We have an EMC Celerra NS-120, which is a NAS. And I was curious to the fact that when I add two luns according the 'celerra' way I end up with a User type of Storage pool in de celerra manager.
So I have a couple of Luns added as clarata_archive which have as type "System". Now I added two more and defined a Pool name but ended up with an "User" type. By the way... when I do nothing after a rescan the extra space is added to the clarata_archive Pool which has auto extend enabled.
I'll be able to make a new Pool with this extra space, but again ended up with an User based type.
I was curious about the two differences. So after some investigation and help from a guy named Rainer which is an EMC Employee from Germany we came up with the following.
It seems to be when you define a pool it's always an User defined pool. System pools are automatically created when matching devices are recognized and there is only one System pool per device type. The differences basically are:
- System pools automatically "expand" when there are new devices
- System pools automatically try to stripe to distribute the IO between LUN's - User defined pools never do
When you add LUN's you don't have to define any pools - the new LUNs will automatically become eligible for use in their matching system pool.
In most cases you would only use User defined pools if:
- I understand and are willing to do the striping myself
- I need to segregate performance and/or capacity between different customers, departments or apps
- I have enough disks for it to make sense
Especially the "no striping" isn't widely understood - there is more harm done by customers not realizing this and working on only one LUN then by just using System pools
The reason that a LUN is not fully pulled into a System pool immediately is to give customers a chance to use them with MVM or User defined pools. When a System pool needs more space then whats available on the already partially used LUNs (stripes) then it goes out and "consumes" eligable unused LUN's of the matching type
The main reason for this article was, I had 10 'old' 320GB (S)ATA disks in a DAE. I did not add them to the system Pool with the other types of disks I used, which are 750 GB (S)ATA. I thought it has an adverse effects on performance. And the second reason was, that I could manage this 320 GB disk because I used a separated Pool. So I did a RAID 5 (8+1) and add them in a 'self made' User Pool. Slice Pool volumes are by Default enabled.
I hope this make sense! Feel free to comment.