You can. You will get the performance that three drives can give you. Not really sure about "Is the perfomance will be degraded" question since you only three drives in the pool and you didn't specify what you are looking for from the performance perspective.

I would suspect the performance would not be anything to write home about, which raises the question of why you would bother putting the SAS drives in there. I'm making an assumption here that you are calling the "big" drives SATA (which are actually NL-SAS) as opposed to EFDs (which actually are SATA - or at least report themselves that way). If you meant EFDs when you said SATA then my question would be: "Why bother with EFDs instead of just sticking with SAS".

If you really do need some level of performance I would think you would stick with either at least 4 drives of each type and use RAID 1/0 or multiples of 5 drives of each type and use RAID5.

It might help if you could provide a bit more info on what you are planning on doing with this pool.

This is my idea:

3x600GB SAS Disk+ 3x2TB NL-SAS disk in one  RAID 5 pool with 4x100GB FLASH disk for FAST Cache.

In this way i will have another 3 SAS and 3 NL-SAS for pool extend in the future.

I can create this pool with 4SAS+4NL-SAS but i don't have the same number of diks(4SAS+4NL-SAS) for future extend of pool.

OK, so if I'm understanding right you actually have 6 each of the 300GB and 2TB drives. I don't know what else you might have in the array already, but instead of going 3 & 3 with RAID 5 right now (with the potential to extend it later)... I would go 5 & 5 right now. The other drives can be used as Hot Spares if  you need them or added along with additional drives in the future. If you think you might get to the point of expanding later then you get more efficient use out of RAID5(4+1) that (3+1). For one thing you will end up with the same usable capacity with less drives if you go (4+1).

If you have a total of 6 drives (3 SAS and 3 NL-SAS) then there are two options:

1. Stick with traditional RAID Groups. This doesn't allow you to use FAST Tiering and two different RAID Groups will be created

2. Create a pool with 3xSAS and 3xNL-SAS drives. Flare will create two private RAID Groups which will be 2+1 RAID 5. You can use FAST Tiering

Good idea to use the EFD drives as FAST Cache.

A real issue here, is that a six drive pool, half of them NL-SAS has a small amount of native IOPs.  In addition, a large proportion of the pool's capacity is taken-up by parity and is unusable for user data.

In another post, the OP has been encouraged to read EMC Best Practices for Performance and Availability: Common Platform and OE Block 31.5, which is available on Powerlink.  I guess, he has not gotten around to that.  

VNX Best Practices does not recommend creating pools with RAID groups smaller than the default 4+1, 4+4 drive numbers.  It also does not recommend creating pools with less than five default RAID groups.  This results in a de facto recommendation of creating pools with 20-drives or more for best performance, capacity utilization, and availability.

A good place for the OP to start would be to find out how many IOPS his LUNs require, and then use the information found in the 'Storage System Sizing and Performance Planning' section of VNX Best Practices to determine, if a 6-drive, two tiered pool will meet his needs.

Absolutely agree. The ideal thing is to buy additional drives but if that's not possible those are his only options at this point.

Thank you.

I know about best practices raid 5 4+1, raid1/0 4+4, raid 6 6+2.

I know about best practices to expand pool with same number of disk.

I agree with you, but my problem is that i don't have enough disk to follow best practices

After migrate my info to my EVA array(now i have small pool on VNX and will migrate to EVA for tests and will bring back to VNX after 5 days) I will have 7 NL-SAS Disk, 11 SAS, and 5 Flash Disk on my VNX 5300.

Will use 3 HotSpare- 1 NL SAS,1SAS,1Flash and will have 6 NL-SAS, 10 SAS and 4 Flash.

I will use 4 Flash disks for FAST Cache and i will have 10 SAS and 6 NL SAS disks and 4 system disk.

I think to use disk 3 and 4 for raid group 1 and to use them for Clone private luns and Snapshot private Lun and Mirror wiev, but this is not best practices.

So according to best practices i can create one pool raid 5 from 5 SAS Disk+ 5 NL SAS disk with 4 Flash disk for Cache and will have 5 SAS disk and from them i can create another pool or raid group and will use for some private luns and other purposes.

I know how to calculate IOPS for Luns according to number of disk raid cache...(i've set up 3 EVA arrays and 5 MSA arrays), but i don't have info what will work on these lun's.

My bos just told me:

"I want bigger capacity and best perfomance"

I asked what will work- database,exchange?

He told me i don't know this is not your business just give what i want this is hardware there...

When you 'return' your data to the VNX, delete the original pool.  Do not expand that pool with the additional 'new' drives.

That pool was created with the smaller private RAID groups.  Expanding the original pool will 'keep' that (2+1) RAID group in the pool. This RAID group will have lower performance than the RAID group(s) created from the newly added drives.  It will also contribute to 'uneven' performance, having RAID groups made-up from different numbers of drives. 

To maximize your IOPS, have consistent performance, and maximize your capacity utilization, you want the default RAID group's larger number of drives in all of the pool's private RAID groups, that is (4+1)s'. Deleting the pool and re-creating it will give you 2x (4+1)s from your 10 SAS drives.

If you are comparing the VNX's performance with the EVA, note that the number of drives used in the VNX's pool was 'fewer than is ideal'.

HTH