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richinsc 01-29-2009 08:26 AM

Disk Storage Systems - SAS,SCSI vs SATA
Morning all,

I am looking to start building a VM server here soon to allow me to play around with more linux distros at the same time and maybe even use it for a production VM Environment. My question is in regards to the data transfer speeds and disk speeds of SATA, SCSI and/or SAS.

While SCSI and SAS can't match the storage size of current sata disks, I have read that the faster the HDD's in for VM Hosting the better. I can't remember where I read this but I do remember this.

If I were to build the VM Server with SCSI or SAS disks. I would be able to get drives that run at 15K RPM. At the risk of lower storage this might be more beneficial. I think that maybe having 15K drives in a 3.0TB Array is better then 7200 RPM ARRAY at greater than 3.0TB.

With the advent of 1.5 TB Disks the array could be built quite large but what good is a large array if the access time cripples your environment.

While the bus transfer speed of SAS and SATA can reach speeds of 3.0Gb/s, wouldn't it be better to have faster disks rather then a faster bus?

Also the added benefit of SAS and SCSI is that you surpass the limitation on number of devices per channel which can decrease the cost of raid system a bit, especially if you have to purchase a raid controller. For SCSI you would only need to have a RAID controller with one 68-pin Ultra320 SCSI port but with SATA you would have to purchase a 16-port SATA controller to get the same device ratio to SCSI.

I would think so but I want to know everyone's opinion. If you were to build a VM Server, what kind of disks would you use?

Would you build it for capacity or speed?

Would you use 1.5TB 7200RPM disks or would you used 15K 300GB Ultra320 68-PIN SCSI or a 15K 300GB SAS?

In all fairness I understand that many may not be able to afford such expensive SCSI disks as the ones I have seen cost about $300 for SCSI disks while the SATA disk are about $180. Pretend that money isn't the issue, which would use for your disk storage system?

If this were a file server, I wouldn't be asking this question as I would go for capacity rather then speed, but being it is a VM Server, I am left unsure of my decision.

lazlow 01-30-2009 06:10 PM

If you want fast then memory based drives are about it for now. Here is one that has been around for a while:

Electro 01-30-2009 07:07 PM

The terabyte hard drives are mainly designed for secondary hard drives or as back ups. Also their latency is poor. Seagate is not good at this range. Western Digital or Hitachi are better.

I suggest use VelociRaptor hard drives connected to hardware RAID that can do RAID-3 or RAID-4. Using RAID-5 or RAID-6 could be used instead but performance will not be as good I think. Another route to take is using computer memory as disks drives. Their latency and throughput provides 1000 times better performance factor than anything. They are more expensive for the performance that you gain. I recommend look for devices that are compatible using ECC memory and does not use proprietary battery sizes.

SATA 3 Gb controllers can provide more ports than what they came with by using a port multiplier. Most port multipliers are five. Actually a 12 port SATA 3.0 Gb controller can be a 60 port controller or 60 hard drives can be connected when a port multiplier is used. The difference between a SCSI and SATA is that SATA is a point to point interface that does not need any termination. SATA is more cost effective even though there are a lot of cables involved and the loyalties to use it is not expensive.

richinsc 01-31-2009 06:09 PM

Any drawbacks or disadvantages to using a multiplier?

Electro 02-01-2009 10:58 PM

I think a SATA multiplier can penalize throughput because it uses one cable. This cable can not handle any more than 300 MB per second or about 60 megabytes per second for each hard drive. Technically the port can not handle any faster than 300 megabytes per second. Rarely hard drives have a throughput of about 60 megabytes per second. If the SATA 3 Gb controller is design right, it should provide over 3 gigabytes per second.

From looking at the SATA connector, it looks flimsy and loose, so you may have to improvise something to make sure the cable connects to the port while routing the cables. Some people said that they use a hot glue gun.

I have not yet used SATA and I have not yet upgraded to SATA.

lazlow 02-01-2009 11:18 PM

A lot of the newer (regular) hard drives are rated(for what that is worth) at 100 megabytes per second(MB/s). The newer dram drives are hitting 250MB/s.

Electro I think you are mixing MB and Mb. Sata 3Gb is rated at 3000 Mbit/s or 300MB/s Megabytes per second:

When you buy the cables make sure it is the spring loaded ones. While MOST of the regular cables do not have an issue certain combination of drive brand and cable brand do not mix well. Some are loose as described, but some are also too tight and end in a broken connector on the drive. The spring loaded ones are physically loose until the spring clamps down.

Electro 02-02-2009 02:49 AM


Originally Posted by lazlow (Post 3428701)
Electro I think you are mixing MB and Mb. Sata 3Gb is rated at 3000 Mbit/s or 300MB/s Megabytes per second:

When I state B, I mean bytes.

When I state b, I mean bits.

I am not getting mixed up and my posts completely states that.

When I state "...over 3 gigabytes per second..." I mean to the bus or PCIe for an example.

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