storage pool

EMC VNX – Pool LUNs – Thin vs Thick

Pool LUNs

The concept of pool LUNs was already mentioned in that post, but let’s go a little deeper into this topic, just to make sure it’s clear to everyone.

Pool LUN is similar to a traditional LUN. For example, most of the Unisphere operations, or secure CLI command can be used on pool LUNs and traditional LUNs in the same way. Most user-oriented functions works the same way, including LUN migration, local or remote protection or LUN properties information.

The main difference between pool LUN and traditional LUN (RAID LUN) is that traditional LUN is created straight on RAID group, whereas pool LUN is a logical size taken from a pool capacity. Creating LUN on pool gives us extra possibilities, such as using FAST VP (mentioned here) or choosing between thin and thick LUNs (Traditional LUNs are always thick).

 Thin LUNs

The most important difference between thin LUN when compared to traditional LUN and/or thick LUN is that thin LUN presents more storage  to an application than is physically allocated. What does that mean? Let me explain it on an example. Let’s say our linux admin whats a 600GB LUN, we create a thin LUN and present it to the host. If the linux host will consume only about 30% of space, so around 200GB the other 400GB will be still available on storage array. Whereas using traditional/thick LUN would consume the total size of LUN (in my example 600GB) during the LUN creation. Presenting storage that is not physically available avoids under-utilizing the storage system’s capacity.

There is a downside of that solution. Thin LUN typically has lower performance than thick LUNs because all of the user capacity is reserved upon creation of a thick LUN so the mapping requirements for a thick LUN are much less compared to a thin LUN.

Thick LUNs

Thick LUNs are fully allocated upon creating. This means that all 1GB slices will be allocated when the LUN is first created. Thick and thin LUNs can share the same pool. When a thick LUN is created, all of its capacity is reserved and allocated in the pool for use by that LUN. Therefore, a thick LUN will never run out of capacity.

Thick LUN concept has beneficial implications for FAST VP, full allocation within thick pool LUN creation gives better control which tier the slice are written to.

How to create a thin LUN

How to create a thin LUN

Creating a thin LUN in Unisphere is really easy, as you can notice on above printscreen. All you have to do is create a LUN from Pool (not RAID group) and check the box Thin. Yep – that’s it. Having thin LUN created in its properties you can check the LUN Capacity that will gives you two values:

  • User Capacity – the capacity presented to the user, e.g. 600 GB LUN
  • Consumed Capacity – the capacity actually consumed by the user, e.g. 200.1 GB



EMC VNX – Create and modify a storage pool

Once you learn the choice of creating RAID groups, and provisioning storage through LUNs created on RAID vs Storage Pool concept I’m sure you would like to learn how to create a Storage Pool. If you  don’t know what that is, try to read my previous post EMC VNX – RAID groups vs Storage Pools. What you should know? Basically what is a Storage Pool, what is the difference between Homogeneous pool and Heterogeneous pool.

 Create a pool

Creating a pool is quite simple, all you need is to provide a pool name, choose the disks and choose the protection level.

How to create Storage Pool

How to create Storage Pool

Presented figure shows an example of creating a heterogeneous storage pool with name Pool 1, and 3 disk types, with RAID5 for Flash and SAS disks, and RAID6 for NL-SAS disks.

The Scheduled Auto-Tiering box is visible only when the FAST enabler is installed and the Pool radio button is selected. Once you select that option, you will include this storage pool in the auto-tiering schedule. I bet you wonder what is in Advanced box, don’t you?

Create Storage Pool - Advanced Tab

Create Storage Pool – Advanced Tab

The Advanced tab can be selected to configure Thresholds and FAST Cache (of course if the FAST Cache enabler is present when the pool is created). Snapshot section will be available if the Snapshot enabler is loaded. As present in the printscreen, you can schedule automatically deletion of snapshots if the monitored space is above chosen threshold.
A VNX can contain one or many pools. Pools can be as large as the maximum number of drives (expect vault drives and hot spares obviously), lathough there are limits of maximum allowed drive increment. For example with VNX5300 you can create a pool with maximum of 40 drives at a time, but later you can expand this pool with another 40 drives to reach 80 disks in total.

Expanding  Pools

You can easily expand RAID Group, and Pools. To do so, you need to have extra hard drives available. Right-click on the pool, and select Expand.

Expanding Pools

Expanding Pools

The operation is simple. From the Expand Storage Pool window, select the number of disks by using the dropdown arrow. Remember the limit of creating big Pools? Depends on the VNX model you can use limited number of disks at once, for example maximum number of disks with VNX5500 is 80, so if you would like to create a pool from 120 disks you need to first create a pool using 80 disks, and then expand this pool with additional 40 disks.

Shrinking Pools

As with RAID groups, the same is with Pools, you cannot shrink the pool. To release some space you would have to completly destroy the pool.