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RAID 5 with SCSI or RAID 10 with SATA

We are in the process of purchasing a new server for our department and I'm doing the specs.  I am the point where I want to get the best performance but maintain price.

In my research, everybody seems to take RAID 10 over RAID 5 for its capability of both performance AND reliability.  I am interested to get feedback from people that have worked with SATA and SCSI drives in RAID configurations.

My question is:  IF you had choice between
a) Three 73GB SCSI 10K RPM in RAID 5 config, or
b) Four 80GB SATA 10K RPM in RAID 10

What would you get?  They both spec-out very close in price.  Is the SCSI performance that much better to for-go the RAID 10 with SATAs?

2 Solutions
In ether case you limit on speed will be the PCI Buss - PCI-Express will change this.  
niltong: two reasons speak for SCSI:
 -> high simultaneous i/o capability
 -> designed to run 24/7
Further, IMHO, a RAID 5 is more flexible (I assume you use hardware raid) and realized with SCSI just works once configured. With SATA raid, check all the problems people have in here.
Just on the money side, something seems to be amiss: SCSI RAID will set you back by something like 500...1000 usd, while a SATA controller can be had for the price of a dinner for two.

Duncan MeyersCommented:
You really need to try both and test, test, test.

Assuming both RAID controllers are *real* hardware RAID (not psuedo-hardware RAID (ie the OS is actually doing the hard work for the controller) as is the case with many of the SATA controllers) then the SATA RAID 10 is likely to outperform the RAID 5 array on write operations - depending on application. To explain: RAID 5 suffers from the RAID 5 write penalty. That is, for every write operation that a RAID 5 operation that a RAID 5 array performs, it must actually do 4 disk I/O's - Read old parity, read old data, XOR old parity and old data to get a partial product, XOR the partial product and new data to get the new parity, write the new parity and (finally!) write the new data. This is why RAID 5 write performance is so poor compared to RAID 10. RAID 10 requires no parity calclulation - just write the data out to mirrors. Easy! Write caching will help to a degree.

So although SATA disks are considerably slower than SCSI (expect approximately one half to one quarter of the performance of the SCSI discs) the performance differential will most likely be offset by the different RAID types. If your budget can stretch to SCSI RAID 10 then you really are on to a winner.  Note that the more random the read/write activity (ie Exchange, File/Print, SQL database etc) the better SCSI disks will perform compared to the SATA.

Like al-hasan, I too have reservations about the reliablity of SATA discs and in particular the SATA "RAID" controllers, and that would be enough to swing me to a SCSI array.

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What are your requirments?
Is performance more important that reliability?
Whaty type of performance do you need?

Are you using on-board (built in) RAID controllers or are you going to buy controllers?  If you are using on-board controllers you need to know if they are connected to the PCI bus or PCI-X bus, if your motherboard has PCI-X.

If you are buying controllers are they PCI or PCI-X, what type of performance do they claim to have.

In either case, if it is PCI, do you have other devices on the PCI bus?  The PCI bus will run as the  speed of the slowest device on the bus.  Guess what happens to SATA on a PCI bus that is running at 33 Mhz because of a old PCI card that only supports 33 Mhz?  Its kinda slow and will cause I/O problems with the old PCI card as the SATA I/O will saturate the bus.

Of couse I am assuming that these are hot pluggable, you don't want to have a second drive fail while you are waiting for the weekend to schedule down time to replace the first drive that failed.

You may also want to think about future growth.  With RAID 10 as you need to grow, you will need to buy two drives everytime.  With RAID-5, you can add 1 drive at a time.

If performance is a real issue then, as meyersd stated, the best thing you could do is setup both, test, test and test, to see which give you what you want.
SATA is just commercial grade kit, its nice for desktops and all that but its not industrial grade kit, its not really designed to run 24x7 flat out or for long term intensive action.  SCSI on the other hand has been around a long time and IS designed to run flat out 24x7x365, its proven and reliable.  In my 15 years of experience most servers run with SCSI and will not be switched off for years at a time and few develop faults.

WhitePhantomIT ProfessionalCommented:
I agree with simonenticott.  Go with the SCSI RAID-5.  It will likely perform a little better, but the bigger reason is the reliability.  A SATA hard drive might last 7 years, or it could die in 6 months.  There's just no way to know.  SCSI drives are made to last for a LONG time, and they have great performance.


Unless you use WD Raptor SATA drives, the HDA's on most modern SCSI drives are superior in terms of relability.

On PCI X bus slots Ultra 320 SCSI (320mb) is more than twice as fast as SATA (150mb)

The fastest SCSI drives are 15000 RPM,SATA is still at 10000.

Most SCSI RAID controllers have a battery backup module,SATA controllers don't.

If you lose power or have to power the system down in an emergency ,the data on the cache module will be lost on a controller without a battery backup.(Can you say data corruption?)  

If you have the budget, SCSI is a no brainer.
Duncan MeyersCommented:
I think al-hasan and I pretty well nailed this one :-)
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