Thursday, July 17, 2008

RAID Alignment and VMware ESX 3i 3.5

Make sure to create your datastores from within the Virtual Infrastructure Client GUI, don't take what the ESX install CD creates (this means deleting any data stores created by the install CD).

It's because the Install CD creates the partitions on the standard Sector 63 starting position (31.5k into the drive. This isn't a proper multiple of any common RAID stripe size (2k, 4k, 8k, 16k, 32k, 64k, etc) so the partition ends up being out of alignment with the RAID stripe elements, and READ/WRITE performance will suffer.

This is all documented in a number of VMware documents.

Creating the partitions in VCI makes them start on sector 128 (64k into the drive), which lines up properly.

Questions on what RAID stripe size and what Windows Allocation Unit size to use will be in another post.

Wednesday, July 9, 2008

Quick and Dirty - LSI 3041E (1064E) SAS PCIe Card vs Dell CERC SATA 1.5/6ch Card


I have a customer with a Dell Poweredge 1800. It comes with the Dell CERC SATA 1.5/6ch RAID controller. It was configured for RAID5 operation.

SAP was killing their server with a lot of SQL requests - They have grown quite a bit in the last year, and their simple little fileserver was now hosting exchange, SQL, and a lot of users.

All my testing pointed to the drives not being able to provide the CPU with enough data. Memory and CPU were always low, while the disk queue was through the ceiling all the time.

I've been doing a lot of SAS drive work lately, and I sold them on a basic RAID mirror with 2 146 gig SAS drives (Seagate Cheetah 15k, 32 meg cache).

I selected the LSI 3041E (1064E based) 4 port SAS card because it was only $190 or so, and I've had good luck with LSI products lately (and less with Adaptec).

After an install and format, I ran a very simple IOmeter tests (the default test set) for 5 minutes. This is what I got;


Operations/Sec 62.98 398.28
Megabytes/Sec .12 .78
Average Response Time(ms) 15.88 2.50

As you can see, the SAS drive array beats the old SATA drive hands down in every test. It's 6.4 times better - 6.4 times more operations per second (reads, writes), it can move 6.4 times more data in a second, and it takes 6.4 times less time to respond to an operation.

Of course, this is a nasty test - It's comparing 7200 RPM 1.5 Gb/sec SATA drives to SAS 15000 RM 3G/sec, it's comparing RAID5 to RAID Mirror. Everything about it is wrong.

But it also shows how much faster life can be for $800 in parts i you have a similar setup. :-)

I'm waiting for real world user comments, and we'll do some SAP end user tests on a number of slow queries shortly to see if the this additional speed is actually being taken advantage of.