[DRBD-user] drbd+lvm no bueno

[Igor Cicimov]

On Sun, 29 Jul 2018 1:13 am Eric Robinson <eric.robinson at psmnv.com> wrote:

>
>
>
>
> > -----Original Message-----
> > From: Eric Robinson
> > Sent: Saturday, July 28, 2018 7:39 AM
> > To: Lars Ellenberg <lars.ellenberg at linbit.com>;
> drbd-user at lists.linbit.com
> > Subject: RE: [DRBD-user] drbd+lvm no bueno
> >
> > > > > Lars,
> > > > >
> > > > > I put MySQL databases on the drbd volume. To back them up, I pause
> > > > > them and do LVM snapshots (then rsync the snapshots to an archive
> > > > > server). How could I do that with LVM below drbd, since what I
> > > > > want is a snapshot of the filesystem where MySQL lives?
> > > >
> > > > You just snapshot below DRBD, after "quiescen" the mysql db.
> > > >
> > > > DRBD is transparent, the "garbage" (to the filesystem) of the
> > > > "trailing drbd meta data" is of no concern.
> > > > You may have to "mount -t ext4" (or xfs or whatever), if your mount
> > > > and libblkid decide that this was a "drbd" type and could not be
> > > > mounted. They are just trying to help, really.
> > > > which is good. but in that case they get it wrong.
> > >
> > > Okay, just so I understand....
> > >
> > > Suppose I turn md4 into a PV and create one volume group
> > > 'vg_under_drbd0', and logical volume 'lv_under_drbd0' that takes 95%
> > > of the space, leaving 5% for snapshots.
> > >
> > > Then I create my ext4 filesystem directly on drbd0.
> > >
> > > At backup time, I quiesce the MySQL instances and create a snapshot of
> > > the drbd disk.
> > >
> > > I can then mount the drbd snapshot as a filesystem?
> > >
> >
> > Disregard question. I tested it. Works fine. Mind blown.
> >
> > -Eric
> >
>
> Although I discovered quite by accident that you can mount a snapshot over
> the top of the filesystem that exists on the device that it's a snapshot
> of. Wouldn't this create some sort of recursive write death spiral?
>

I think this can help properly understanding lvm snapshots and answer your
question https://www.clevernetsystems.com/lvm-snapshots-explained/


> Check it out...
>
> root at 001db01a /]# lvdisplay
>   --- Logical volume ---
>   LV Path                /dev/vg_under_drbd1/lv_under_drbd1
>   LV Name                lv_under_drbd1
>   VG Name                vg_under_drbd1
>   LV UUID                LWWPiL-Y6nR-cNnW-j2E9-LAK9-UsXm-3inTyJ
>   LV Write Access        read/write
>   LV Creation host, time 001db01a, 2018-07-28 04:53:14 +0000
>   LV Status              available
>   # open                 2
>   LV Size                1.40 TiB
>   Current LE             367002
>   Segments               1
>   Allocation             inherit
>   Read ahead sectors     auto
>   - currently set to     8192
>   Block device           253:1
>
>   --- Logical volume ---
>   LV Path                /dev/vg_under_drbd0/lv_under_drbd0
>   LV Name                lv_under_drbd0
>   VG Name                vg_under_drbd0
>   LV UUID                M2oMNd-hots-d9Pf-KQG8-YPqh-6x3a-r6wBqo
>   LV Write Access        read/write
>   LV Creation host, time 001db01a, 2018-07-28 04:52:59 +0000
>   LV Status              available
>   # open                 2
>   LV Size                1.40 TiB
>   Current LE             367002
>   Segments               1
>   Allocation             inherit
>   Read ahead sectors     auto
>   - currently set to     8192
>   Block device           253:0
>
> [root at 001db01a /]# df -h
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sda2        30G  3.3G   27G  12% /
> devtmpfs         63G     0   63G   0% /dev
> tmpfs            63G     0   63G   0% /dev/shm
> tmpfs            63G  9.0M   63G   1% /run
> tmpfs            63G     0   63G   0% /sys/fs/cgroup
> /dev/sda1       497M   78M  420M  16% /boot
> /dev/sdb1       252G   61M  239G   1% /mnt/resource
> tmpfs            13G     0   13G   0% /run/user/0
> /dev/drbd0      1.4T  2.1G  1.4T   1% /ha01_mysql
> [root at 001db01a /]#
> [root at 001db01a /]# ls /ha01_mysql
> lost+found  testfile
> [root at 001db01a /]#
> [root at 001db01a /]# lvcreate -s -L30G -n drbd0_snapshot
> /dev/vg_under_drbd0/lv_under_drbd0
>   Logical volume "drbd0_snapshot" created.
> [root at 001db01a /]#
> [root at 001db01a /]# mount /dev/vg_under_drbd0/drbd0_snapshot /ha01_mysql
> [root at 001db01a /]#
> [root at 001db01a /]# cd /ha01_mysql
> [root at 001db01a ha01_mysql]# ls
> lost+found  testfile
> [root at 001db01a ha01_mysql]# echo blah > blah.txt
> [root at 001db01a ha01_mysql]# ll
> total 2097172
> -rw-r--r--. 1 root root          5 Jul 28 14:50 blah.txt
> drwx------. 2 root root      16384 Jul 28 14:10 lost+found
> -rw-r--r--. 1 root root 2147479552 Jul 28 14:20 testfile
> [root at 001db01a ha01_mysql]# cd /
>  [root at 001db01a /]# umount /ha01_mysql
> [root at 001db01a /]# ls /ha01_mysql
> lost+found  testfile
> [root at 001db01a /]#
>
> What? I know nothing.
>
> --Eric
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