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3.5'' Vs. 2.5'' SAS HDDs: In Storage, Size Matters

3.5'' Vs. 2.5'' SAS HDDs: In Storage, Size Matters
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Diminutive 2.5” form factors are favored in the enterprise storage world for many reasons, but how do high-end 2.5” SAS drives compare to the established 3.5” products when it comes to performance and power? We requested some drives and did the analysis.

With all of the hype surrounding SSDs (solid state drives) these days, you might think that hard drive technology is already dead. Indeed, flash-based storage will certainly replace high-end hard drives in the months and years to come. One of the first victims will be the 3.5” form factor in enterprise applications. Specifically, 10,000 and 15,000 RPM SAS drives will be replaced by 2.5” drives. We compared both form factors at 15,000 RPM.

While 3.5” desktop and 2.5” notebook drives are very different in most things, save recording density, there are significant similarities between 3.5” and 2.5” enterprise hard drives. These share spindle speeds and capacity points for a reason. Internally, 2.5” and 3.5” enterprise drives are based on the same platter diameter. The actual platters still differ, though, since a higher spindle speed requires more solid platters.

The main difference between the two form factors can be found in their total capacities. The 3.5” form factor allows more platters to be crammed into a roughly 26 mm z-height. Four platters can create 600GB, 15,000 RPM, 3.5” SAS hard drives, while a comparable 2.5” model runs on only two platters. A 2.5” enterprise drive running at only 10,000 RPM is usually based on three platters.

A smaller drive requires less power to operate because there's less mass needing to be moved. But how do 2.5” high performance drives compare to their 3.5” brothers? We grabbed several Hitachi enterprise drives and ran some tests.

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  • -2 Hide
    steddy , May 8, 2010 3:44 PM
    If the 3.5 inch drives have twice the number of platters as the 2.5 inch drives, shouldn't their performance be similar to that of two 2.5 inch drives in RAID? Or does the performance of multiple platters not scale that way?
  • -1 Hide
    Anonymous , May 8, 2010 11:31 PM
    Nice article.

    "Two 2.5” SAS drives in RAID will outperform one fast 3.5” SAS drive in every workload."

    I would like to see proof of this, as I don't believe the "in every workload" bit. RAID-0 and -5 don't always deliver performance as promised. As far as I know, it depends on the file sizes, among other factors. Applications that use thousands of little files (web servers come to mind) can't always get a good performance boost from data striping, as the overhead required to read/write many small files outweighs the speed gains achieved by spreading the load across multiple drives. In this case, one faster 3.5" drive might outperform two slower 2.5" drives.
  • 1 Hide
    Alvin Smith , May 9, 2010 2:56 AM

    SO ... We ALL DO have 5.25" drive bays as a common option ... Can you guess what I am gonna say next ??

    Perhaps mechanical drive manufactureres should revisit the 5.25" form-factor for super-perpendicular, ultra-dense, high-performance desktop apps, such as video editing and graphics (and other HD+ media content apps).

    UltraRaptor 5"x10K dual-platter short-stroked.

    = or not =
  • -1 Hide
    paperfox , May 10, 2010 5:07 AM
    lol, Alvin you beat me to it
    Ive always wondered what if the latest hard drive technology was applied to the old 5.25 standard. I can easily see 300 MB/s transfer speeds and at least 1 TB platters with enough room for maybe 6 or so in one drive. Although realistically the price per gigabyte would be unmatched and the price per performance and efficiency would be hideously low compared to the trend I see on page 8... unless you add another actuator arm :)  but thats a whole other article.
  • 0 Hide
    distortion , May 11, 2010 12:33 AM
    try spinning a stack of 5.25" disks at 10k rpm.
  • 1 Hide
    davidb77380 , May 12, 2010 2:08 PM
    "2.5” and 3.5” enterprise drives are based on the same platter diameter"
    LMAO ...another drive "expert" who doesn't know ____ from shinola.
  • 1 Hide
    GullLars , May 16, 2010 11:15 AM
    @Alvin Smith: I'm more interrested in high-density low-RPM 5,25" drives for desktops and fileservers. I would easily buy sine 10TB 5,25" 3600RPM (or 1800RPM?) drives for RAID-5 to store huge ammounts of media and other files. Given the price pr GB was right ofc. 50MB/s sequential read/write pr drive would be sufficient for mass storage.
  • 0 Hide
    Anonymous , June 4, 2010 12:11 PM
    " Two 2.5” SAS drives in RAID will outperform one fast 3.5” SAS drive in every workload"

    ...and what about a drive failure in your striped array? You should consider the whole, bigger picture, not just speed and capacity.
  • 0 Hide
    sohelm , April 6, 2013 10:38 PM
    WHICH ONE IS BATTER 2.5” SAS OR 3.5” SAS