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SATA II vs SAS

Posted on 2006-11-01
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SATA II 7.2K RPM vs. SAS 10K RPM

I know that SAS is probably faster but how much faster is it than SATA II ?

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Question by:Kurt4949
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I read that they are both 3Gb/s.  So data transfer is the same speed but the faster RPM will provide better seek time?
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"... So data transfer is the same speed but the faster RPM will provide better seek time? " ==> NO !!!!!!!   Transfer will NOT be at the same speed !!  (except for the very small % of transfers from the disk's buffer)

Seek time and sustained data rate are two DIFFERENT parameters.   It is true that in general higher rpm drives have better access times, because the head positioning mechanisms are typically faster (a simple manufacturing choice of the drive makers).  So ... the seek time is almost certainly better with the 10K drive.

In addition ... the transfer rate is nearly 50% faster for the 10,000 rpm drive => the sustained transfer rate of a drive is almost entirely a function of the rpm;  the faster it spins, the higher the sustained transfer rate.

The interface speed only matters for transfers between the disk's buffer and the PC's memory.  MOST transfers are from the platters ... so the sustained transfer rate is much more important than which interface you're using.   The SAS 10K will be much faster ==> but because it's a 10k drive;  NOT because it's SAS.

Where SAS begins to shine is with large RAID arrays ... where the combined transfer rate of the drives in the array can take advantage of the SCSI bus's disconnect features and the higher sustainable transfer speeds.
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by:Kurt4949
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So the 3Gb/s is the interface speed?  This number seems meaningless.  Why don't they give us the sustained transfer rate then?  The only specs they seem to give are 3Gb/s and RPM.  Thanks for the info.
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I am planning to use Raid 1
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"... So the 3Gb/s is the interface speed?  This number seems meaningless " ==>  Yes, it's the interface speed;  and although it's not completely "meaningless", I agree it's deceptive.  A lot of people think that means it's a much faster drive ... but in practice there's virtually NO difference in performance between the same drive with an ATA100 interface and a SATA-II interface (the difference is measurable;  but almost certainly not noticeable).
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by:Duncan Meyers
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>The SAS 10K will be much faster ==> but because it's a 10k drive;  NOT because it's SAS.
That needs a little qualification. SAS supports the full SCSI command set and enhancements - things like Native Command Queueing, Seek re-ordering and a whole host of other. What all this means is that the SAS drive can do clever stuff like re-order a sequence of reads depending on where the disc heads happen to be at the time, so it is faster than SATA II which only has a subset of these enhancements. It is important to note that SATAII (particularly with some SATAII RAID controllers) drives will often show simply staggering I/O results. The devil, as always, is in the detail. One of the big differences between SCSI (which includes native SCSI, Fibre Channel and SAS) and SATA drives is that SATA II drives will reports that a write operation is complete as soon as that write is copied into the drive's write cache - which means that it isn't safely committed to the magnetic surface of the disc. SCSI discs, on the other hand, must commit the data to disc before they return a write complete - which is what you want on a system where data integrity is important. But the point is that this 'quirk' *really* skews performance figures.

Another thing to be aware of is that SATA II drives will usually show better (in the order of 10 -20%) sequential read and write performance simply because the data is written more densely (referred to as the areal density) onto the disc's surface. SATA drive (and IDE) performance will also start to degarde as you stack them up, because the transferred vibration of spinning drives sitting next to each other tends to throw them ever so slightly off-track. The vibration issue get worse as you have a number of drives seeking at the same time. SCSI drives are built with a more rigid casing and a different type of head actuator motor that reduces the effects of this induces vibration. Its one of the things that makes them more expensive than SATA and IDE drives.

>So the 3Gb/s is the interface speed?  This number seems meaningless.  Why don't they give us the sustained transfer rate then?
Yep - pretty much so. But isn't 3Gb/s much more exciting than 30MB/sec? :-) The sustained transfer rate is usually buried somewhere in the specifications if its given at all. I've noticed that Seagate stopped providing that information for their range of SATA/ATA/IDE drives abount 6 months ago. I don't know if they've started publishing it again - I haven't looked recently.

So - in short. If you're building a server that a number of users will be accessing at once, go with SCSI/SAS. If this is for your own PC, then SATA II is a better choice.
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I noted this idea above, but it's nicely stated in the article mcp_jon referenced:  "... It's important to emphasize that SAS is an "interface" -- the circuitry and interconnection used to move data to or from the drive. SAS has virtually no impact on the mechanical design or operation of the drive itself (e.g., the platters, heads or other drive components)."

... in other words, it's the 10,000rpm DRIVE that's faster --> NOT the fact that it's SAS.   You can get equivalent performance with a nice 10,000rpm SATA drive => a WD Raptor 150GB comes to mind (it's "only" SATA-150, but will outperform virtually any SATA-II drive).
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by:mcp_jon
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The major difference is that SCSI is attached to the same channel, and Data must run in the same channel. SAS is like a Backplane where every disk interconnect and eliminates cabling between disks, SERVER Speaking :). In Workstations it works just like Sata, where every disk goes to a Physical Port on a Controller, wether it's Onboard or on a PCI card .

Hope this lightens up your mind !

Best Regards !
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by:Kurt4949
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Thanks for the help.  I think I will go with Raid 1, 300GB, 10KRPM, SAS drives.  I want this server to be fast and reliable.
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by:garycase
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Good choice ... it will indeed be both fast and reliable :-)
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by:mcp_jon
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You bet it will !

Best Regards !
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