sata vs scsi for a server
I read the following article
http://www.pugetsystems.com/articles.php?id=19 where it says that sata are actually faster than scsi. My questions are the following: 1) What are the differences between sata and scsi? 2) Is it correct that sata are made for computers that doesn´t work 24 hours/day all year long and scsi are made to work all the time without interruption? 3) Is it difficult to make a scsi-system work? Thanks. XpucTo |
Re: sata vs scsi for a server
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and has been around for quite a while. One of the biggest differences will be the number of devices you can hook up to a SCSI controller, a modern SCSI chain will hold 14 devices (14+1 [the controller]) and most controllers will have 2 channels. The other thing is the sustained data transfer rate, current SCSI controllers will be able to maintain 320MBit/s Quote:
and 5 year warrantys are quite normal, most ATA and SATA drives will be warranted for 1 or 3 years. Quote:
And a couple of general notes on the comparison you posted. a) They forgot to mention what controller type they were using for the SCSI drives b) They are comparing apples with pears if they don't use the same hardware (minus the controller and HDD, of course) for both tests. And as for real-life experience: I've had more IDE/SATA drives die on me than SCSI ones, and that's not only due to the proportionally bigger number of the first, but in relation to "dead IDE"/"IDE" and "dead SCSI"/"SCSI" ... Cheers, Tink |
Re: sata vs scsi for a server
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They are two different protocols/interfaces. All modern motherboard have a SATA controller integrated so you just connect the drive to the motherboard. You need to get a SCSI controller in order to connect a SCSI peripheral. SATA have a buffer-to-host of 150MB/sec (300 for SATA II). SCSI has reached 360MB/sec for regular parallel scsi and 3Gb/sec for SAS (Serial Attached SCSI) A regular SATA controller provides 4 ports for 4 peripherals (for now hard disks and plextor dvd-rw) A regular wide SCSI controller provides support for 14 peripherals per channel (15 minus the controller) (As Tinkster mentioned) Also SCSI has a much wider product range (hard disks,tape drives,scanners,etc) (Actually it used to have. for example scanners are usb now) SCSI protocol specification is more carefully written (written is not the best word for this). (now SATA II and SATA 2.0 are coming which are two different things) 2) SCSI drives production is more thorough and carefully planned. This is also because SCSI drives have to implement a much broader set of instructions. Generally, one can say that a SCSI drive will outlast a SATA drive, but now all drives (if properly cooled) are very reliable and will not have any problem. SATA drives are now a product of mass production and so they follow the rules of mass production product (some people say that because they have to be produced in masses, they don't get the attention SCSI drives get. In general this has some point) 3) I have 3 SCSI controllers in my boxes and i have a Plextor SCSI CD-writer, a SCSI tape drive, SCSI disks and SCSI scanner There are only a few rules you must follow to set a SCSI system. In some extremely complicated cases of different SCSI devices you can create a mess easily, but generally you only set the SCSI ID and you are set. The rotation speed of a disk is a great factor for its speed, so generally SCSI disks (15000 RPM now) are faster than SATA disks, but a 147GB 15000RPM Ultra 320 disk is 4-5 times more expensive than a 300GB SATA disk. If you want to have a 24/7 server with medium to large load, i would suggest you to get a small SCSI drive (36GB) for your OS drive which will be faster, more robust,versatile and get a large SATA drive for storage. You can get two SATA drives and have them in RAID1 and it will still be more cheap than a single large SCSI drive. I hope i helped. |
Two of my PCs and one server have SATA drives. That being said,
there is a lot of discussion relative to your post. Only one thing I want to say, though. AHCI and NCQ are only available with a few motherboards and drives right now. See the information posted on Jeff Garzik's site. He's the kernel developer for the libata driver, which runs the SATA drives in Linux. The information in the article you linked is pertaining to Windoze. The hardware manufacturers are not all offering their source code for the Linux developers to use, so you'll not see the true performance of a SATA drive with the present Linux kernels. The other men gave you more meat to chew on. As Tinkster said, there really is no comparison between SATA and SCSI drives. You can look at SATA drives as just a bit better than ATA for now. When AHCI is used in more boards, and NCQ is standard on SATA drives, then they'll be more attractive. The best results I've gotten in Linux on any of my SATA drives so far: Code:
root@silas:~# hdparm -tT /dev/sda /dev/sdb Code:
bruce@silas:~$ /sbin/lspci -v /dev/sda = Seagate ST3160827AS Barracuda 7200.7 SATA /dev/sdb = Hitachi Deskstar 7K250 HDS722525VLSA80 SATA That's really not a fair assessment of what the drives are capable of, because the kernel doesn't yet have support, and the motherboard that I'm using doesn't fully support what the drives will do, either. |
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The disk read result is almost the same. I have seen better results (62,65,etc) but generally the results are these. They vary from 52-62MB/sec P.S this result from hdparm has nothing to do with real life speed. To measure real life speed you need a io benchmark tool and in order to have a meaning for you, you choose the parameters according to your use. hdparm just gives an image of the speed. Anyway, maybe i am wrong but Chinaman's results seem good to me. P.S-2 If you run hdparm on a SCSI drive you will get the same results. But real life speed will be entirely different (way faster IMHO) |
Well, thanks a lot for your answers. It helped a lot. I would have 1 last question: what about SCA?
Hier is what I read about SCA: http://www.pcmech.com/show/harddrive/152/ If I understand well, SCA is cheaper, slower and not so reliable than SCSI? But still better than SATA? Is SCA an interesting compromise? Thanks again. XpucTo |
How about you describe what the system is going to be for,
and what expected loads there are going to be, and then we can tell you which system is most appropriate for you? SCA is a dumbed down SCSI, and you'll require external termination and quite likely some adapters, too. Cheers, Tink |
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XpucTo. |
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SCA drives are just the same as normal SCSI drives. The difference is the connector. The connector is 80pin which includes the 68pins of the wide scsi and the ID pins and the power pins. This make hotplugging easy. SCA drives are used in hotplug arrays. In most cases, you use external boxes in which you connect the SCA drives, but server cases include hotplug trays too. (for example my Siemens primergy server case has) You can get SCA drives but they won't do you any good, because you don't have these boxes. You will have to get a 80pin->68pin adaptor as Tinkster mentioned and you won't gain anything. Now, let me clarify some things mentioned in the article which are not true anymore (the article is rather old) 1) "SCA doesn't have termination while 68pin has." All drives conforming to protocols from Ultra2 (80MB/sec) to Ultra320 are LVD (Low Voltage Differential). LVD drives don't have internal termination. You need an external terminator which goes to the end of the cable. This made things easier for setup too, because you don't have to remember this device had the termination on, oh that device had the termination on. So, if you buy a hard disk either 68pin or 80pin (SCA) it won't have terminator. 2) "Slower than Non-SCA drives because of the use of adapter." This is true only in your case for example where you will need the adapter in order to connect the drive to the 68pin connector. The high end arrays that use SCA don't have this "problem". Even in your case the difference in speed will be minimal (if there is one) The problem is that there are adaptors from 1Euro to 20Euro. There are cheap adaptors which cause interference problems and lead to slow speed and/or crashes. In 98% of the cases its the adaptors problem and not the drives. (btw the proper term for this is adaptor and not adapter) 3) About the benchmarks I don't take "Hawk" into consideration because it has lower RPM than the other two. As for the other two disks: a) The difference is not so huge b) If you look at the specifications from the seagate site you will see differences in the two drives. for example the first drive has internal speed of 80-122mbit/sec while the second has 92-140mbit/sec. This alone can give reason for the difference in the benchmark and not because he used the adaptor. My most polite opinion about the article is "Don't take what he says seriously" These things don't concern your setup even if you go for SCSI. If you read them for the sake of knowledge, there are many good info sites. For example wikipedia is one. It describes terms and also has some links with good info. |
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[edit] The reason that I'm asking :} Code:
su -c "/sbin/hdparm -tT /dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde " [/edit] Cheers, Tink |
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XpucTo |
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The sda is a bit slow but the sdd,sde is a bit fast. Well way fast :) These are single disks ? Is the 2265 so fast ? My box is not the fastest around. I have a P4 3.0GHz LGA775 with a 925 Intel chipset and a Western Digital SATA hard disk. Quote:
hotplugging easier since they use only a connector. Generally, you won't be able to hotplug internal hard drives. |
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