On 18 July 2013 23:03, Martin Wilck <mwilck@xxxxxxxx> wrote: > On 07/18/2013 10:37 PM, Francis Moreau wrote: >> Hello, >> >> Sorry if the question is stupid but I'm a rookie in md things, but I'd >> like to understand the big picture here. >> >> I've been told to use mdadm whenever possible even if my raid is >> handled by the bios (fake raid) which use the ddf metadata format. >> (unfortunately it seems that I can't desactive this fake raid in >> favour of linux soft raid). It's RAID1 BTW. >> >> So my question is rather simple: in my understanding the bios is doing >> the mirroring, but when setting up the md device, linux (kernel or >> userspace, I don't really know) also handles the mirroring for RAID1 >> personnality. Is Linux clever enough to see that the mirroring is done >> by the bios in my case ? >> >> Could anybody teach me the big picture ? > > Fake RAID uses a part of every disk to record information about the RAID > arrays. This is called meta data, and your BIOS uses it for setting up > the drives. > > Under Linux, first you need a low level SATA or SAS driver that detects > your physical drives, e.g. the ahci driver. > > md can then detect the DDF meta data on your disk just like the BIOS, > assemble the array(s), mirror the data, and do other RAID operations. > > Distributions can set this up automatically. Currently most distros > don't do this for DDF (they do it only for fake RAID using the Intel > Matrix (IMSM) format). For DDF, for historical reasons, most > distributions will setup a mapping using dmraid (device mapper based > mirroring). That will also basically work, but it isn't a > fully-functional RAID implementation such as MD. The magic to set up > either MD or dmraid automatically as disks are detected is hidden in the > distro's udev rules, and possibly in the distro's installer logic. > There patches posted to debian bug tracker to enable using mdadm in the installer to assemble/setup IMSM/DDF raid arrays, and thus using mdadm. I haven't integrated those, but am planning to work on merging them soon. At the moment dmraid is used by default for both IMSM/DDF on Debian/Ubuntu. My experience with these fakeraid arrays is very limitted, and I'd want to enquire of proper migration strategies from dmraid to mdadm. Whilst looking at the udev rules, at the moment, i have disable IMSM/DDF from assembly in mdadm udev rules, because dmraid has a nice property of "activating anything it finds". I suppose having both mdadm & dmraid racing to activate those drives wouldn't be nice. How would one migrate from dmraid to mdadm? I was pondering about drastic measures: patch out ISMS support out of dmraid, make dmraid package depend on mdadm and make mdadm activate ISMS drives by default. But that sounds harsh, as I wouldn't want to cripple dmraid package for those who still prefer to use it. Are there distributions which switched to mdadm by default for ISMS? Suse?! Regards, Dmitrijs. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
