Re: Question about mdadm commit d6508f0cfb60edf07b36f1532eae4d9cddf7178b "be more careful about add attempts"

[NeilBrown <neilb@suse.de>]

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On Mon, 31 Oct 2011 10:57:25 +0200 Alexander Lyakas <alex.bolshoy@gmail.com>
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@suse.de> wrote:
> > On Thu, 27 Oct 2011 11:10:54 +0200 Alexander Lyakas <alex.bolshoy@gmail.com>
> > 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@suse.de> wrote:
> >> > On Wed, 26 Oct 2011 19:02:37 +0200 Alexander Lyakas <alex.bolshoy@gmail.com>
> >> > 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
> >> >
> >> >
> >> --
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> >
> >
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