Can different drive speeds and sizes be used in a hardware RAID configuration w/o affecting performance?

Specifically, I have a RAID 1 array configuration with two 500gb 7200rpm SATA drives mirrored as logical drive 1 (a) and two of the same mirrored as logical drive 2 (b). I'd like to add two 1tb 5400rpm drives in the same mirrored fashion as logical drive 3 (c). These drives will only serve as file storage with occasional but necessary access, and therefore, space is more important than speed.

In researching whether this configuration is doable, I've been told and have read that the array will only see the smallest drive size and slowest speed. However, my understanding is that as long as the pairs themselves aren't mixed (and in this case, they aren't) that the array should view and use all drives at their actual speed and size.

I'd like to be sure before purchasing the additional drives. Insight anyone?

You will be constrained by the slowest speed and smallest size for disks in the same RAID group. Since you're talking about having three separate mirrors, this doesn't apply to you. Each mirrored pair is a separate array/group/whatever-you-want-to-call-it. You'll end up with three logical volumes, two with 500GB and the other with 1TB with no performance hit to the 7200 RPM disks.

If the disks are grouped together in one array, your capacity and performance will be determined by the smallest, slowest disk.

Here's an example from a setup that started with 4 x 750GB disks in a RAID 1+0. As disks failed, HP sent 1TB disks as replacements, so now the array is comprised of 2 x 750GB disks and 2 x 1TB disks. This is what HP does, so the practice is definitely supported. While the physical layout shows:

The controller treats it like 4 x 750GB disks:

Smart Array P410i in Slot 0 (Embedded)    (sn: 50014380054E9DA0)

   array A (SAS, Unused Space: 1188170  MB)

      logicaldrive 1 (72.0 GB, RAID 1+0, OK)
      logicaldrive 2 (400.0 GB, RAID 1+0, OK)

      physicaldrive 0:0 (box 0:bay 0, SAS, 750 GB, OK)
      physicaldrive 0:0 (box 0:bay 0, SAS, 750 GB, OK)
      physicaldrive 0:0 (box 0:bay 0, SAS, 750 GB, OK)
      physicaldrive 0:0 (box 0:bay 0, SAS, 750 GB, OK)

But in your specific situation

Your Smart Array controller will see the new array as another pair of mirrored drives, independent of the existing two logical drives. This is fine. There may be some considerations depending on the controller and drive layout (e.g. whether a SAS expander is in the mix or not), but that only affects the ProLiant DL180 G5/G6 servers for now.

Which HP server model, generation and controller are you using?

For example, here is a setup with a pair of 300GB SAS disks and four SATA solid-state disks in a RAID 1+0. It's a set of mixed drive types in the same enclosure, but grouped with like-members. There's no impact on perfomance:

[root@abc ~]# hpacucli ctrl all show config

Smart Array P410i in Slot 0 (Embedded)    (sn: 500143801664FE50)

   array A (SAS, Unused Space: 0  MB)

      logicaldrive 1 (279.4 GB, RAID 1, OK)

      physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SAS, 300 GB, OK)
      physicaldrive 2I:1:5 (port 2I:box 1:bay 5, SAS, 300 GB, OK)

   array B (Solid State SATA, Unused Space: 0  MB)

      logicaldrive 2 (447.1 GB, RAID 1+0, OK)

      physicaldrive 1I:1:3 (port 1I:box 1:bay 3, Solid State SATA, 240.0 GB, OK)
      physicaldrive 1I:1:4 (port 1I:box 1:bay 4, Solid State SATA, 240.0 GB, OK)
      physicaldrive 2I:1:7 (port 2I:box 1:bay 7, Solid State SATA, 240.0 GB, OK)
      physicaldrive 2I:1:8 (port 2I:box 1:bay 8, Solid State SATA, 240.0 GB, OK)