Here is my brain-teaser of the day. I'm wondering if somebody can
help me figure out how this works and why somebody would set things up
this way. It's possible that I'm just confused, too.
My environment is:
* HP BL460c G6
* Qlogic QMH2462 4Gb FC HBA for HP c-Class BladeSystem
* some kind of SAN (sorry, I'm a contractor here, and new)
* RHEL 5.6
There are two volumes coming from a SAN. I have no idea how things
are configured on that end, such as zoning, etc. (...yet; I have an
email into the storage folks). However, I do know that multipath is
configured like so:
root@server:~:# multipath -ll groundwork
groundwork (360060e8005bb97000000bb970000a2ff) dm-0 HP,OPEN-V*2
[size=60G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=2][active]
\_ 0:0:0:0 sda 8:0 [active][ready]
\_ 1:0:0:0 sde 8:64 [active][ready]
So, if I understand the docs correctly, this means that /dev/sda and
/dev/sde are SAN volumes multipathed together in a round-robbin
active-active configuration, and that /dev/mpath/groundwork and
/dev/mapper/groundwork are the same thing, just logical names created
by different parts of the system, which in turn refer to a combination
of sda and sdb.
Now enter the confusion (unless, of course, I got that first part
wrong and I'm already off track).
There is a volume group called "vgnagios" which is comprised of two
physical volumes (as VGs are wont to be...). But look at the PVs that
make up vgnagios:
root@server:~:# vgdisplay -v vgnagios | grep -C 4 PV\ Name
Using volume group(s) on command line
Finding volume group "vgnagios"
- currently set to 256
Block device 253:10
--- Physical volumes ---
PV Name /dev/mapper/groundworkp1
PV UUID iMiPcg-usxi-hhwu-VFFC-4Z8q-mDoE-Uc7wOQ
PV Status allocatable
Total PE / Free PE 15358 / 0
PV Name /dev/mpath/groundwork
PV UUID wP3Eus-ULCd-mS1m-aMbb-WvEX-St3V-rW1Oz6
PV Status allocatable
Total PE / Free PE 15360 / 7678
What? Am I wrong to be confused here?
RedHat's DM-Multipath docs say *not* to use the /dev/mpath/ path when
creating LVMs, because it is created by udev, and therefore may not be
available when booting. But that is done here. What's far more
confusing to me, however, is that /dev/mpath/groundwork is the same
volume as /dev/mapper/groundwork. /dev/mapper/groundworkp1 is the
PARTITION that takes up the volume /dev/mapper/groundwork. So
essentially, both the full disk and the partition that takes up that
disk are used as if they were two separate disks. LVM allows this?
And more: /dev/mpath/groundwork (or /dev/mapper/groundwork) is 64 GB.
The partition /dev/mapper/groundworkp1 takes up the entire volume, so
it's also 64 GB. But aren't these two the same thing? Even more mind
blowing, is that this seems to work, because LVM is telling me that
the partition and the volume together make up a 120 G VG:
root@server:~:# vgdisplay -v vgnagios | grep VG\ Size
Using volume group(s) on command line
Finding volume group "vgnagios"
VG Size 119.99 GB
Is this some kind of a trick to concatenate the round-robin
multi-pathed I/O back into one large disk, or am I just totally
reading the situation incorrectly?
Here's the output of fdisk, for (hopefully) clarity:
root@server:~:# fdisk -l /dev/mapper/groundwork
Disk /dev/mapper/groundwork: 64.4 GB, 64425492480 bytes
255 heads, 63 sectors/track, 7832 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/mapper/groundworkp1 1 7832 62910508+ 8e Linux LVM
root@server:~:# fdisk -l /dev/mpath/groundwork
Disk /dev/mpath/groundwork: 64.4 GB, 64425492480 bytes
255 heads, 63 sectors/track, 7832 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/mpath/groundworkp1 1 7832 62910508+ 8e Linux LVM
root@server:~:# fdisk -l /dev/mapper/groundworkp1
Disk /dev/mapper/groundworkp1: 64.4 GB, 64420360704 bytes
255 heads, 63 sectors/track, 7831 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/mapper/groundworkp1 doesn't contain a valid partition table
root@server:~:# fdisk -l /dev/sda
Disk /dev/sda: 64.4 GB, 64425492480 bytes
255 heads, 63 sectors/track, 7832 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 7832 62910508+ 8e Linux LVM
root@server:~:# fdisk -l /dev/sde
Disk /dev/sde: 64.4 GB, 64425492480 bytes
255 heads, 63 sectors/track, 7832 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sde1 1 7832 62910508+ 8e Linux LVM
Clearly Confused,
Chris
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