dmraid - where is the raid done?
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.
2008/10/17 Brian J. Murrell <[email protected]>:
Quoted 2 lines
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: [email protected]
Signature · 5 lines
Drew Victoria, BC "Nothing in life is to be feared. It is only to be understood." --Marie Curie
Quoted 4 lines · Drew
Try the folks over at the software raid list. They'd be your best bet.
list: [email protected]
Thanx! Subscribed and asked there (for the information of anyone wanting to follow along).
b.
Quoted 8 lines · Drew
2008/10/17 Brian J. Murrell <[email protected]>:
Quoted 2 lines
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: [email protected]
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.
Quoted 6 lines
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.
Signature · 4 lines
Drew "Nothing in life is to be feared. It is only to be understood." --Marie Curie
Quoted 7 lines · Drew
Quoted 6 lines
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.
Quoted 7 lines
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.