[linux-lvm] what is the IOPS behavior when partitions of single disk(raid5 backend) are used in an LVM?

[Heinz Mauelshagen]

On 10/12/18 5:46 PM, Sherpa Sherpa wrote:
> Thank you for reply i dont mind if fstab sees partitions.  I read this 
> "To avoid striping performance
>  problems LVM can't tell that two PVs are on the same physical disk, so if
> you create a striped LV then the stripes could be on different 
> partitions  on the same disk resulting in a *decrease* in performance 
> rather than an increase." in the tldp.org <http://tldp.org> but does 
> this apply to disks made from RAID backend ?


If you use partitioning, only create one partition per backing device 
and use it as a PV.
This avoids striping across multiple PVs on the same backing device.

The same config flaw (i.e. use multiple partitions on the same backing 
device as PVs thus potentially
stripe across them) may apply to any backing store allowing for 
partitioning.  So don't do it on SW/HW RAID either.

Heinz


>
> Warm Regards
> Urgen Sherpa
>
>
> On Fri, Oct 12, 2018 at 9:09 PM Heinz Mauelshagen <heinzm at redhat.com 
> <mailto:heinzm at redhat.com>> wrote:
>
>     On 10/11/18 4:31 PM, Emmanuel Gelati wrote:
>>     If you use sdb only for data, you don't have need to use
>>     partition on the disk.
>
>     Though that's true, keeping 1 partition per disk for each LVM PV
>     adds additional
>     'visibility' by tools like fdisk/[cs]fdisk, parted etc. showing
>     the partition type to be 'Liinux LVM'.
>
>     Using the whole disk, blkid or lsblk will provide that information
>     still,
>     e.g. 'blkid --match-token TYPE=LVM2_member'.
>
>     Heinz
>
>>
>>     Il giorno gio 11 ott 2018 alle ore 16:26 David Teigland
>>     <teigland at redhat.com <mailto:teigland at redhat.com>> ha scritto:
>>
>>         On Thu, Oct 11, 2018 at 08:53:07AM +0545, Sherpa Sherpa wrote:
>>         > I have LVM(backed by hardware RAID5) with logical volume
>>         and a volume group
>>         > named "dbstore-lv" and "dbstore-vg" which have sdb1 sdb2
>>         sdb3 created from
>>         > same sdb disk.
>>
>>         > sdb                                8:16   0 19.7T  0 disk
>>         > ├─sdb1                             8:17   0  7.7T  0 part
>>         > │ └─dbstore-lv (dm-1)              252:1    0  9.4T  0 lvm 
>>         /var/db/st01
>>         > ├─sdb2                             8:18   0  1.7T  0 part
>>         > │ └─dbstore-lv (dm-1)              252:1    0  9.4T  0 lvm 
>>         /var/db/st01
>>         > └─sdb3                             8:19   0 10.3T  0 part
>>         >   └─archive--archivedbstore--lv (dm-0)     252:0   0 
>>         10.3T  0 lvm
>>
>>         > I am assuming this is due to disk seek problem as the same
>>         disk partitions
>>         > are used for same LVM or may be its due to saturation of
>>         the disks
>>
>>         You shouldn't add different partitions as different PVs.  If
>>         it's too late
>>         to fix, it might help to create new LV that uses only one of the
>>         partitions, e.g. lvcreate -n lv -L size vg /dev/sdb2, and
>>         then copy your
>>         current LV to the new one.
>>
>>         _______________________________________________
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>>         linux-lvm at redhat.com <mailto:linux-lvm at redhat.com>
>>         https://www.redhat.com/mailman/listinfo/linux-lvm
>>         read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>>
>>
>>
>>     -- 
>>       .~.
>>       /V\
>>      //  \\
>>     /(   )\
>>     ^`~'^
>>
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