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The_Great_Sephiroth
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PostPosted: Fri Jun 23, 2017 4:38 pm    Post subject: Strange issue, possibly GRUB? Reply with quote

I am attempting to install Gentoo onto a PowerEdge T20 system to act as a small business server with AD, DHCP, DNS, file-sharing (Samba/NFS), and possibly more. However, after completing the live CD phase, I cannot boot into the system I have double and triple-checked the fstab file and the UUIDs and such are correct. However, after GRUB says "Booting Linux 4.6.19..." It says "unknown command /dev/sdb2". Never seen this before. The system is BTRFS and /dev/sda2 AND /dev/sdb2 are the RAID1 partitions, specified in the fstab file via UUID, but I believe grub-mkconfig must be doing something wrong to get this error.

Video of the boot process
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PostPosted: Fri Jun 23, 2017 10:12 pm    Post subject: Reply with quote

That's... quite special.

Can you post the offending grub.cfg?
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PostPosted: Fri Jun 23, 2017 10:35 pm    Post subject: Reply with quote

Likely grub-mkconfig got confused and did not know how to set up a btrfs config - enough so that it causes a grub.cfg syntax error.

It probably then booted into the fallback that does not set up root properly - basically completely ignoring it - because the first boot failed.

Now my problem: I don't know what I'm talking about because I've never set up a btrfs-raid system, but from what I've read, an initramfs is needed. Likely grub-mkconfig screwed up setting up the initramfs root -- it always screws up my initramfs root. It "works" but is NOT optimal, I end up having to hand fix the grub.cfg.

Yes that grub.cfg is the hint to figure out how to get grub-mkconfig generate it right, or you have to edit it manually...
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PostPosted: Mon Jun 26, 2017 2:56 pm    Post subject: Reply with quote

I will try to post captures of the configuration file in a bit. BTRFS has nothing to do with it. I build all local hardware into my kernels and use BTRFS and it has worked on every system up to this one. I have it on this laptop in fact, and it works fine. So no initramfs, and BTRFS has worked on RAID1 before, so I doubt that is the issue.
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PostPosted: Mon Jun 26, 2017 3:06 pm    Post subject: Reply with quote

There's a difference between getting btrfs working and getting btrfs automatically working with grub-mkconfig.

We all know grub-mkconfig is an abomination and likely there's some special case in the code that deals with btrfs RAID that it probably screwed up.
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PostPosted: Mon Jun 26, 2017 3:13 pm    Post subject: Re: Strange issue, possibly GRUB? Reply with quote

The_Great_Sephiroth wrote:
...to act as a small business server with AD...


I'm assuming this means active directory and you'll be serving windows clients. If so, is there any chance you could open a thread letting us know how it goes after you get your grub problem fixed?
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PostPosted: Mon Jun 26, 2017 4:45 pm    Post subject: Reply with quote

Mrbassie, I have numerous clients all over Fayetteville, NC and surrounding areas that have been using Samba4 for years to save them the costs of Windows Server and has worked quite well, spare a few hiccups here and there after upgrades. It is ALMOST all we deploy for AD now. If you have questions feel free to PM me.
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PostPosted: Mon Jun 26, 2017 6:15 pm    Post subject: Reply with quote

I took a picture of what I believe the issue to be.

Grub configuration

The thing is, sda and sdb are in a RAID1 mirror. You have /dev/sdX2 as BTRFS with the following subvolumes.

  • @root
  • @boot
  • @source
  • @home

They are mounted via fstab with @root being /, @boot being /boot, @source being /usr/src, and @home being /home. The GRUB entry does not look correct, but I am NOT familiar with GRUB configuration files.
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PostPosted: Mon Jun 26, 2017 8:23 pm    Post subject: Reply with quote

Yeah grub-mkconfig royally fscked up, typical. It never gets labeled partitions right. How do you normally specify a btrfs raid1 as root on a kernel command line? Did you do any manual setup files for grub?

You could just manually type that in right there and get rid of that extraneous line, and then "delete" grub-mkconfig (i.e., never use it again)... That would be a start, then would be a stepping board to get mkconfig to do it correctly..
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PostPosted: Tue Jun 27, 2017 1:29 pm    Post subject: Reply with quote

I figured that line was my issue. I saw sda referenced above it and then that line, which wasn't indented or anything. I knew something was up. Is it possible to use UUID there instead of sda/sdb?
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PostPosted: Tue Jun 27, 2017 3:02 pm    Post subject: Reply with quote

If things haven't changed, UUID is an initramfs construct; only PARTUUID can be taken directly by kernels.

That's why I was a bit confused - a RAID1, supposedly the two disks match minus perhaps metadata - somehow the RAID needs to be assembled first and then mounted. This is where I don't know how btrfs needs to assemble the RAID first before mounting root.

Again this is beyond me as I have never set up a native RAID1 btrfs system so I don't know how to specify root=XXX for such system. All the references I've found online requires initramfs to do this work - which is similar to the hoop jumping needed to get root on ext3 over lvm over mdraid which is my setup. The initramfs will also deal with the UUID= aspect as it can work with that too.
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PostPosted: Tue Jun 27, 2017 10:31 pm    Post subject: Reply with quote

I figured it out, I think. When grub-mkconfig queried the UUID for the root filesystem it returned TWO lines, one for /dev/sda and one for /dev/sdb. All devices in a mirror in BTRFS have the same UUID, hence it getting two lines back. This is apparently NOT handled, so it simply plopped both lines into the configuration file. Removing the line break and one of the two devices should fix the issue. The thing is, grub-mkconfig should handle receiving more than one device back, even if it simply fails.
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PostPosted: Wed Jun 28, 2017 7:03 pm    Post subject: Reply with quote

I might have to try out btrfs...though I wish raid5 is an option but it seems unsafe at the moment.

My current stacking of ext3 -> lvm -> mdraid5 seems like a disaster waiting to happen, though it hasn't yet (in fact, I've been able to recover from disk failures just fine.)
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PostPosted: Thu Jun 29, 2017 9:50 am    Post subject: Reply with quote

eccerr0r wrote:
I might have to try out btrfs...


Beware of the raid performance issues too. The scrubbing is certainly an advantage but my experience with multi device raid is that performance suffers massively. This seems to be due to the fact that all of the raid is done above the block layer and so the scheduler is not able to make intelligent decisions. I'm not sure why I found it so bad, but in a 14TB btrfs raid 0 system I was seeing multiple second write latencies.

The switch to lvm2 raid + 'plain' btrfs has restored performance to close to the theoretical maximum the hardware can deliver.

If you really need the scrubbing consider zfs, it has the same theoretical problems as btfrs raid but has also implemented the necessary solutions.

Edit: I can also recommend lvm2 over mdraid, it removes a component and I find it very stable and reliable both for sw raid and for thin provisioning of VM snapshots.

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PostPosted: Thu Jun 29, 2017 2:00 pm    Post subject: Reply with quote

Check your hardware. I have BTRFS RAID in place in many areas including my home and it works fine. Granted, our most complicated setup is a four-disk RAID10 (like at home) but it is on par with other RAID solutions in that configuration. If you have ten disks or if they are mixed sizes, I don't know. I just know that ZFS is a memory hog and lacks the features BTRFS has. I do agree with you on RAID5/6 support though. It has been a long time coming!

Now, new problem! Correcting GRUB as above gets me right into the boot, and a kernel panic.

Photo of the panic

BTRFS support is built into the kernel, same as on this laptop. Not sure why it is losing its mind. This is working on far weaker, home-brew servers. Maybe it has something to do with the more advanced hardware in the PowerEdge, but I just don't understand it. Below is the GRUB configuration file, obtained via SSH session to System Rescue CD, which is what I used to install Gentoo.
Code:

root@sysresccd /root % cat /mnt/gentoo/boot/grub/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="0"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    save_env saved_entry
  fi
}

function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

if [ x$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_gpt
insmod part_gpt
insmod btrfs
set root='hd0,gpt2'
if [ x$feature_platform_search_hint = xy ]; then
  search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  --hint-bios=hd1,gpt2 --hint-efi=hd1,gpt2 --hint-baremetal=ahci1,gpt2  13cf66b1-42e3-48ca-9f56-96ab1766f934
else
  search --no-floppy --fs-uuid --set=root 13cf66b1-42e3-48ca-9f56-96ab1766f934
fi
    font="/@root/usr/share/grub/unicode.pf2"
fi

if loadfont $font ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  set locale_dir=$prefix/locale
  set lang=en_US
  insmod gettext
fi
terminal_output gfxterm
if [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=5
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/10_linux ###
menuentry 'Gentoo GNU/Linux' --class gentoo --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-13cf66b1-42e3-48ca-9f56-96ab1766f934' {
        load_video
        if [ "x$grub_platform" = xefi ]; then
                set gfxpayload=keep
        fi
        insmod gzio
        insmod part_gpt
        insmod part_gpt
        insmod btrfs
        set root='hd0,gpt2'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  --hint-bios=hd1,gpt2 --hint-efi=hd1,gpt2 --hint-baremetal=ahci1,gpt2  13cf66b1-42e3-48ca-9f56-96ab1766f934
        else
          search --no-floppy --fs-uuid --set=root 13cf66b1-42e3-48ca-9f56-96ab1766f934
        fi
        echo    'Loading Linux 4.9.16-gentoo ...'
        linux   /@boot/vmlinuz-4.9.16-gentoo root=/dev/sda2 ro rootflags=subvol=@root 
}
submenu 'Advanced options for Gentoo GNU/Linux' $menuentry_id_option 'gnulinux-advanced-13cf66b1-42e3-48ca-9f56-96ab1766f934' {
        menuentry 'Gentoo GNU/Linux, with Linux 4.9.16-gentoo' --class gentoo --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.9.16-gentoo-advanced-13cf66b1-42e3-48ca-9f56-96ab1766f934' {
                load_video
                if [ "x$grub_platform" = xefi ]; then
                        set gfxpayload=keep
                fi
                insmod gzio
                insmod part_gpt
                insmod part_gpt
                insmod btrfs
                set root='hd0,gpt2'
                if [ x$feature_platform_search_hint = xy ]; then
                  search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  --hint-bios=hd1,gpt2 --hint-efi=hd1,gpt2 --hint-baremetal=ahci1,gpt2  13cf66b1-42e3-48ca-9f56-96ab1766f934
                else
                  search --no-floppy --fs-uuid --set=root 13cf66b1-42e3-48ca-9f56-96ab1766f934
                fi
                echo    'Loading Linux 4.9.16-gentoo ...'
                linux   /@boot/vmlinuz-4.9.16-gentoo root=/dev/sda2 ro rootflags=subvol=@root 
        }
        menuentry 'Gentoo GNU/Linux, with Linux 4.9.16-gentoo (recovery mode)' --class gentoo --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.9.16-gentoo-recovery-13cf66b1-42e3-48ca-9f56-96ab1766f934' {
                load_video
                if [ "x$grub_platform" = xefi ]; then
                        set gfxpayload=keep
                fi
                insmod gzio
                insmod part_gpt
                insmod part_gpt
                insmod btrfs
                set root='hd0,gpt2'
                if [ x$feature_platform_search_hint = xy ]; then
                  search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  --hint-bios=hd1,gpt2 --hint-efi=hd1,gpt2 --hint-baremetal=ahci1,gpt2  13cf66b1-42e3-48ca-9f56-96ab1766f934
                else
                  search --no-floppy --fs-uuid --set=root 13cf66b1-42e3-48ca-9f56-96ab1766f934
                fi
                echo    'Loading Linux 4.9.16-gentoo ...'
                linux   /@boot/vmlinuz-4.9.16-gentoo root=/dev/sda2 ro single rootflags=subvol=@root
        }
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###

Again, BTRFS support is compiled into the kernel. Not sure what the issue is.
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PostPosted: Thu Jun 29, 2017 3:51 pm    Post subject: Reply with quote

It can't find the root partition... So you have another raid1 setup that this works with, or this one is not raid?
Looks like probably disk drivers are missing or somehow pointing to wrong partition.
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PostPosted: Thu Jun 29, 2017 4:16 pm    Post subject: Reply with quote

Figured it out. The in-kernel BTRFS support is behind. I made a minimalist initramfs and all is good. I will post the steps I did to achieve this shortly. I am in the OS now and all is good.

*UPDATE*

OK, I made /usr/src/initramfs and created the following files in the directory. The kernel BTRFS code cannot deal with multiple devices during boot, which is the whole problem. No idea why it cannot do this yet, so it means BTRFS single devices work fine but multiple devices require an initramfs, for now.
initramfs_list
Code:

dir /proc       755 0 0
dir /usr        755 0 0
dir /bin        755 0 0
dir /sys        755 0 0
dir /var        755 0 0
dir /lib64      755 0 0
dir /sbin       755 0 0
dir /mnt        755 0 0
dir /mnt/root   755 0 0
dir /mnt/boot   755 0 0
dir /etc        755 0 0
dir /root       700 0 0
dir /dev        755 0 0

file /bin/busybox               /bin/busybox            755 0 0
file /sbin/fsck                 /sbin/fsck              755 0 0
file /lib64/libmount.so.1       /lib64/libmount.so.1    755 0 0
file /lib64/libblkid.so.1       /lib64/libblkid.so.1    755 0 0
file /lib64/libc.so.6           /lib64/libc.so.6        755 0 0
file /lib64/libuuid.so.1        /lib64/libuuid.so.1     755 0 0
file /lib64/ld-linux-x86-64.so.2  /lib64/ld-linux-x86-64.so.2 755 0 0
file /sbin/fsck.ext3            /sbin/fsck.ext3         755 0 0
file /lib64/libext2fs.so.2      /lib64/libext2fs.so.2   755 0 0
file /lib64/libcom_err.so.2     /lib64/libcom_err.so.2  755 0 0
file /lib64/libe2p.so.2         /lib64/libe2p.so.2      755 0 0
file /lib64/libpthread.so.0     /lib64/libpthread.so.0  755 0 0
file /sbin/btrfs                /sbin/btrfs             755 0 0
file /sbin/btrfs-convert        /sbin/btrfs-convert     755 0 0
file /sbin/btrfs-debug-tree     /sbin/btrfs-debug-tree  755 0 0
file /sbin/btrfs-find-root      /sbin/btrfs-find-root   755 0 0
file /sbin/btrfs-image          /sbin/btrfs-image       755 0 0
file /sbin/btrfs-map-logical    /sbin/btrfs-map-logical 755 0 0
#file /sbin/btrfs-show-super    /sbin/btrfs-show-super  755 0 0
file /sbin/btrfs-zero-log       /sbin/btrfs-zero-log    755 0 0
file /sbin/btrfsck              /sbin/btrfsck           755 0 0
file /sbin/btrfstune            /sbin/btrfstune         755 0 0
file /sbin/mkfs.btrfs           /sbin/mkfs.btrfs        755 0 0
file /lib64/libz.so.1           /lib64/libz.so.1        755 0 0
file /lib64/liblzo2.so          /usr/lib64/liblzo2.so   755 0 0
file /sbin/nano                 /usr/bin/nano           755 0 0
file /lib64/libncursesw.so.6    /lib64/libncursesw.so.6 755 0 0
file /lib64/libdl.so.2          /lib64/libdl.so.2       755 0 0
file /sbin/mknod                /bin/mknod              755 0 0
file /sbin/more                 /bin/more               755 0 0
file    /init                   /usr/src/initramfs/init 755 0 0
file    /etc/fstab              /usr/src/initramfs/fstab        644 0 0

init
Code:

#!/bin/busybox sh

rescue_shell() {
        echo "$@"
        echo "Something went wrong. Dropping you to a shell."
        busybox --install -s
        exec /bin/sh
}

mount_root() {
        echo "scanning for btrfs filesystems.... will take about 5-10 seconds"
        /sbin/btrfs device scan
        echo "mounting /mnt/root"
        mount /mnt/root
}

mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs none /dev

echo 0 > /proc/sys/kernel/printk
clear

mount_root || rescue_shell "Error with uuidlabel_root"
echo "All done. Switching to real root."

umount /proc
umount /sys
umount /dev

exec switch_root /mnt/root /sbin/init

fstab
Code:

/dev/sda2       /mnt/root               btrfs   device=/dev/sda2,device=/dev/sdb2,defaults,subvol=@root         0 0
/dev/sda2       /mnt/boot               btrfs   device=/dev/sda2,device=/dev/sdb2,defaults,subvol=@boot         0 0
/dev/sda2       /mnt/root/usr/src       btrfs   device=/dev/sda2,device=/dev/sdb2,defaults,subvol=@source       0 0
/dev/sda2       /mnt/root/home          btrfs   device=/dev/sda2,device=/dev/sdb2,defaults,subvol=@home         0 0

I then added the initramfs_list to my kernel config, rebuilt, all is good!
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eccerr0r
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PostPosted: Thu Jun 29, 2017 4:54 pm    Post subject: Reply with quote

The kernel has never been able to deal with multiple devices bound as a single for rootfs as far as I knew, from the start... And I have a feeling it will never be able to start without initramfs.

The only possible exception is mdraid 0.9 superblocks and even that had caveats. Plus this feature was removed in 1.2 superblocks assuming that initramfs was available to do this.
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Telemin
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PostPosted: Fri Jun 30, 2017 1:39 pm    Post subject: Reply with quote

The_Great_Sephiroth wrote:
Check your hardware.


I did, everything checks out. It is an SAS controller with sata disks in a dell dual socket workstation, and with lvm2 and zfs I get basically full theoretical throughput, and btrfs thrashes and crawls. I don't fully understand it, because like you I have other 2 disk btrfs raid arrays that work great on commodity hardware, but moving to lvm2 the difference was night and day. I'd love to know why btrfs is so terrible on that hardware but I just don't the time to screw around debugging on a production machine.
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PostPosted: Sat Jul 01, 2017 6:52 pm    Post subject: Reply with quote

Might be due to that being professional hardware and BTRFS tries to do things the hardware already does, thus killing performance. I have not tried BTRFS on a SAS (Perc) controller yet. Maybe I should!
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