On Wed, 21 Nov 2012 12:26:00 +0000 Benjamin ESTRABAUD <be@xxxxxxxxxx> wrote: > On 20/11/12 21:16, NeilBrown wrote: > > On Tue, 20 Nov 2012 12:36:52 +0000 Benjamin ESTRABAUD <be@xxxxxxxxxx> wrote: > > > >> Hi! > >> > >> I recently updated mdadm-2.6.9 to mdadm-3.2.6 on a system (own busybox > >> based distro) that had its kernel version updated from 2.6.35 to 3.4. > >> > >> Everything works well apart from a feature I used that I seem to be > >> unable to reactivate: > >> > >> In the past, to ensure knowing the number of possible RAID devices that > >> could be created on the system, I used static major:minor mappings by > >> building the /dev/md/dXX and /dev/md/dXXpXX entries (with major 254, and > >> three minors for each device for 3 partitions), allowing me to be > >> certain that 64 RAID devices could be created at all times. > >> > >> When creating an array, the /dev/md/dXX and dXXpXX devices "nodes" would > >> be used (not recreated) and the major/minor number these devices point > >> to would be used for the actual MD block device. > >> > >> This was a very handy feature. With the latest mdadm, I simply cannot do > >> that, as creating a /dev/md/dXX will in fact remove that device file and > >> symlink it to /dev/md_dXX. I then created /dev/md_dXX devices prior to > >> creating an array, and got the following error message: "mdadm: > >> /dev/md_d3 exists but looks wrong, please fix". > >> > >> Looking through the source I can see that mdadm basically verifies if > >> the file's major/minor matches the one that it had planned for the > >> device, which in this case doesn't. > >> > >> Is there any way to work around that? Or in fact, I don't actually > >> *need* to use static major/minor numbers, but I need to know in advance > >> how many RAIDs I'll be able to create provided I'll always use 2 > >> partitions for them and can provide the -amdp2 argument to mdadm to make > >> sure that only 2 partitions devices nodes are created. > >> > >> Is there a way to know the maximum number of RAIDs that can be created, > >> provided that nothing else uses major 9 and 254? > >> > >> Thank you very much in advance for your help! > > I don't really understand what your problem is. > > > > mdadm should create any devices it needs - unless it detects udev, in which > > case it leaves the device creation to udev. > > > > So simply don't create any md devices in /dev and let mdadm do whatever is > > required. > > Does that approach not suit your needs? If not, why not? > > > > NeilBrown > Hi Neil, > > mdadm creates the devices it needs quite well (there is no udev on this > system), but how can I tell how many maximum RAID devices mdadm will be > able to create before running out of major/minors, since their > allocation is done on the fly? > > Will mdadm use all of "md" registered major (9) minor numbers (9:0-254)? Actually I think it is 9:0-4194303. There are 22 bits for minor numbers. And yes, md will use as many as needed (Though I haven't tested millions of arrays - or even hundreds - so there might be bugs). > Will it also use the "mdp" registered ones (254) when running out? mdp uses 4 bits for the partition, so 18 bits available for arrays. Though you don't really need mdp any more as 'md' devices can now be partitioned. > > I need to know how many MD devices I'll be able to create in advance on > that system. Creating the devices beforehand helped with that, since I > had md_d0 254:0 up to md_d127 254:252, I knew that I could create a > maximum of 128 devices. Creating the device files in /dev doesn't really have any effect on whether the devices can be created. It is a bit like creating a whole bunch of symlinks to non-existent files, and assuming that means that the files can later be created. > > What's the maximum number of MD devices that can be created on a > udev-less system provided we specify 2 partitions in mdadm --create? millions. NeilBrown > > Thank you very much in advance for your help. > > Regards, > Ben.
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