Performance with many logical volumes

[Pippo_9]

Hi all.

I've got a very general question about disks and performance.
I need to add many logical volumes (more than 100) to my VG, which is made-up by 4 phisical volumes. I am not planning to add more disks as well.

In your opinion, how can that influence the overall performances?
What are the aspect/areas I should focus on in order to minimize the possible problems related to the increased number of LV?

Thanks a lot

JPM

[Thierry Poels_1]

Hi,

I'm not concerned about the performance issue.
I'm more wondering why to have so many LVOLs on just four disks.

regards,
Thierry.

[The Real MD]

I think once you have gone down the path of using logical volume manager. it can't be that much more of an overhead no matter how many logical volumes you use. If you think of it as an abstract layer that sits between the O/s and the physical disks then it gives you easier overall management of the disks. If you dont use LVM then there will be some performance benefits but then manageability becomes more difficult.

I hope this helps

Martin.

[Mobeen_1]

JPM,
I don't think it would make any significant influence over the performance that you are experiencing now.

Having said that, i would like to know why is that you need to create somany lvs? This would most certainly be interesting to know :-)

regards
Mobeen

[Pete Randall]

JPM,

We have a similar situation. Our Informix database engine still requires units of disk space (they call them chunks) that are only 2GB in size. We, therefore, have hundreds of little 2GB lvols.

Performance seems fine. As long as you have a valid need for this type of setup, I wouldn't hesitate in implementing it.


Pete

[Sanjay Kumar Suri]

Generally one remember which LV volume contains what? Have more than 100 means, difficult to remember and manage.

Have a look at physical extent size for the space given to each lvol.

In case HFS is used, check the HFS block size (default is 8 KB). You can make it smaller if the stored files are small.

sks

[Bill Hassell · Accepted solution]

Performance is no different between 5 logical volumes and 250 lvols. Each I/O request is sent to the driver with information from the lvol device file, so there aren't any issues to worry about. If these are mounted filesystems, make sure they are VxFS so if an accidental powerfail or reboot occurs, the time to recover will be short. Tools like bdf and df will take a some time to run. vgcreate automatically allows 255 lvols per volume group.

For raw access (no mounted lvols on this VG), I would put some comments in /etc/fstab and be careful about looking for unused disk space since raw volumes will not have any protection from newfs or mkfs.

[Ted Buis]

If you mount the lvols, then consider the appropriate mount options that can have a significant performance impact. Naturally, there is a trade off between performance and reliability.

[Rory R Hammond]

Performance could be affected, if your active lvols are physically apart on your disk platters, causing disk thrashing. If all of your LVOLS are only 2 gigs, large I/O buffers might help your application. The usual issues or disk reads, writes, striping, mirroring and data placement should apply. I think small lvols can impact the mix.


Rory