Re: Overhead of LVM (Re: Upgrade Problem)

[Reco <recoverym4n@enotuniq.net>]

	Hi.

On Fri, Jan 04, 2019 at 11:03:48PM -0500, rhkramer@gmail.com wrote:
> On Friday, January 04, 2019 08:23:59 AM Reco wrote:
> > # pvs
> >   PV             VG     Fmt  Attr PSize  PFree
> >   /dev/md10      nas    lvm2 a--  14.55t 10.43t
> > 
> > # hdparm -Tt /dev/md10
> > /dev/md10:
> >  Timing cached reads:   1224 MB in  2.00 seconds = 612.05 MB/sec
> >  Timing buffered disk reads: 1210 MB in  3.00 seconds = 403.28 MB/sec
> > 
> > # hdparm -Tt /dev/nas/root
> > /dev/nas/root:
> >  Timing cached reads:   1224 MB in  2.00 seconds = 611.55 MB/sec
> >  Timing buffered disk reads: 1154 MB in  3.00 seconds = 384.42 MB/sec
> > 
> > I see a difference, but I's something I can live with.
> 
> Thanks very much!
> 
>  I need to learn more about pvs (I did google the man page), but I assume the 
> /dev/md10 and /dev/nas/root are two ways of referring to the same partition, 
> one within LVM, and one not?

md10 is a RAID10 consisting of 4 HDDs. No partitions, just
straightforward block devices.
/dev/nas/root is a logical volume (/ filesystem in this case), it
resides on this md10, as 'pvs' output shows.
Storage hierarchy is a as follows (lsblk, same for sdb, sdc and sdd):

NAME                 MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda                    8:0    1  7.3T  0 disk
└─md0                  9:0    0  7.3T  0 raid1
  └─md10               9:10   0 14.6T  0 raid0
    ├─nas-root       254:2    0 15.3G  0 lvm   /

So yes, both md10 and /dev/nas/root are two ways of referring to the
same set of bytes on disks.

Reco