[CentOS] LVM hatred, was Re: /boot on a separate partition?

[Lamar Owen]

On 06/25/2015 01:20 PM, Chris Adams wrote:
> ...It's basically a way to assemble one arbitrary set of block devices 
> and then divide them into another arbitrary set of block devices, but 
> now separate from the underlying physical structure. 

> Regular partitions have various limitations (one big one on Linux 
> being that modifying the partition table of a disk with in-use 
> partitions is a PITA and most often requires a reboot), and LVM 
> abstracts away some of them. .... 

I'll give an example.  I have a backup server, and for various reasons 
(hardlinks primarily) all the data needs to be in a single filesystem.  
However, this is running on an older VMware ESX server, and those have a 
2TB LUN size limit.  So, even though my EMC Clariion arrays can deal 
with 10TB LUNs without issue, the VMware ESX and all of its guests 
cannot.  So, I have a lot of RDMs for the guests.  The backup server's 
LVM looks like this:
[root at backup-rdc ~]# pvscan
   PV /dev/sdd1   VG vg_opt       lvm2 [1.95 TB / 0    free]
   PV /dev/sde1   VG vg_opt       lvm2 [1.95 TB / 0    free]
   PV /dev/sdf1   VG vg_opt       lvm2 [1.95 TB / 0    free]
   PV /dev/sda2   VG VolGroup00   lvm2 [39.88 GB / 0    free]
   PV /dev/sdg1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdh1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdi1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdj1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdk1   VG bak-rdc    lvm2 [1.47 TB / 0    free]
   PV /dev/sdl1   VG bak-rdc    lvm2 [1.47 TB / 0    free]
   PV /dev/sdm1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdn1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdo1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdp1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdq1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdr1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdb1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   PV /dev/sdc1   VG bak-rdc    lvm2 [1.95 TB / 0    free]
   Total: 18 [32.27 TB] / in use: 18 [32.27 TB] / in no VG: 0 [0   ]
[root at backup-rdc ~]# lvscan
   ACTIVE            '/dev/vg_opt/lv_backups' [5.86 TB] inherit
   ACTIVE            '/dev/VolGroup00/LogVol00' [37.91 GB] inherit
   ACTIVE            '/dev/VolGroup00/LogVol01' [1.97 GB] inherit
   ACTIVE            '/dev/bak-rdc/cx3-80' [26.37 TB] inherit
[root at backup-rdc ~]#

It's just beautiful the way I can take another 1.95 TB LUN, add it to 
the volume group, expand the logical volume, and then expand the 
underlying filesystem (XFS) and just dynamically add storage.  Being on 
an EMC Clariion foundation, I don't have to worry about the RAID, 
either, as the RAID6 and hotsparing is done by the array.  SAN and LVM 
were made for each other.   And, if and when I either migrate the guest 
over to physical hardware on the same SAN or migrate to some other 
virtualization, I can use LVM's tools to migrate from all those 1.95 and 
1.47 TB LUNs over to a few larger LUNs and blow away the smaller LUNs 
while the system is online.  And the EMC Clariion FLARE OE software 
allows me great flexibility in moving LUNs around in the array for 
performance and other reasons.