On Mon, 31 Oct 2011 10:57:25 +0200 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> wrote: > Thank you for the clarification, Neil! > > I was looking at raid_disk, because I am trying to see whether it is > possible to control into which raid slot a disk is being added (not > re-added). > > Let's say we had raid6 with 4 drives (a,b,c,d), and drives c and d > failed.Now let's say, that it is decided to replace drive d with a new > drive e (--add for drive e). Then it's possible that drive e will take > the raid slot of drive c. So later, if we want to bring back drive c > into the array, it will have to go into a different slot, resulting in > a full reconstruction, not bitmap-based reconstruction. While if we > would have re-added drive c first, it would have done a bitmap-based > reconstruction, so only the e drive would have required a full > reconstruction. > > So do you think it makes sense to somehow (perhaps through > disc.raid_disk) instruct the kernel into which slot to add a new > drive? You should be able to do that via sysfs. echo frozen > sync_action echo $major:$minor > new_dev echo $slot > dev-$DEVNAME/slot echo idle > sync_action something like that. Seems and odd sort of scenario though. Note that with RAID1 this is not an issue. A re-added drive can take any position in a RAID1 array - it doesn't have to be the same one. NeilBrown > > Thanks, > Alex. > > > > > > On Mon, Oct 31, 2011 at 1:16 AM, NeilBrown <neilb@xxxxxxx> wrote: > > On Thu, 27 Oct 2011 11:10:54 +0200 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> > > wrote: > > > >> Hello Neil, > >> it makes perfect sense not to turn a device into a spare inadvertently. > >> > >> However, with mdadm 3.1.4 under gdb, I tried the following: > >> - I had a raid6 with 4 drives (sda/b/c/d), their "desc_nr" in the > >> kernel were respectively (according to GET_DISK_INFO): 0,1,2,3. > >> - I failed the two last drives (c & d) via mdadm and removed them from the array > >> - I wiped the superblock on drive d. > >> - I added the drive d back to the array > >> So now the array had the following setup: > >> sda: disc_nr=0, raid_disk=0 > >> sdb: disc_nr=1, raid_disk=1 > >> sdd: disc_nr=4, raid_disk=2 > >> So sdd was added to the array into slot 2, and received disc_nr=4 > >> > >> - Now I asked to re-add drive sdc back to array. In gdb I followed the > >> re-add flow, to the place where it fills the mdu_disk_info_t structure > >> from the superblock read from sdc. It put there the following content: > >> disc.major = ... > >> disc.minor = ... > >> disc.number = 2 > >> disc.raid_disk = 2 (because previously this drive was in slot 2) > >> disc.state = ... > >> > >> Now in gdb I changed disc.number to 4 (to match the desc_nr of sdd). > >> And then issued ADD_NEW_DISK. It succeeded, and the sdc drive received > >> disc_nr=2 (while it was asking for 4). Of course, it could not have > >> received the same raid_disk, because this raid_disk was already > >> occupied by sdd. So it was added as a spare. > >> > >> But you are saying: > >> > If a device already exists with the same disk.number, a re-add cannot > >> > succeed, so mdadm doesn't even try. > >> while in my case it succeeded (while it actually did "add" and not "re-add"). > > > > We seem to be using word differently. > > If I ask mdadm to do a "re-add" and it does an "add", then I consider that to > > be "failure", however you seem to consider it to be a "success". > > > > That seems to be the source of confusion. > > > > > >> > >> That's why I was thinking it makes more sense to check disc.raid_disk > >> and not disc.number in this check. Since disc.number is not the > >> drive's role within the array (right?), it is simply a position of the > >> drive in the list of all drives. > >> So if you check raid_disk, and see that this raid slot is already > >> occupied, then, naturally, the drive will be converted to spare, which > >> we want to avoid. > > > > It may well be appropriate to check raid_disk as well, yes. It isn't so easy > > though which is maybe why I didn't. > > > > With 0.90 metadata, the disc.number is the same as disc.raid_disk for active > > devices. That might be another reason for the code being the way that it is. > > > > Thanks, > > NeilBrown > > > > > > > >> > >> And the enough_fd() check protects us from adding a spare to a failed > >> array (like you mentioned to me previously). > >> > >> What am I missing? > >> > >> Thanks, > >> Alex. > >> > >> > >> > >> > >> > >> > >> > >> On Wed, Oct 26, 2011 at 11:51 PM, NeilBrown <neilb@xxxxxxx> wrote: > >> > On Wed, 26 Oct 2011 19:02:37 +0200 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> > >> > wrote: > >> > > >> >> Greetings everybody, > >> >> I have a question about the following code in Manage.c:Manage_subdevs() > >> >> > >> >> disc.number = mdi.disk.number; > >> >> if (ioctl(fd, GET_DISK_INFO, &disc) != 0 > >> >> || disc.major != 0 || disc.minor != 0 > >> >> || !enough_fd(fd)) > >> >> goto skip_re_add; > >> >> > >> >> I do not underatand why the checks: disc.major != 0 || disc.minor != 0 > >> >> are required. This basically means that the kernel already has an > >> >> rdev->desc_nr equal to disc.number. But why fail the re-add procedure? > >> >> > >> >> Let's say that enough_fd() returns true, and we go ahead an issue > >> >> ioctl(ADD_NEW_DISK). In this case, according to the kernel code in > >> >> add_new_disk(), it will not even look at info->number. It will > >> >> initialize rdev->desc_nr to -1, and will allocate a free desc_nr for > >> >> the rdev later. > >> >> > >> >> Doing this with mdadm 3.1.4, where this check is not present, actually > >> >> succeeds. I understand that this code was added for cases when > >> >> enough_fd() returns false, which sounds perfectly fine to protect > >> >> from. > >> >> > >> >> I was thinking that this code should actually check something like: > >> >> if (ioctl(fd, GET_DISK_INFO, &disc) != 0 > >> >> || disk.raid_disk != mdi.disk.raid_disk > >> >> || !enough_fd(fd)) > >> >> goto skip_re_add; > >> >> > >> >> That is to check that the slot that was being occupied by the drive we > >> >> are trying to add, is already occupied by a different drive (need also > >> >> to cover cases of raid_disk <0, raid_disk >= raid_disks etc...) and > >> >> not the desc_nr, which does not have any persistent meaning. > >> >> > >> >> Perhaps there are some compatibility issues with old kernels? Or > >> >> special considerations for ... containers? non-persistent arrays? > >> > > >> > The point of this code is to make --re-add fail unless mdadm is certain that > >> > the kernel will accept the re-add, rather than turn the device into a spare. > >> > > >> > If a device already exists with the same disk.number, a re-add cannot > >> > succeed, so mdadm doesn't even try. > >> > > >> > When you say in 3.1.4 it "actually succeeds" - what succeeds? Does it re-add > >> > the device to the array, or does it turn the device into a spare? > >> > I particularly do not want --re-add to turn a device into a spare because > >> > people sometimes use it in cases where it cannot work, their device gets > >> > turned into a spare, and they lose information that could have been used to > >> > reconstruct the array. > >> > > >> > That that make sense? > >> > > >> > NeilBrown > >> > > >> > > >> -- > >> 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 > > > > > -- > 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
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