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"). 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. 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
