual server is 
running on physical machine 1 and I need to work on physical machine 2, 
I can turn it off and when it gets turned back on the AoE disk 
reconnects and dm-raid replays the missed writes from it's log. If I 
need to work on physical machine 1, I can move the virtual server to 
physical machine 2, and now I can work on physical machine 1 in a 
similar way, all the time with no interruption to the virtual server. 
This however relies on the ability to replay a small set of changes 
rather than having to rebuild the disk each time.

Thanks,

Tony Wright.

ted creedon wrote:
> The dm-raid drives are also portable. Comes in handy when doing a
> motherboard upgrade.
>
> edc
>
> -----Original Message-----
> From: ataraid-list-bounces at redhat.com
> [mailto:ataraid-list-bounces at redhat.com] On Behalf Of Phillip Susi
> Sent: Monday, September 25, 2006 12:26 PM
> To: ATARAID (eg, Promise Fasttrak, Highpoint 370) related discussions
> Subject: Re: dm-raid vs md-raid
>
> md-raid is the older pure software raid kernel driver.  dm-raid is a 
> utility that scans for the configuration tables written to disks by 
> hardware fakeraid controlers, and configures the device-mapper kernel 
> driver to access the raid volumes.  device-mapper is the newer kernel 
> driver that LVM uses to support raid functions.  Most of the modern 
> cheap ide or sata "hardware raid" cards are not really hardware raid. 
> They have software raid support in their proprietary drivers and the 
> system bios.
>
>  From a practical standpoint, the advantages of dm-raid with a hardware 
> fakeraid card over traditional md-raid are:
>
> 1) Can boot directly from a raid5 or raid0
> 2) Can failover and still boot from a raid5 or raid1 with a damaged boot 
> area
> 3) Can dual boot with windows
>
> dm-raid however, is not very well supported yet compared to md-raid.
>