Working with extent

[benutzername]

Hi,

i was just crawling here in the Community about extents. I found lots of older post where all sayd: "Do not use extents, create a second Datastore".

I just found the Document VMFS Best Practices http://communities.vmware.com/docs/DOC-9276

Here I found the following conclusion: Page15 "It’s generally best to begin with a single LUN in a VMFS volume. To increase the size of that resource pool, you can add additional capacity by adding a new extent to the VMFS volume. This will add the additional LUN to an existing VMFS volume"

We are using VC 2.5 and ESX 3.5 machines.

So, what are u thinking about extents? Is it a high risk to use them? In our environment with shared customers, the VM´s of a customer are only allowed in a datastore that is reserved exclusively for that single customer. So it would be very unuseful to create a lot of datastores or having a big datastore unused for a long time.

Do you use extents?

Has anybody experience of loosing a LUN from a extent? What happened. Was the complete Datastore unusable, or only the parts of the extendt that were lost?

[AntonVZhbankov]

I used datastores with multiple extents. One of them was made from 17 equal extents 144GB each - no problems at all.

As for my opinion, if you use regular backups and place all the extents on a single array, it's unlikely you'll get serious problems. Just do not use LUNs from different arrays in one datastore.

In your case I think NFS datastore is the best solution in the meaning of disk usage.

---

http://blog.vadmin.ru

[Lightbulb]

The problem with extents is a SAN management problem. You are essentially putting all your eggs in one basket, which is OK as long as you watch that basket. We do not use them but if you design properly, have good backups and good monitoring you can use them. That is alot of "ifs" in most organizations, you work in a shared environment which has special needs (I work for a hosting provider) so do what is best for you.

I find in a shared environment that SAN management is more difficult so you might want to consider avoiding extents and just adding new datastores from your shared storage poll when needed.

[Drewsky]

In my environment, we extensively use extents and have never experienced issues related to the use of them. ESX 2.x made it a little hazardous as the metadata was in only one place. ESX 3.x VMFS has metadata on every LUN and does only LUN-level locks instead of VMFS-level locks. My experience/advice:

1) Watch LUN sizing compared to average # of guests per LUN. Too dense, and you can start getting reservation conflicts (I'm told...I've hit guest density of 15 guests on a single LUN across 5 hosts with no RC's). I think the convention is to keep guests/LUN <10. Note this isn't per DATASTORE, this is per LUN.

2) Make sure you get your block size right at the outset, as the only way to fix it later is to migrate everything away and destroy the datastore and on a big datastore, this can take a very long time.

3) Keep datastore extents on the same array (echoing advice above)

4) Pay close attention to storage balancing. IE, on a Clariion, set the paths to alternate across HBA's, and have LUN ownership alternate between SP's.

5) Also pay attention to storage backend: work to design the meta's so you dont have a single disk allocated multiple times on a datastore-this can lead to IO overloads on a disk. Since datastores are concatenated LUNS, the same array/disk across LUNs in a datastore is ok.

6) Regularly watch your logs and performance to make sure you dont have a "hot LUN", meaning higher IOPS than others. If so, migrate machines to other datastores to even the load out.

Hope that's helpful!

[Texiwill]

Hello,

Also remember that once an extent is allocated, you can not delete just the extent, you have to delete the entire VMFS and all its extents.

Not using extents is more a 'management concern' than it is a VMFS/ESX concern.

Best regards,
Edward L. Haletky
VMware Communities User Moderator, VMware vExpert 2009
====
Author of the book 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', Copyright 2008 Pearson Education.
Blue Gears and SearchVMware Pro Blogs -- Top Virtualization Security Links -- Virtualization Security Round Table Podcast

[benutzername]

Hi,

because our SAN Team can´t give use the guarantee that they will be able to give use a LUN for an extents on the same array, we just decided that we won´t use extends at all. Thanks for the good Discussion that made our decision easier.

By the way: I talked also to a an VMware Senior System Engineer. He also recommended not to use extents.