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xpucto 11-28-2005 02:10 PM

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

Tinkster 11-28-2005 03:15 PM

Re: sata vs scsi for a server
 
Quote:

Originally posted by xpucto
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?

They're different interfaces, SCSI stands for Small Computer System Interface
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:

Originally posted by xpucto
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?

Well, SCSI has been geared towards durability for ages, 150000 MTBF
and 5 year warrantys are quite normal, most ATA and SATA drives will
be warranted for 1 or 3 years.

Quote:

Originally posted by xpucto
3) Is it difficult to make a scsi-system work?

I've never had any problems ;}


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

imitheos 11-28-2005 04:56 PM

Re: sata vs scsi for a server
 
Quote:

Originally posted by xpucto
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

1)
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.

Bruce Hill 11-28-2005 07:38 PM

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

/dev/sda:
 Timing cached reads:  2104 MB in  2.00 seconds = 1052.16 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device
 Timing buffered disk reads:  166 MB in  3.01 seconds =  55.21 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device

/dev/sdb:
 Timing cached reads:  2088 MB in  2.00 seconds = 1042.07 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device
 Timing buffered disk reads:  166 MB in  3.02 seconds =  54.96 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for device

Here's some specs on this box for comparison:
Code:

bruce@silas:~$ /sbin/lspci -v
00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA RAID Controller (rev 80)
        Subsystem: ASUSTeK Computer Inc. A7V600/K8V Deluxe/K8V-X motherboard

and the drives are:
/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.

cs-cam 11-29-2005 12:47 AM

Quote:

Originally posted by Chinaman
<snip>
If I were you I'd be extremely disappointed with those results. Either that or I've been mislead. I am of the understanding that SATA disks are exceedingly quicker than what us old schoolers have in our PCs but this doesn't seem to be the case. That same test (which I already know is extremely inconclusive) on my PC, the cached reads were convincingly quicker but you beat me out on the buffer reads by just under 2MB/sec. Is this right?

imitheos 11-29-2005 08:37 AM

Quote:

Originally posted by cs-cam
If I were you I'd be extremely disappointed with those results. Either that or I've been mislead. I am of the understanding that SATA disks are exceedingly quicker than what us old schoolers have in our PCs but this doesn't seem to be the case. That same test (which I already know is extremely inconclusive) on my PC, the cached reads were convincingly quicker but you beat me out on the buffer reads by just under 2MB/sec. Is this right?
Maybe i am wrong but these are not disappointing results. (Or mine are too :) )

Code:


/dev/sda:
 Timing cached reads:  4532 MB in  2.00 seconds = 2265.86 MB/sec
 Timing buffered disk reads:  180 MB in  3.00 seconds =  59.92 MB/sec

My cached data result is twice which is weird but cached result doesn't matter much.
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)

xpucto 11-30-2005 12:14 PM

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

Tinkster 11-30-2005 12:26 PM

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

xpucto 11-30-2005 12:45 PM

Quote:

Originally posted by Tinkster
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?

Well, I just would like to understand things and have better idea what is what for.
XpucTo.

imitheos 11-30-2005 05:41 PM

Quote:

Originally posted by xpucto
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

SCA doesn't mean cheaper and not reliable.
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.

cs-cam 11-30-2005 06:02 PM

Quote:

Originally posted by imitheos
Maybe i am wrong but these are not disappointing results. (Or mine are too :) )

Code:


/dev/sda:
 Timing cached reads:  4532 MB in  2.00 seconds = 2265.86 MB/sec
 Timing buffered disk reads:  180 MB in  3.00 seconds =  59.92 MB/sec

My cached data result is twice which is weird but cached result doesn't matter much.
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)

Fair enough, I don't know a lot about hardware, I was under the impression that SATA was Superman-faster but I've heard a bunch of conflicting opinions there so I'll just leave it. I can't use it so why bother? ;)

Tinkster 11-30-2005 07:29 PM

Quote:

Originally posted by imitheos

Code:

/dev/sda:
 Timing cached reads:  4532 MB in  2.00 seconds = 2265.86 MB/sec
 Timing buffered disk reads:  180 MB in  3.00 seconds =  59.92 MB/sec


What type of CPU/Chipset are you getting the 2265.86 from? :}

[edit]
The reason that I'm asking :}

Code:

su -c "/sbin/hdparm -tT /dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde "
Password:

/dev/sda:
 Timing buffer-cache reads:  2428 MB in  2.00 seconds = 1214.00 MB/sec
 Timing buffered disk reads:  56 MB in  3.01 seconds =  18.60 MB/sec

/dev/sdb:
 Timing buffer-cache reads:  2480 MB in  2.00 seconds = 1240.00 MB/sec
 Timing buffered disk reads:  234 MB in  3.01 seconds =  77.74 MB/sec

/dev/sdc:
 Timing buffer-cache reads:  2476 MB in  2.00 seconds = 1238.00 MB/sec
 Timing buffered disk reads:  232 MB in  3.01 seconds =  77.08 MB/sec

/dev/sdd:
 Timing buffer-cache reads:  2476 MB in  2.00 seconds = 1238.00 MB/sec
 Timing buffered disk reads:  332 MB in  3.00 seconds = 110.67 MB/sec

/dev/sde:
 Timing buffer-cache reads:  2496 MB in  2.00 seconds = 1248.00 MB/sec
 Timing buffered disk reads:  338 MB in  3.01 seconds = 112.29 MB/sec

And that's on a Dual-Xeon 3.6 ...
[/edit]


Cheers,
Tink

xpucto 12-01-2005 05:32 AM

Quote:

Originally posted by imitheos
SCA doesn't mean cheaper and not reliable.
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.

Well, I kind of inherited of a server with 3 SCA hard drives. That´s why I was asking, I thought that SCSI could also be hotplugg. Does SCSI mean "not hotplug" or can it be also hotplug?

XpucTo

imitheos 12-01-2005 09:29 AM

Quote:

Originally posted by Tinkster
What type of CPU/Chipset are you getting the 2265.86 from? :}

[edit]
The reason that I'm asking :}

Code:

su -c "/sbin/hdparm -tT /dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde "
Password:

/dev/sda:
 Timing buffer-cache reads:  2428 MB in  2.00 seconds = 1214.00 MB/sec
 Timing buffered disk reads:  56 MB in  3.01 seconds =  18.60 MB/sec

/dev/sdb:
 Timing buffer-cache reads:  2480 MB in  2.00 seconds = 1240.00 MB/sec
 Timing buffered disk reads:  234 MB in  3.01 seconds =  77.74 MB/sec

/dev/sdc:
 Timing buffer-cache reads:  2476 MB in  2.00 seconds = 1238.00 MB/sec
 Timing buffered disk reads:  232 MB in  3.01 seconds =  77.08 MB/sec

/dev/sdd:
 Timing buffer-cache reads:  2476 MB in  2.00 seconds = 1238.00 MB/sec
 Timing buffered disk reads:  332 MB in  3.00 seconds = 110.67 MB/sec

/dev/sde:
 Timing buffer-cache reads:  2496 MB in  2.00 seconds = 1248.00 MB/sec
 Timing buffered disk reads:  338 MB in  3.01 seconds = 112.29 MB/sec

And that's on a Dual-Xeon 3.6 ...
[/edit]


Cheers,
Tink

Hm.
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:

Originally posted by xpucto
Well, I kind of inherited of a server with 3 SCA hard drives. That�s why I was asking, I thought that SCSI could also be hotplugg. Does SCSI mean "not hotplug" or can it be also hotplug?

XpucTo

If your controller supports hotplug (not everyone does) then you can hotplug the SCSI drives. the SCA drives just make
hotplugging easier since they use only a connector. Generally, you won't be able to hotplug internal hard drives.


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