In episode #24 of the BYOB podcast we discussed traditional hard drives in detail. Read on for an overview of the subject and some great information on hard drive specifications.
Conventional Hard Disk Drive Sermon
Since 1956, the hard drive has gone from a mere 3.75 MB to 3.0 TB, and shows no sign of slowing down any time soon.
My Favorite Drives
Mass storage hard drives have changed over the years but the basic concepts still hold true for these spindle type hard drives. A spinning platter with a head that reads and writes data is the norm.
Some hard drives may have more than one platter, so think almost of a record player with multiple records on the turntable.
Courtesy Wiki Image
Hard Drive Form Factor
There are three mainstream types of hard drive form factors. These drive types are:
- Desktop – 3.5” form factor – Speeds vary
- Laptop – 2.5” form factor – Speeds vary
- Enterprise / Server – 3.5” form factor usually with 10,000 RPM speeds with SCSI or SATA connections. These drives are typically used only for server backplanes and usually not in a home consumer environment. Some hard drives like the WD VelociRaptor series are actually 2.5” drives encased in an aluminum heat sink. I have a 300GB model and they are now affordable.
Connection Types
There are two basic consumer types of hard drives. There are the older Parallel ATA drives and the newer Serial ATA drives. We call them PATA and SATA. Lets briefly review each one.
PATA
- AKA IDE/EIDE by WD, Seagate and others were PATA, eventually also called Ultra ATA
- Two devices on one ribbon of 80 wires (40 each device) with 18” length max
- If you had two devices on one ribbon you had to designate one as a master (devices 0 and 1)
- Connections were different for laptop drives and desktops
- Four devices max per standard PATA controller
- Four pin Molex type power connection +5 (red) and +12 volt (yellow) wires
- ATA-6 (the last mainstream adopted spec) is at 1 Gbit/s – PATA drives are just old technology.
PATA desktop and laptop drive
SATA
- 7 wires with one meter max length
- Connections are the same on laptops and desktop drives
- One cable per drive
- Fifteen pin power with +3, +5 and +12v
- SATA provides NCQ (Native Command Queuing) if both the controller/chipset and the hard drive support it. Now think of a record player again, if you tell your drive you want to get songs 4, 8, 2 and 1, NCQ will read the songs 1, 2, 4 and 8 based on the location of the data. Not the best analogy of course but I hope you get the idea. Just think of NCQ as a 10% performance boost in your system.
Jumper in use on WD Advanced Format SATA drive used in Windows Home Server
Wires
Red new. Grey old. Need I say more?
Drive Advanced Formatting
Standard drive blocks are currently 512 bytes long but, as you have heard so much recently, the drive manufacturers are moving towards 4,096 byte blocks. This new sector formatting is called Advanced Format sectors.
To maintain compatibility, Advanced Format media emulates a 512-byte device by maintaining a 512-byte sector at the drive interface. This is important and will eventually be covered in detail in another show. Check with the Forums in the meantime for information on these drives.
By the way, the blocks are done at the factory. This formatting is done during the manufacturing “low level” format. What we do in Disk Management is called “high-level” formatting.
GPT Formatting
Information on UEFI can be found at:
Realistic Drive Performance
Now, before I start talking numbers here, let me explain something about spindle hard drives:
There is a delay in getting to the data on the drive, and then there is the throughput or the actual writing of the data to the disk. There are many factors when trying to validate the drive performance. All things being equal, you can use the manufacturer specs but your mileage may vary. Tests and benchmarks assume your drive is already spinning and not asleep or in some power saving mode.
Western Digital WD30EZRS Drive Benchmark
Disk Delays
Three speed measurements Seek Time, Rotational Delay and Transfer Rate.
Seek Time
Think of that record player again. You want to play a song but it is not currently under the needle so the head has to move to it. That move delay is measured in milliseconds but depends on where the head is currently, so try and look for an average seek time. The lower the better, and you want to see around 9ms or less for performance drives.
Rotational Delay
Think of a record player yet again. Once the “needle” has moved you have wait for the start of the song to spin around again on the record. This delay is the same for each RPM class of drives (all 7,200 RPM drives are the same, 5,400 RPM drives etc) so it is really not a comparative number.
Side note: The average ms time has increased over time from 600ms in the 70’s to 20ms in the 80’s to around 4ms on drives currently. These times are based on the RPMs of the drive!
Everything is relative so keep in mind that math shows that the maximum rotational delay of drive spinning 7,200 RPMs a minute is 60/7200, which is 8 milliseconds.
Manufacturers can also say that the “average” of the rotational delay is half of that maximum number so when you see the “average latency” listed on a 7,200 drive is 4.20 ms then you know where it came from!
60 seconds ÷ 7,200 RPMs = .008 – Then take half of that!
Seagate has all of the delay specs on their website while WD does not list delay or random seek times!
Transfer Rates
When talking about transfer rates for drives people typically quote the maximum theoretical cable rate, which is just a simple formula based on the clock rate. You should know that due to file overhead and real life use you will not see those speeds.
In addition, those numbers do not include the drive latencies you will encounter as I just talked about.
Video File Transfer Speeds Between Drives
Serial Advanced Technology Attachment Spec
Three speed specs for SATA – They are confusing with the numbers, so here you go:
- Version 1.0: SATA 1.5Gb/s - Initial interface speed for 150MB/s data transfer rate
- Version 2.0: SATA 3Gb/s - Enhanced interface speed for 300MB/s data transfer rate, backward compatible
- Version 3.0: SATA 6Gb/s - Most recent interface speed increase providing 600MB/s data transfers and backward compatibility
Want to fully understand the above I/O (input/output) SATA specs?
Here is another math problem: The Version 3.0 spec is 6 Gigabits. That is 6,000,000 bits, but divide it by 8 to get 750 MB (750,000). But you can only get 80% due to 8b/10b encoding, so you end up with 750 MB x .80 = 600MB which is the spec that manufacturers put on their pages.
Disk Buffer
Per the Western Digital Knowledge Base “Cache memory is the data buffer or cache between the hard drive and the actual platters in the drive where data is temporarily stored. Access to data in the memory cache is much faster than accessing data on the platters in the hard drive. The larger the memory cache, the more data can be stored which can be accessed faster.”
In preparing this sermon I had to search to really find any performance data that relates to the disk buffer. The performance distinction between disks that have over 8MB is not necessarily a determining factor when buying a drive. Most buffers were at 2MB in 2001 and have worked their way up to 64MB for the latest high-end drives available. Even most consumer drives now come with 16MB or more so the performance comparison it is not really an issue.
You probably will not notice a performance difference with disk buffer amounts. Again, I could not find any reliable data for buffer performance and will just reference the WD quote above.
Power Consumption and Sound
Power is rated in watts. Green drives run fewer watts. Black Caviar and Seagate XTs run more watts. There is also a relationship to watts and heat, so more watts usually means more heat.
If power is important then do the math (watts x 8.76 x kWh cost) for a yearly cost. For example, the 9 watt Seagate Barracuda XT at 15 cents a kWh will cost me $11.82 per year. It would cost me half that in Canada, right Diehard?
Watts it all about?
Sound could also be a factor. More RPMs means more sound. In addition, desktop drives usually make more noise than laptop drives.
Sounds good to me?
Reliability
MTBF is often used to gauge drive reliability. A vendor-quoted MTBF implies that half the drives in a large population will fail within that time of operation. This does not mean that you will get those hours out of your drive.
Another way to check a drive manufacturer’s reliability is with the Annualized Failure Rate. This rate is the relation between the mean time between failure (MTBF) and the hours that a number of devices are run per year, expressed in percent.
Both ways can be irrelevant to your situation or system so just backup your data!
Conclusion – Overall Drive Performance
Get the best drive you can for the money. Find a brand you like and do the research. Check the specs.
For example:
- Using sustained transfer rates for comparison is good if you are going to use your drive for massive storage and file transfers but may not be indicative of your actual performance
- Latency figures should all be about the same for each RPMs category so watch them
- Random read and seek times are a great way to see how daily usage will be affected
- I/O data transfer times are theoretical and nowhere near realistic – Overhead
- If power is important then compare watt ratings
- If sound is important the check the dB ratings
And there you have it.
References
| WD Caviar Blue | WD Caviar Green | WD Caviar Black | |||||||||||||||||||||||||||
|
|
|
![clip_image015[1]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0151.gif){kind=small}
![clip_image015[2]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0152.gif){kind=small}
![clip_image015[3]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0153.gif){kind=small}
![clip_image015[4]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0154.gif){kind=small}
![clip_image015[5]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0155.gif){kind=small}
![clip_image015[6]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0156.gif){kind=small}
![clip_image015[7]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0157.gif){kind=small}
![clip_image015[8]](http://homeservershow.com/wp-content/uploads/2010/12/clip_image0158.gif){kind=small}
Recommended Comments
There are no comments to display.
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now