There is a machine: [root@SERVER ~]# df -mPFilesystem                 1048576-blocks   Used Available Capacity Mounted on/dev/sda2                          124685   7462    110897       7% /tmpfs                               12016      0     12016       0% /dev/shm/dev/sda1                             485    102       358      23% /boot/dev/sdb1                           32131     48     30444       1% /var/foobar1/dev/sdb2                           16009    420     14770       3% /var/foobar2/dev/sdb3                             988      6       930       1% /var/foobar3/dev/sdb5                             988      2       935       1% /var/foobar4/dev/sdb6                             988     17       919       2% /var/foobar5/dev/mapper/datvg-FOO             125864  81801     37663      69% /var/FOOBAR61.2.3.4:/var/FOOBAR7         193524 128878     54816      71% /var/FOOBAR7[root@SERVER ~]# vgs  VG     #PV #LV #SN Attr   VSize   VFree   datvg   1   1   0 wz--n- 204.94g 79.94g[root@SERVER ~]# lvs  LV   VG     Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert  FOO  datvg -wi-ao---- 125.00g                                                    [root@SERVER ~]# lsblkNAME                  MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTsr0                    11:0    1  1024M  0 rom  sda                     8:0    0   128G  0 disk sda1                  8:1    0   500M  0 part /bootsda2                  8:2    0 123.7G  0 part /sda3                  8:3    0   3.8G  0 part [SWAP]sdb                     8:16   0   256G  0 disk sdb1                  8:17   0    32G  0 part /var/foobar1sdb2                  8:18   0    16G  0 part /var/foobar2sdb3                  8:19   0     1G  0 part /var/foobar3sdb4                  8:20   0     1K  0 part sdb5                  8:21   0     1G  0 part /var/foobar4sdb6                  8:22   0     1G  0 part /var/foobar5sdb7                  8:23   0   205G  0 part   datvg-FOO (dm-0) 253:0    0   125G  0 lvm  /var/FOOBAR6[root@SERVER ~]# [root@SERVER ~]# grep ^Red /etc/issueRed Hat Enterprise Linux Server release 6.8 (Santiago)[root@SERVER ~]# Question: how can we migrate it to use LVM for the / and /var, etc. FS too? Just creating the same partition with LVM, copy the files in the old FS? How will be machine boot? /boot could stay in /dev/sda1. 
How to migrate to LVM?
rhel;lvm
You're only using around 7.4GB on / and have 79GB free in LVM so, yes, you can create a new LV for / (and another one for /var) and copy the files from / and /var to them.  I recommend using rsync for the copies.e.g. with the new / and /var mounted as /target and /target/var:rsync  --archive --sparse --one-file-system --delete-during --delete-excluded \  --force --numeric-ids --hard-links / /var /target/optionally use these options too:--human-readable --human-readable --verbose --stats --progressYou can repeat this as often as you like, until you have enough free time to reboot to single user mode and complete the procedure, which is:reboot to single-user modemount / and /boot RW if they're not already RWmount /target and /target/var as aboveone final rsync as abovefor i in proc dev sys dev/pts boot ; do mount -o bind /$i /target/$i ; donechroot /targetedit /etc/fstab and change the device/uuid/labels for / and /varrun update-grubexitfor i in proc sys dev/pts dev boot var /; do umount /target/$i ; donerebootIf everything's working fine after the reboot, you can add /dev/sda2 (the old root partition) to the LVM VG datvg.If you wanted to, you could also create an LV for /boot (mounted as /target/boot) and rsync it etc along with / and /var (except remove boot from the for loop around mount -o bind, you don't want the original /boot bind-mounted over /target/boot).Then create an LV for swap too, and you can add the entire /dev/sda to the LVM VG (delete all partitions on sda, create /dev/sda1 and add that).An Alternative: Clonezilla is great!BTW, another way to do this is to boot with a Clonezilla CD or USB stick, create the LVM partitions, and use cz to clone / and /boot to LVM.  It's been a while since I last did this so I can't remember exactly, but you'll probably still have to mount /target (and /target/var, /target/boot), do the bind-mounts and editing fstab and update-grub.In fact, the for loops above are copied and slightly modified from aliases made by the customisation script for my tftp-bootable clonezilla image...so, it's a pretty good bet that it is needed.It boots up with aliases like this available to the root shell:alias prepare-chroot-target='for i in proc dev sys dev/pts ; do mount -o bind /$i /target/$i ; done'Optional Extra Reading (BTRFS Propaganda)If you're not using LVM because you need to create partitions for VMs (partitions and most other block-devices are faster than .qcow2 or raw files for VMs), I recommend using btrfs  instead of LVM.  It's a lot more flexible (and, IMO, easier to use) than LVM - e.g. it's trivial to grow (or shrink) the allocations for sub-volumes.btrfs has the very useful feature of being able to perform an in-place conversion from ext3 or ext4 to btrfs.Unfortunately, that wiki page now has a warning:Warning: As of 4.0 kernels this feature is not often used or well tested anymore, and there have been some reports that the conversion doesn't work reliably. Feel free to try it out, but make sure you have backups.You could do an in-place conversion of /, migrate your /var/foobar* directories to btrfs, and then add half of sdb to btrfs / (as a RAID-1 mirror) and use the other half for extra storage (also btrfs) or for LVM. It's a shame you don't have a pair of same-sized disks.If you choose not to do the in-place conversion (probably wise), the procedure would be similar to the rsync etc method above, except instead of creating LVM partitions for /target/{,boot,var} you create a btrfs volume for /target and sub-volumes for /target/boot and /target/var.  You'd need a separate swap partition (or forget about disk swap and use zram, the mainline-kernel compressed-RAM block device. or add more RAM so you don't swap.  or both.)But it would be easier to backup your data & config-files, boot an installer CD or USB for your distro, re-install from scratch (after carefully planning the partition layout) and then restore selected parts of your backup (/home, /usr/local, some config files in /etc).More Optional Extra Reading (ZFS Propaganda)Given your mismatched drives, btrfs is probably a better choice for you but I find it impossible to mention btrfs without also mentioning ZFS:If you need to give block-devices to VMs AND want the flexibility of something like btrfs AND don't mind installing a non-mainline kernel module, use ZFS instead of btrfs.It does almost everything that btrfs does (except rebalancing, unfortunately. and you can only add disks to a pool, never remove them or change the layout) plus a whole lot more, and you can create ZVOLs (block-devices using storage from your pool) as well as sub-volumes.Installing ZFS on Debian or Ubuntu and several other distros is easy these days - the distros provide packages (including spl-dkms and zfs-dkms to auto-build the modules for your kernels).    Building the dkms modules takes an annoyingly long time, but otherwise it's as easy and straight-forward as installing any other set of packages.Unfortunately, converting to ZFS wouldn't be as easy as the procedure above.  Converting the rootfs to ZFS is a moderately difficult procedure in itself.  As with btrfs, it would be easier to backup your data and config files etc and rebuild the machine from scratch, using a distro with good ZFS support (Ubuntu is probably the best choice for ZFS on Linux at the moment).Worse, ZFS requires that all partitions or disks in a vdev be the same size (otherwise the vdev will only be as large as the smallest device in it), so you could only add sda and approx half of sdb to the zpool.  the rest of sdb could be btrfs, ext4, xfs, or even LVM (which is kind of pointless since you can make ZVOLs with ZFS).I use ZFS, on Debian (and am very pleased that ZFS has finally made it into the distribution itself so I can use native debian packages).  I've been using it for several years now (since 2011, at least).  Highly recommended.  I just wish it had btrfs's rebalancing feature, that would be very useful in itself AND open up the possibility of removing a vdev from a pool (currently impossible with ZFS) or maybe even conversions from RAID-1 to RAIDZ-1.btrfs's ability to do online conversions from, e.g. RAID-1 to RAID-5 or RAID-6 or RAID-10 is kind of cool, too but not something I'd be likely to use.  I've pretty much given up on all forms of RAID-5/RAID-6, including ZFS' RAID-Z (the performance cost AND the scrub or resync time just isn't worth it, IMO), and I prefer RAID-1 or RAID-10  - I can add as many RAID-1 vdevs to a pool as I like, when I like (which effectively converts RAID-1 to RAID-10, or just adds more mirrored pairs to an existing RAID-10).I make extensive use of ZVOLs for VMs, anyway, so btrfs isn't an option for me.