SAS vs. SATA Differences, Technology and Cost

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One of our resources at HP Lefthand Networks (thanks Ben!) made the following comment to one of our customers and I thought it’d be a perfect post for the blog as it contains some useful information that some might not be aware of.

Here are the high-level differences between SAS and SATA disk drives:

Capacity:

  • SATA disk drives are the largest on the market.  The largest SATA drives available with widespread distribution today are 1.5TB-2TB.
  • SAS disk drives are typically smaller than SATA.  The largest SAS drives available with widespread distribution today are 450GB.
  • So, for capacity, a SATA disk drive is 3X-4x as dense for capacity than SAS.
  • A good way to quantify capacity comparison is $/GB.  SATA will have best $/GB.

Performance:

  • SATA disk drives spin at 7.2k RPMs.  Average seek time on SATA is 9.5msec.  Raw Disk IOPS (IOs per second) are 106.
  • SAS disk drives spin at 15k RPMs.  Average seek time on SATA is 3.5msec.  Raw Disk IOPS (IOs per second) are 294.
  • So, for performance, a SAS hard drive is nearly 3X as fast as SATA.
  • A good way to quantify performance comparison is $/IOP.  SAS will have best $/IOP.

Reliability: there are two reliability measures – MTBF and BER.

  • MTBF is mean time between failures.  MTBF is a statistical measure of drive reliability.
  • BER is Bit Error Rate.  BER is a measure of read error rates for disk drives.
  • SATA drives have a MTBF of 1.2 million hours.  SAS drives have a MTBF of 1.6 million hours.  SAS drives are more reliable than SATA when looking at MTBF.
  • SATA drives have a BER of 1 read error in 10^15 bits read.  SAS drives have a BER of 1 read error in 10^16 bits read.  SAS drives are 10x more reliable for read errors.  Keep in mind a read error is data loss without other mechanisms (RAID or Network RAID) in place to recover the data.

Here are some good links for comparing disk types:
http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_es_2.pdf
http://www.seagate.com/docs/pdf/datasheet/disc/ds_cheetah_15k_7.pdf
http://h18004.www1.hp.com/products/servers/proliantstorage/drives-enclosures/index.html

7 thoughts on “SAS vs. SATA Differences, Technology and Cost”

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  5. There are models of SAS drive that are 1 or 2TB and spin at 7200 RPMs.
    As far as I know the difference between the 7200RPM SAS drives and the 7200RPM SATA drives is just the SAS/SATA interface.

    Suppose you have two disk drives; identical capacity and RPMs…. one is SAS, one is SATA; I wonder if either is technologically better / how much better?

  6. it’d also be good to talk about many sata drives are now marketed as ‘nearline sas’ or ‘midline sas’ where the sata protocol and drive characteristics are used but they have the same 6G/sec bandwidth available as the sas drive.

  7. Good thing most SANs have hot spares! If the difference in BER and/or MTBF made a difference in real world business, I would think there would be a lot more businesses complaining about data loss. Putting some perspective on the that 1.2 million hours for SATA MTBF, that’s 50000 days or 137 years.

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