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SAS vs SATA

Posted on 2010-09-12
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Last Modified: 2012-06-27
Hi, I was doing some Sunday afternoon reading and starting thinking about SAS again.  It got me thinking maybe I the reason I have had so many HDD failures in RAID 1 & RAID 5 on some of my servers is due to moving to SATA rather than sticking with SAS.

So, I would like to take a poll to find out who all has made the switch to SATA on their servers?

We have nearly moved completely over to SATA, due to cost-savings, but now I am wondering if we should at least rethink our very critical servers (SQL, Exchange, Web) should be SAS for more reliablity.

Here is what the wiki says about SAS vs SATA :


SAS vs SATA
 This section does not cite any references or sources.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2009)

SAS is full-duplex, SATA is half-duplex.
Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide Name (WWN).
SAS protocol supports multiple initiators in a SAS domain, while SATA has no analogous provision.
Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing, each of which has its pros and cons.
SATA uses the ATA command set; SAS uses the SCSI command set. ATA directly supports only direct-access storage. However SCSI commands may be tunneled through ATA for devices such as CD/DVD drives.
SAS hardware allows multipath I/O to devices while SATA (prior to SATA 3Gb/s) does not. Per specification, SATA 3Gb/s makes use of port multipliers to achieve port expansion. Some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.
SATA is marketed as a general-purpose successor to parallel ATA and has become[update] common in the consumer market, whereas the more-expensive SAS targets critical server applications.
SAS error-recovery and error-reporting use SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.
SAS uses higher signaling voltages (800-1600 mV TX, 275-1600 mV RX) than SATA (400-600 mV TX, 325-600 mV RX). The higher voltage offers (among other features) the ability to use SAS in server backplanes.
Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, SATA has a cable-length limit of 1 m (3 ft) or 2 m (6.6 ft) for eSATA.

So, chime in and let me know what you think about this debate.

Thanks for your time.


Bob
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Question by:rsnellman
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by:rsnellman
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Found this too...I know it a few years back, but still interesting when debating the SAS vs SATA.

http://www.tomshardware.com/reviews/UNIFIED-SERIAL-RAID-CONTROLLERS-PCI-EXPRESS,1665-3.html

Looks like I need to pay more attention to the specific HDD makes/models I decide on when it comes to server performance & longevity.


Bob
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by:PowerEdgeTech
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SAS drives are very robust, and are generally used for high speed applications, as their spindle speed is generally 10K or 15K, but they do not have the capacity of a SATA drive.  If it is raw storage capacity you are looking for, then SATA has their place in Enterprise storage.  If performance also enters the equation, then one has to weigh the pros and cons of their particular application and situation (financial, etc.).

To give you an idea on SAS vs SATA longevity ... Dell's Basic warranty on servers only covers SATA drives for 1 year, regardless of the actual length of the system's warranty.  (This does not apply to "advanced" contracts, SAS drives, or SATA drives in desktops/laptops.)  
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dlethe earned 300 total points
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Your very premise is flawed.  Your assumption is that all SATA (and for that matter, SAS) disks are the same.  SATA manufacturers have two classes of SATA disk. Consumer/desktop class and enterprise class.   This is not to take more money from you (entirely), but there are real differences in terms of reliability and operating characteristics.  Look at things like the # of ECC bits, duty cycle, and factory-tuned settings relating to error recovery.

Were you using $50 HDDs designed for desktops, meaning 8 hours a day, 300 days a year, or 24x7x365?  If so, stop there.  You have bad results because you are using disks in a way that they were not designed.  

The physical interface does not dictate operational characteristics and reliability.  Maybe you are equating SAS with high reliability because there aren't any cheap desktop class SAS drives that are only designed for light duty?

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by:tenetics
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I think the key probably is the type of SATA disk you're using. There's a reason why enterprise disks are more expensive than desktop ones. If you're buying disks yourself rather than manufacturer's ones try www.span.com.

The other thing is how hard are the disks having to work. Often people see the huge capacities of SATA drives, rather than going for more spindles of a smaller size - impacts reliability as well as performance.

They both have their place - SATA predominantly for near-line storage rather than day to day use.

The cost of the disks in any event are negligible compared to the cost of managing the data - a no-brainer in most circumstances.
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by:rsnellman
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Yes, I ran across another article about standard & enterprise versions of SATA drives, which I never knew existed until now.  Makes me go hmmmmm...

I will have to look into the drives on the servers to verify what version they are.

I know they are Seagates, but ill have to dig further.  If my sales rep supplied me a server with desktop HDDs I will be ticked.

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by:rsnellman
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So, file servers, Exchange servers, Web / SharePoint servers, SQL servers, SCCM/SCOM servers, etc. should be running SAS for the speed & reliablity, correct?

Or would you go another route?  Just trying to pick the brains of the other admins out there.
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by:dlethe
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You should read this, it touches on a lot of this stuff ..
http://www.experts-exchange.com/Storage/Misc/A_2757-Disk-drive-reliability-overview.html

For your exchange, SQL servers, etc. .. then nothing wrong with mixing ENTERPRISE-class SATA and SAS, SCSI, fibre channel, or even SSDs.   enterprise class SATA has it's place.  Think SATA as an appropriate storage pool for large block sequential, and the premium drives give you better random, IOPs, and response time.   RAID level certainly comes into play.  

But for reliability, read the specs, and not the data sheet.
Look at ECC bits, MTBF, and consider your RAID strategy.  All disks have 100% probability of drive failure, so look at RAID6 technology for greatest availability for any given disk size, not the SAS vs SATA (again enterprise class SATA).

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by:dlethe
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P.S. if your sales rep did indeed sold desktop class drives with a server that he/she knew was going to be in 24x7 operation, and did not tell you to go with server class drives, then they either don't know what they are doing or don't care what they are doing.

While you are at it, look at the specs on the controller card, and see if the manufacturer provides a list of qualified disks & firmware for it.  If the system is misconfigured, then you have a legitimate beef with them. If this is an ongoing problem, and you have been dealing with this for years, then personally, I think you have a legal claim if you can prove damages (but i would just try to get them to give you credit on your drives and upgrade you at cost for appropriate disks designed for your type of operation)
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by:oztrodamus
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There is another class of disk out there called Near-Line SAS. nSAS disks have the reliability of SAS with the capacity of SATA. Please take note I did not say performance of SAS. Performance wise it's closer to SATA, but not quite as bad. The good news is they are much cheaper than SAS.
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by:dlethe
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Actually nSAS disks have the reliability of enterprise SATA disks ... because that is what they are.   THe added performance comes from the SAS interface, which is full duplex, and the SCSI command-set which is more robust than ATA command set.  Unless one is running benchmarks then most people will never see the performance benefit.
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by:andyalder
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I thought near-line SAS had better reliability than the equivalent SATA because at least with Seagate they went into read-after-write mode if they got hot (which hammers performance due to 1.5 turns around the platter per I/O) - http://www.seagate.com/docs/pdf/whitepaper/wp_barracuda_es2_lsi_sas-sata_adapter.pdf has a bit about it.
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by:dlethe
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There are vendor-specific firmware features that manufacturers use to differentiate their products from others, but the mech is still an enterprise-class SATA HDD with a SAS electronics package. It *HAS* to be this way, because there are no SAS 2TB or 3TB mechanisms in the first place.
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by:andyalder
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Oh yes, the enterprise SATA from Seagate also have workload manager, I'd forgotten that.
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by:BigSchmuh
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I will summarize the SAS vs SATA arguments as below:
SAS pros:
  • Most SAS HDD are designed with a low UBE (1 unrecoverable sector per 10e15 bits), many are even claiming for a UBE of 1 per 10e16; of course, those drives are storing less data than their technology equivalent alternatives models
  • 15krpm drives always have a SAS or FC interfaces; the only 10krpm SATA are the WD Raptors series
  • Better UBE and faster rotation speed explain why SAS drives are smaller than SATA's
  • Some SAS / FC HDD has a dual port interface allowing for controller failover capability (multipath feature)
  • Most SAS HDD are coming with a read/write timeout with a default value of less than 7 seconds that allows their raid controller to not kick the drive out of the array; all SATA "Enterprise" or "RAID" drives has this feature in place; some "Desktop" SATA can enable this kind of low timeout using their manufacturer low level utilities
  • SAS cables can be longer than SATA's (due to a higher voltage)
  • (SATA are full-duplex as well...not a "pros" any more)
SATA pros:
  • Most SSD are using SATA
  • Some SSD claims for a UBE of 1 per 10e17 bits
  • The SATA market is cheaper (more volumes) than the SAS one
  • Most Desktop SATA HDD comes with an incredibly poor UBE of 1 per 10e14 (8% of an unreadable sector per 1TB read!!! ) which, to my opinion, explains why we read so many "raid array failed" posts of poor guys using large desktop SATA HDD
  • Most "Enterprise" class SATA HDD comes with a UBE of 1 per 10e15 bits and are still cheaper (about 10%) than their SAS alternative (same drive, same UBE, same warranty, only the interface differs)
  • No SAS drive reach the density of a 2TB (UBE 1 per 10e15) SATA
My personal opinion for the next 5 years:
  • SAS is to become a niche market for those that requires "controller failover" capability (dual port drives)
  • SATA rules the IOPS world (using SSD) and the storage space world (using large SATA drives)
  • Most storage racks will allow mixing 2U stacks of 3,5" (24-36x per 4U; 12x per 2U) and 2,5" form factor (Most SSD are that size + less energy used + allows for 48-72x drives per 4U; 24x drives per 2U)
  • Using an array scrubber (Ex: MD5 checking) is mandatory (except for ZFS based array because a checksum is already included in ZFS)
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by:BigSchmuh
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Some other SAS pros:
  • SAS is full-duplex where SATA is half-duplex is not a rumor...but does not offer a real difference in performance
  • SAS drives usually have 128 queue depth capability where SATA looks being bound to 32...but an avg io execution QD looks like being around 8 and one may expect a 5% IOPS increase by going from 32 QD to 128 QD (+risk of a very long access time due to a TCQueued io getting 120th io delay)
  • SAS drives checks the IOECC at READ and WRITE time where SATA checks only at WRITE time...That is a good reliability point that may lower the need of a SCRUBber for SAS
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by:rsnellman
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Good answers.
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by:dlethe
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Curious .. were you being sold desktop-class SATA drives with your servers?  If so, who was your vendor??  People need to know who to avoid!
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