Whats the relationship between transfer speed and access time?

[sindows]

I know that with a faster access time, the faster an OS can get information off of a harddrive. However I'm not entirely sure what transfer speeds do. I'm sure that the higher the transfer speed, the quicker it is to write/copy files but does transfer speed have anything to do with a drive's ability to get information faster? ie load apps or the OS faster?

 

[Idontcare]

Have you used an ATM (automated teller machine) before?

So latency is the time it takes you to dig your card out of your wallet, put it in the machine, punch in your password, navigate the menu, and select the option that you want to withdraw $100. Bandwidth is how fast the ATM then spits out $100 in the tray.

Your total time to extract $100 was the bandwidth (transfer speed) plus the latency (access time).

Notice that your access time for the ATM is the same whether you withdraw $20 or $500. However the bandwidth of the machine, its ability to spit out $20 bills faster or slower, determines how long you must wait for your $500 to pile up in the tray.

On the computer a request to read a file (or write) incurs a minimum processing time equal to the access time (latency). In addition to that there is further processing time necessary to actually read the file or write the file, this portion goes faster if you have higher bandwidth.

 

[Rubycon]

Originally posted by: Idontcare
Have you used an ATM (automated teller machine) before?

So latency is the time it takes you to dig your card out of your wallet, put it in the machine, punch in your password, navigate the menu, and select the option that you want to withdraw $100. Bandwidth is how fast the ATM then spits out $100 in the tray.

Your total time to extract $100 was the bandwidth (transfer speed) plus the latency (access time).

Notice that your access time for the ATM is the same whether you withdraw $20 or $500. However the bandwidth of the machine, its ability to spit out $20 bills faster or slower, determines how long you must wait for your $500 to pile up in the tray.

On the computer a request to read a file (or write) incurs a minimum processing time equal to the access time (latency). In addition to that there is further processing time necessary to actually read the file or write the file, this portion goes faster if you have higher bandwidth.

I went to use the ATM and it was out of order. Must be Maxtor drive. :laugh:

 

[n7]

:laugh:

Or perhaps a newer Seagate

 

[Idontcare]

Originally posted by: n7
:laugh:

Or perhaps a newer Seagate

Yeah that's the atm that keeps your bank card at the end of the transaction and says you need to contact the bank's help center to get it back :laugh:

 

[Golgatha]

Originally posted by: Idontcare

Originally posted by: n7
:laugh:

Or perhaps a newer Seagate

Yeah that's the atm that keeps your bank card at the end of the transaction and says you need to contact the bank's help center to get it back :laugh:

My Dad actually had this happen at a BoA ATM machine last month.

 

[Rubycon]

Seagates of late are worrisome and somewhat Maxtoresque. Except for the Cheetah/Savio/Constellation line. Which in case you have not heard Constellation is their new "enterprise" class SATA product.

 

[sindows]

So I've done a bit more research on the topic and I'm come to the following conclusion which I would like to ask if its correct. Assume that I have a single folder with 10,000 photos. A faster access time with everything else equal means that I can view the thumbnails quicker whereas if I had a quicker transfer speed and all other things held equal, I could move the folder to another location quicker.

 

[taltamir]

maybe.,.. windows caches all the thumbnails into a single thumbs.db file (and mac has their own cache), and there is also fragmentation and other factors that come into play.

 

[Idontcare]

Originally posted by: sindows
So I've done a bit more research on the topic and I'm come to the following conclusion which I would like to ask if its correct. Assume that I have a single folder with 10,000 photos. A faster access time with everything else equal means that I can view the thumbnails quicker whereas if I had a quicker transfer speed and all other things held equal, I could move the folder to another location quicker.

Along the lines of what Talta is discussing - unless thumbnail images are created and cached, viewing the image will still be dependent on both access time (time it takes to locate the image's data) and the bandwidth (time it takes to read all of the image's data).

A copy involves the same work as an image view but adds on the extra work of an additional latency/access for locating the position of the freespace for the copy to be stored plus the write-bandwidth time it takes to write the copied data to the disk.

In all cases, even with cached thumbnails, access time + transfer time (= filesize/read_bandwidth) is the time it takes before you can view the images.

In the case of cached thumbnails the filesize is reduced, thus reducing the transfer time (smaller filesize/read_bandwidth = smaller transfer time) but the access time remains the same unless the thumbnails are also stored in a compiled catalogue which represents a contiguous file which has been defragmented.

 

[sindows]

Alright I give up now, all I know is that you want the lowest possible access time with the highest transfer rate. Thats pretty much as I understand it now...

So which of the following would be faster overall?
1. Same transfer speeds(say 80megs/sec) but different access times(13ms vs 16ms)
2. Same access time but different transfer speed(using the same numbers as example number 1)?

 

[Idontcare]

Originally posted by: sindows
Alright I give up now, all I know is that you want the lowest possible access time with the highest transfer rate. Thats pretty much as I understand it now...

So which of the following would be faster overall?
1. Same transfer speeds(say 80megs/sec) but different access times(13ms vs 16ms)
2. Same access time but different transfer speed(using the same numbers as example number 1)?

The answer depends entirely on the size of the files you are accessing.

If you are accessing 1GB files then the access time of either 13ms or 16ms is nothing compared to the 12.8seconds it will take to read the file (1GB/80MB/s = 12.8 seconds).

The total time it took your hard-drive to read the 1GB file (assuming it is defragmented) is 13ms + 12.8s = 12.813 seconds.

Had you been using the crappy higher access drive (but still with 80MB/s bandwidth) then your total time for reading the file (assuming it is defragmented) is 16ms + 12.8s = 12.816 seconds.

Notice how the access time is only changing that digit in the third decimal place?

Now let's say you aren't interested in doing things with 1GB files, but rather you've got bunches of 20KB files you like to read. In this case your access time remains the same (13 or 16ms) but the time it takes your drive to read the file is drastically lower as 20KB/80MB/s = 0.000244 seconds...that is a really small number, as in 0.24 ms or 244 µs.

Thus in this case the total time it took your hard-drive to read the 20KB file (assuming it is defragmented) is 13ms + 0.000244s = 0.013244 seconds = 13.244 ms

In other words the read time is now almost entirely dominated by the access time. A higher access time drive would markedly increase the time it takes to read the file, although the total time would be on the order of milliseconds so you won't likely notice a 13ms versus 16ms access time for a single file.

But if you transfer 1GB worth of 20KB files (~52,428 files) then the total time to read that 1GB of small files is now 52,428 x 13.244 ms = 694.37 seconds with the 13ms drive but increases to 851.66 seconds with the 16ms drive. Now the access time makes a huge difference but the bandwidth does not.

For 1GB worth of 20KB files whether you have an 80MB/s 13ms drive or a 800MB/s 13ms drive the time is only improved to 682.85 seconds with the 10x higher bandwidth drive.

But take that 1GB worth of 20KB files and transfer them with a crappy 30MB/s 0.01ms Intel SSD and suddenly your total time is a mere 34.66 seconds.

Am I making this any clearer or am I just muddying the waters with this example?