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SAS vs SATA

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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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rsnellmanIT Manager

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Commented:
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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Commented:
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.)  
President
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rsnellmanIT Manager

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Commented:
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.

rsnellmanIT Manager

Author

Commented:
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.
DavidPresident
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DavidPresident
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Commented:
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)
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.
DavidPresident
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Commented:
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.
andyalderretired saggar maker
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Commented:
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.
DavidPresident
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Commented:
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.
andyalderretired saggar maker
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Commented:
Oh yes, the enterprise SATA from Seagate also have workload manager, I'd forgotten that.
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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
rsnellmanIT Manager

Author

Commented:
Good answers.
DavidPresident
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Commented:
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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