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How fast Hard Drives relly are ?

Posted on 2003-03-19
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Last Modified: 2010-04-26
I want to know what is the rael speed of hd.

does a hd ATA66 fast relly is 66.6MBPS(IMO - no ;) )?

could someOne please direct me to a site that explains this turf.

thank u.
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Question by:swift87
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10 Comments
 
LVL 3

Expert Comment

by:ShadowWarrior111
ID: 8171850
This sites explain what you need to know about ATA66 http://www.directron.com/faqata66.html
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Author Comment

by:swift87
ID: 8173350
Well,
I don't think u understood me right.
i want to know the real speeds of hd,not necessarily ata66-it was just an example,and not the general of hd.
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LVL 12

Expert Comment

by:pjknibbs
ID: 8173637
The real speed of a hard drive is limited to how fast the drive can physically move data off its platters. At the moment this is around 40-50 Mb/sec for a 7200rpm IDE drive. You could theoretically get the full ATA66 bandwidth if the data being transferred off the drive didn't have to come off the physical disk (e.g. is coming from the built-in cache on the drive), but this isn't all that likely because the built-in cache is very small compared to the size of the drive.
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Expert Comment

by:Naito
ID: 8175960
The MAXIMUM speed of of hard drives is limited by the interface speed, 33MB/s for Ultra ATA 33, 66MB/s for Ultra ATA 66, and so forth.  This only happenns however when the hard drive is bursting data from it's cache.
The ACTUAL sustained transfer speed of the hard drive is much lower than that.  Most hard drives transfer about 40-60MB/s.  Use a program like HDTach to test your drive.
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Expert Comment

by:ShadowWarrior111
ID: 8178252
The speed of hard disk depends on a few factor. One of the is the interface speed, for example Ultra ATA 66 tranfer speed at 66MB/s. Besides, the real speed of data transer also depends on the cache size the hard disk have. Most hard disk have 2MB of cache but newer hard disk come equip with 8MB cache. Cache is used to store temporarily data that you need to transfer from the hard disk to other medium or destination. Besides this, the speed of hard disk also depends on the seek time of the disk. The faster the seek time of the disk, there faster data can be located in the hard disk and the faster you can start to transfer data from the disk.
Hard disk also can be specified by it's write speed. The faster the write speed, the faster data can be written to the disk.
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Author Comment

by:swift87
ID: 8178604
As far as Naito was the closest , it steel far a way.

I want the core of this matter explained widly.
i tried to look up in google , but no such like so far :(

i looked at HDTach ,didn't help alot. though it showed
the the transfare rate of the hd , it was not understandable Graph.that doesn't explain you how does the speed of hd works.
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Expert Comment

by:Naito
ID: 8179087
core of the matter:
speed of hard drives can be measured many ways
Manufacturers like to simply quote the maximum theoretical speed, 100 or 133MB/s in case of Ultra ATA, 150MB/s for SATA

in real-life, it is affected primarily by these factors:
1 spindle speed - the faster the physical disks in the drive spin, the faster the read heads can reach any point on the disk to retrieve data.  also, it will allow that data to be streamed to that read head faster.

2 access speed - the speed the read head of the hard drive can move from the any points on the platters.  average access speeds, known as seek times, are generally around 10-12ms, meaning it takes 10-12ms to move to any point on the disk.

3 data density - hard drives are built with a varying number of platters, that is the physical number of disks inside the drive that actually store data.  The greater the density, the more data is stored per physical area of the disk, meaning the less distance the read heads of the drive have to move to retrieve a corresponding amount of data.

4 cache size - the larger the cache size on the drive, the more data can be read ahead by the hard drive's control firmware, and this data can be bursted to the CPU as fast as the connection, be it Ultra ATA or SATA, will allow.

it's rather hard to define the "real-speed" of an HD, because of all these things.  Some drives have lower access times, making them faster at storing data that is randomly accessed.  Drives with large caches, high spindle speeds and data density can sometimes be slower in access time, but they will be faster when accessing linear data like video/audio streams.

----

Simple version:
if I take your question as asking whether a hard drive advertised as Ultra ATA-133 compliant can actually transfer data at 133MB/s:
Yes, if the data the CPU is looking for is in the hard drive's cache, it can be sent to the CPU at 133MB/s, usually slightly less however because nothing ever works at the maximum theoretical speed.
No, if the data is not in the cache, as then the hard drive will have to search for the data.  and that's where those other factors come in.  if it so happens that the data is found immediately, most current hard drives will be able to read and send that data at around 40 to 60MB/s.
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LVL 1

Accepted Solution

by:
Naito earned 150 total points
ID: 8179092
core of the matter:
speed of hard drives can be measured many ways
Manufacturers like to simply quote the maximum theoretical speed, 100 or 133MB/s in case of Ultra ATA, 150MB/s for SATA

in real-life, it is affected primarily by these factors:
1 spindle speed - the faster the physical disks in the drive spin, the faster the read heads can reach any point on the disk to retrieve data.  also, it will allow that data to be streamed to that read head faster.

2 access speed - the speed the read head of the hard drive can move from the any points on the platters.  average access speeds, known as seek times, are generally around 10-12ms, meaning it takes 10-12ms to move to any point on the disk.

3 data density - hard drives are built with a varying number of platters, that is the physical number of disks inside the drive that actually store data.  The greater the density, the more data is stored per physical area of the disk, meaning the less distance the read heads of the drive have to move to retrieve a corresponding amount of data.

4 cache size - the larger the cache size on the drive, the more data can be read ahead by the hard drive's control firmware, and this data can be bursted to the CPU as fast as the connection, be it Ultra ATA or SATA, will allow.

it's rather hard to define the "real-speed" of an HD, because of all these things.  Some drives have lower access times, making them faster at storing data that is randomly accessed.  Drives with large caches, high spindle speeds and data density can sometimes be slower in access time, but they will be faster when accessing linear data like video/audio streams.

----

Simple version:
if I take your question as asking whether a hard drive advertised as Ultra ATA-133 compliant can actually transfer data at 133MB/s:
Yes, if the data the CPU is looking for is in the hard drive's cache, it can be sent to the CPU at 133MB/s, usually slightly less however because nothing ever works at the maximum theoretical speed.
No, if the data is not in the cache, as then the hard drive will have to search for the data.  and that's where those other factors come in.  if it so happens that the data is found immediately, most current hard drives will be able to read and send that data at around 40 to 60MB/s.
0
 

Author Comment

by:swift87
ID: 8184519
thanks Naito u were the closest of all and that last paragraph ,was exacly what i meant.

but still, i'd rather have a site to explain it , as there it would be explained more widely :) .
and i'll tell you why,

because when i used HDtach, the graph shows a very unclearly schema ,as it showed at the this link:
http://planet.nana.co.il/iso9660/hd%2Egif 

it starts at ~ 15000 goes up to ~ 25000 and downs gradually
to ~ 22000
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LVL 1

Expert Comment

by:Naito
ID: 8184630
that's an effect of the data density between the centre of the hard drive and the outer edges.

sorry for lack of link, there's lots but I just can't find them.
try this tho, it'll explain the HDtach result somewhat
http://www.lostcircuits.com/advice/satacuda/

thanks for the points!
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