LV creation with the distributed option

[Sana Chaarani]

Hi,

I have a VG with only ONE PV in it.
I need to create three LVs in this VG. Can I use
the distributed option with lvcreate ?? even if
I have only one disk in the vg ??

Thanks,

Sana

[Patrick Wallek · Accepted solution]

No you can't use the Distributed (-D) option to lvcreate with just one disk in the VG. There would really be no point in doing this with just 1 disk. You wouldn't gain anything.

Just create your LVs normally.

[S.K. Chan]

You can't. From the man pages of lvcreate ..
"When the distributed allocation policy is turned on, only one free extent is allocated from the first available physical volume.The next free extent is allocated from the next available physical volume. Allocation of free extents proceeds in round-robin order on the list of available physical volumes."

[John Poff]

Hi,

I've never tried it with just one PV, so I don't know for sure, but my guess is that lvcreate will complain about just having one PV in the VG. The distributed option will allocate extents in round robin order from the available PVs. Since you only have one PV, it really is a moot point. If you are planning to add more PVs to the VG later, you can always try turning on the distributed option when you have the new PVs in place. What problem are you trying to solve by using the distributed option?

JP

[Sana Chaarani]

Thanks for all your responses. I know that it doesn't give anything to do that and that it is impossible to do it while there is only one PV in the VG. There is one person over here at work that is saying otherwise and he is being bad at it and I wanted a confirmation from you gurus!

Another quick one : what will the distributed option give me, performance wise when my disks are already on an EMC frame (i.e : data is already hardware striped, mirrored...)

Thanks again, I appreciate your fast reponses

Sana

[Patrick Wallek]

If your data is already striped and mirrored on an EMC array, then distributed striping probably will not benefit you much.

Distributed striping was primarily designed with the JBOD (Just a Bunch Of Disks) architecture in mind. Specifically it allows you to have stripes (of a sort) and mirrors at the same time since MirrorDisk does not allow TRUE striping and mirroring.

While it does work great in a JBOD environment, I probably wouldn't use it with a disk array.

[Michael Steele_2]

Regarding "...what will the distributed option give me; performance wise when my disks are already on an EMC frame..."

Most EMC array's out perform other disk array and I haven't seen or heard of many (* if any *) EMC users implementing striping to get better performance or to load balance.

Striping is used for I/O performance enhancement and implemented over multiple disks and controllers and more common on other disk arrays, but not really the EMC family.

[Sana Chaarani]

Thanks to everyone. When I forwarded your messages to this guy here this was
his response :

"It will further spread your data across the frame providing better
utilization of spindles, back end controllers and cache."

I don't really know what else to do to prevent from using the distributed option when creating lvs with disks in EMC.

Thanks anyways for all your
help, you have been great !

Sana

[Steve Valvasori]

You already mention that your EMC frame is hardware striped, which I assume you are referring to EMC's meta volumes. If this is the case you are already going across many spindles (depending on your meta size). I can't see any gain in putting software on top of this config.
If not using meta's - they will give you the performance you want, and should keep your friend happy. Is he a dba by chance - they always know how to config everything. :)