The HP Smart Array P410 is a fine controller, but you will get poor performance out of it if you don't have the battery-backed or flash-backed cache units installed. The cache makes a tremendous difference in that writes are buffered by the cache memory before being committed to disk. You get the write confirmation to the application without having to incur the latency of the physical disk drives.
Here's a 4GB dd on a similarly-spec'd system (DL380 G7 with 24GB RAM and a p410 with 2 x SAS disks and 1GB Flash-Backed Write Cache). The RAM helps a lot in a test like this, but you get the idea...
[root@xxxx /]# dd if=/dev/zero of=somefile bs=1M count=4096
4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB) copied, 3.70558 seconds, 1.2 GB/s
But realistically, your write performance with two SAS drives in a RAID 1 on that controller with the appropriate cache should be between a sustained 130-170 megabytes/second. A quick iozone test on the above server configuration shows:
[root@xxxx /]# iozone -t1 -i0 -i1 -r1m -s16g
Write
Avg throughput per process = 166499.47 KB/sec
Rewrite:
Avg throughput per process = 177147.75 KB/sec
Since you're using ESXi, you can't run online firmware updates. You should download the Current Smart Update Firmware DVD, burn it to disk and make sure your system is patched to a relatively recent level.
Here are the controller's quickspecs:
http://h18004.www1.hp.com/products/quickspecs/13201_na/13201_na.html
You will want to purchase one of the following, ranging from $350-$600 US:
512MB BBWC
512MB Flash Backed Write Cache
1G Flash Backed Write Cache
To answer your question, the cache solution will help the most. Additional disks won't make much of a difference until you handle the caching situation.
*Note for other users. If you have cache memory on recent HP controllers with up-to-date firmware, there is a write cache override available if you have RAM on the controller but no battery unit. It's slightly risky, but can be an intermediate step in testing what performance would be like on the way to buying a battery or flash unit.
1) The D2600 isn't supported by the P600, although you could, if you really wanted to you could cable two servers to the disk enclosure, though that isn't what this box was designed for I'm almost certain it wouldn't be supported in this configuration - I'm also pretty sure you could easily corrupt your data almost immediately this way too.
2) Yes, the P411 can create a single array of all your disks and then create multiple 2TB logical disks for ESXi. You don't mention what tools you're trying to do this with but I would suggest using the ACU tool on the SmartStart CD if you can, it's a GUI and very easy for beginners to deal with compared to ORCA or CPQONLIN.
3) The D2600 isn't supported by the P600, though the array will happily move between other P411, P212 and P812 controller with ease.
Best Answer
Is this for production use, or test?
if it's for test or non-critical production, check out QNAP or SYNOLGY for a decent NAS box that supports iSCSI.
If you need something for production, checkout Equallogic, they make a nice SAN and are a real bang for the buck in the enterprise.
Edit1:
To answer your question, yes it will support SATA. See the following link: http://h18000.www1.hp.com/products/quickspecs/12247_na/12247_na.HTML
3Gb/s SAS technology delivers high performance and data bandwidth up to 300 MB\s per physical link and contains full compatibility with 1.5G SATA technology.
I will say though, it probably will be cheaper per GB and a lot more flexiable to go with a NAS as you can run more generic drives. You'll also get the added benifits that come with shared storage in vmware such as vmotion, storage migration, etc.