The goal of this article is to show how modern hard drives can read and write data. From this article you would know about zones, tracks, sectors, servo system, variable density and some over stuff.
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As you know information stored on platters and itís written by heads while platters are spinning. As any device which can store and retrieve information a hard drive needs to write data by special rules in order to read it back. HDD writes certain pieces on information in certain time frames, so to read data back HDD just calcs and synchs time, reads the pieces of information in certain time frames and decodes them. Letís imaging generator which generates pulses in certain and equal periods of time, with each pulse writing head writes piece of information in certain format. If frequency of pulses in such generator is constant then all time frames to write data on the drive will be the same and it will be very easy to write data on platters and read it back.
Thatís how it would look on a platterís surface.
Black dots represent the beginning of writing with each pulse from the generator. The red circle represents path (one of them) which makes writing head during writing. Black circles represent inner and outer diameter of a platter (ID and OD).
As you can see if the frequency of writing is the same for the whole surface of the platter but the density of writing wouldnít be the same because the further head goes toward OD the faster is linear speed of the platter in that point (even if the radial speed of the platter is constant). The density which lies on the path of the head measured in Bits Per Inch or BPI. The path which makes head when flying above the surface called Track.
And here comes a problem
As you can see on this graph with one writing frequency drive looses almost half of the platterís surface.
What would be if we virtually divide platter on two rings. And the outer ring would have twice higher writing frequency than the inner ring.
Letís see how it helps:
I highlighted our imaginary rings with different colors. Frequency in outer ring is twice higher than before which helps to fit more data in it.
This is how it would be on the graph:
Imaginary rings with different writing frequencies called Zones. Zone0 represents outer ring and Zone1 represents inner ring. Zoning helps to fit more data on the platterís surface and makes platters usage more efficient.
Modern hard drives can have around 15-20 zones which makes platters usage very efficient.
But if you take real zone table from a drive and make a graph, you would see that this graph has a strange shape.
Something like this:
BPI is not even in different zones. Why would this happen? Actually it is a bunch of problems.
BPI drops a lot towards OD and there are two main reasons for this feature.
First thing: in order to keep BPI high drive has to increase writing frequency toward outer diameter. High frequency makes read channel work harder when drive reads data back. Beyond certain frequency limit read channel just cannot distinguish data pieces and starts making too many errors. Such errors ratio called Bits Error Rate or BER. BER is a one big nightmare for each and every HDD manufacturer.
To keep decent data consistency level manufacturers have to keep BER as much flat as possible regardless of the disk radius. And this leads to some cuts in frequencies toward OD.
Second thing: tracks actually have different width. They narrow toward OD and may narrow a little bit toward ID (which also causes small BPI drop toward ID). How would this happen, we know for sure that writing head has a constant width?
Yes, writing head width is constant but the angle of writing head to a track tangent is variable and depends of the disk radius.
With less width track delivers less magnetic power to the reading head which makes data harder to distinguish and to make reading stable drive has to use lower writing frequencies on narrower tracks.
Letís talk about another density which called Tracks Per Inch or TPI. TPI measures how many tracks can be fitted into each inch of the disk radius. TPI is also non-linear and it also drops mostly toward OD. Why would this happen? Well, the thing is tracks are not round, with different width and they may be non-concentric and which causes many problems.
Take a look on the next picture:
Tracks have such a shape because of many factors. In general all these factors divided in two groups. The first group has a name Repeatable Runout or RRO, examples of factors of this group are: platters shift and platters geometry irregularity. The second group has a name Non-Repeatable Runout or NRRO and examples would be: vertical and horizontal platters vibrations/noise and air fluctuations. Because RRO and NRRO higher on OD (radius is bigger) driveís manufacturer has to increase space between tracks toward OD which drops TPI. Also as we know toward OD tracks are narrower and this feature drops TPI too.
Because realistic tracks are not really round heads need to move left and right a little bit to stay in the middle of a track when itís necessary. To do that drive generates Position Error Signal or PES which shows how far offtrack heads are in a specific moment of time.
To generate PES and move from track to track or seek any track or even tune up rotation speed drive uses special coordinate system which called Servo System. Modern drives use embedded servo system which means servo system information located on all surfaces evenly. Servo System can be written only on a factory and cannot be fixed or re-written during driveís regular work and if Servo System got severely damaged drive is basically dead which is not very cool but thatís the way drives work.There are three common ways to write servo on the platters:
Next picture will show you example of servo system:
Black dots represent servo sectors. The red line represents servo-track and the green line represents servo sectors with the same number one all tracks, such a line called a Wedge.
We have new word here - a Sector: sector is logically completed piece of data; sectors of similar type usually have the same logical length. Servo sectors also called Servo Frames or Servo Marks.
As you can see servo sectors written with one predetermined writing frequency and thatís the main rule of servo system. Using this knowledge drive can tune up rotating speed and also synchronize internal clocks with data frequencies in each zone. On modern drives frequency of servo sectors can be around 100 KHz. Usually there are 200-300 servo sectors per servo-track but as you also can see all the servo-tracks on particular drive (actually all drives of particular model) have exactly the same amount of servo sectors and in addition all servo sectors in a particular wedge have the same sector number. This knowledge helps when drive needs to know current position.
Real life wedges actually donít look like lines they look more like arcs. Itís because when HSA or STW moves to create next servo-track, platters turn a little at the same time.
Take a look on next picture:
This is how real life wedges would look on a platter.
What is inside of a servo sector? Letís see:
This is usual servo sector. Format of the sector may vary in different models.
AGC preamble or just preamble itís a signal written with constant frequency and amplitude. Itís used for automatic gain control Ė preparing drive to receive information from servo sector.
AM&Index stands for Address Mark and Index signal. AM itís a special signal which basically marks the beginning of Gray Code field. Index signal located only in servo sectors of wedge number 0 (the other sectors have only AM), so such the signal is generated only once per revolution. Using Index signal drive can easily tune up rotation speed or read the whole track into an internal buffer.
Gray Code field encodes servo-track number. The servo-track number encoded with redundancy and with Gray code (special noise immunity code). Drive needs such encoding for very fast and consistent reading of a servo-track number, even in some seek modes. All servo sectors on a servo-track have the same servo-track number encoded.
Signals A,B,C,D also known as position signals Ė special signals written with constant frequency and amplitude but some of them written offtrack (see picture below). Drive uses A-B-C-D to generate PES.
Padding marks end of servo sector Ė itís a signal with constant frequency and amplitude.
Next picture will explain how drive uses A-B-C-D for positioning on the center of a servo track:
This picture shows three servo sectors from the same wedge.
Servo tracks on this picture shown w/o space in between but all tracks have some space between each other. Imaging that shown tracks include actual tracks and the empty space.
As you can see some of the A-B-C-D signals written offtrack and to make that happen STW or a drive writes each servo track in several passes.
For example, drive needs to stay in the middle of Servo Track N: Signal A should be read with half amplitude, considering increasing amplitude when head moves to the right off the track. Signal B should be read with half amplitude considering increasing amplitude when head moves to the left off the track. Signal C should have full amplitude and Signal D should have amplitude close to nothing. Thatís how drive knows current heads position.
Now letís talk about data. How drive stores it? The user data stored in data sectors and data sectors grouped in data tracks.
Next picture shows how a data sector would look on a modern HDD:
AGC preamble is used for Automatic Gain Control. Sync Mark is a special field with constant digital info in it. Because itís a predetermined field and itís the same in all data sectors drive can use this to tune up PRML decoder (Modern hard drives use PRML encoding/decoding to store data on platters) also drive knows that data goes right after sync mark and drive uses it for data decoding synchronization. User data may be followed by several fields, in this sector data followed by DIF - Data Integrity Field, IOEDC Ė Input/Output Error Detection Code and ECC Ė Error Correction Code. Data sector may be closed by padding field.
ECC detects and corrects on the fly (If possible) errors which may occur when drive reads data sector from platterís surface.
IOEDC field checks data when it goes from driveís formatter (R/W system) then through de-scrambler until it outs driveís cache memory and goes to driveís interface frame buffer.
DIF field checks data when it goes from the frame buffer then through interface cable and finally to the host (DIF check available only on certain interfaces, like SAS or FC). And even for more data protection drive generates CRC when data goes out of the frame buffer and through the cable. All modern interfaces support this CRC check feature. This CRC is not written into data sector but generated on the fly.
As you see you never can be too paranoid to protect your data.
When host sends data to a drive to write, the actual data will not be written as is. It will be scrambled. Usually that would mean data will go through randomizer and RLL encoder, why? Letís imaging that you want to write one sector with only zeroes inside and letís imaging drive writes it as is and then trying to read it back. And here is the problem Ė drive may lose synchronization when reading signal is not changing for a long period of time, drive will become blind and will fail to read data. To fix such a problem drive uses RLL encoder. RLL encoder mixes up data with redundant info and makes sure that critical for sync bunch of bits never occur. Randomizer before RLL encoding makes driveís life easier and drive can use less complex RLL encoders.
The ECC field is a vital necessity for HDD because data written with high frequency (up to GHz) almost on the border of driveís limits. When drive reads data back it is very possible that data in the sector would have minor errors and ECC makes it invisible correcting it on the fly. Such technology helps to fit as much data as possible. But heads for a drive cannot be made exactly similar; each head has unique electric, magnetic and geometric characteristics. And here comes another problem: letís say we have a drive with 4 heads inside and Head 0 can fit 1000 sector on a particular data track, Head 1 however can fit only 950 sectors on a data track with the same number but located on a surface for Head1; and it is very possible that Head 2 can fit only 900 sectors but Head 3 is a champion and can fit 1050 sectors. In order to keep writing frequency similar in a particular zone for all heads we would have to select Sector Per Track or SPT as 900 sectors for all heads and that means we donít use all capabilities of a hard drive. Such schema were used in older HDDs but moderns drives can actually select unique writing frequency for each head in each zone and this schema called Variable Bits Per Inch or VBPI. Zone tables for VBPI drives store SPT for each head in each zone.
Same thing about data tracks some heads can write with larger TPI but some wonít and modern hard drives may use different TPI for different heads in different zones, such feature called Variable Tracks Per Inch or VTPI. Having VTPI on a drive means that empty space between data tracks varies from head to head and zone to zone and that leads to a logical conclusion Ė centers of servo tracks and data tracks do not match.
Take a look on the following picture:
Servo and data tracks on this picture include space between tracks.
Because space between data tracks likely will be wider than space between servo tracks, the data tracks look thicker. As we know drive can only position on a servo track because only servo sectors have A-B-C-D positioning fields but the goal is to read and write data, how drive knows where a data track is? It is simple Ė drive calcs it. VCM controller has ability to step aside from the center of a track by adding certain amount of current into actuator circuit. Such step called DAC (basically itís digital to analog conversion). Each servo track can be virtually divided longitudinal on particular number of DACs, like 64, 128 or 256 DACs. When drive needs to read a data sector, first drive calcs closest servo track to a data track thus jump to the center of data track from the servo track would be less or equal half of the servo track width. Then drive seeks desired servo track and positions to its center and after that drive shifts heads position on the calculated number of DACs to get positioned on the center of the data track. Red lines on the picture represent path heads making to get on data tracks.
HDD is a precise device and all inside works under watch of clocks. Drive reads every servo sector on a servo track even when drive needs to read or write data. Drive knows when next wedge will appear because drive calcs time. Reading each servo sector helps when drive needs to correct heads position. To do such a complex job drive separates servo sectors reading and data sectors reading. Servo sectors read by Servo Processor and special signal Ė Servo Gate tells Servo Processor when to read next servo sector the same Servo Gate signal pauses the Formatter. Formatter responsible for reading and writing data sectors. Formatter can generate two signals Read Gate Ė for reading data sectors and Write Gate Ė to write them. Gate signals are hardware related and this helps to start and stop R/W channel and preampís work when necessary. For example: until Write Gate signal comes drive will not be able to write, physically, because preamp will not allow heads to write. As you may notice because Servo Gate pauses the Formatter servo sectors can never be re-written in regular mode, if drive works properly.
Because reading each servo sector is such a critical job data sectors reading considered less important task and a data sector can be split into pieces by wedges. Now you know how drive calculates position for the center of data track and if data track for some reason will not be in calculated position Ė data will be unreadable thatís one of the reasons why hard drives sensitive for vibrations. If drive vibrates too much RRO and NRRO can cause offtrack for the data track positioning and itís especially dangerous when drive writes.
Another problem what drive has to fight is reading and writing cannot be done at the same time for the same track even if we do full duplex hardware support it is still impossible.
And the reason of that problem described on next picture:
Writing head is magnetically shielded from reading head and this causes a gap between heads.
When heads flying above track in the middle of a platter, centers of reading and writing head will be above center of the same track and drive can read and write on the same track without moving heads. But if, for example, heads move toward OD, centers of reading and writing heads will not be above the same track, one of the heads will be far offtrack (sometimes several tracks aside). Such a difference between centers of reading and writing heads called MR Offset. MR Offset is not a constant it varies from track to track; it also can be negative (for ID) and positive (for OD). MR Offset depends of track location and track width. Another parameter called PLO Delay or Read-Write Delay. It shows difference between Read Gate and Write Gate appearances. On different tracks PLO Delay will be different for Write Gate. As we know each head has unique parameters and MR Offset and PLO Delay also will be unique for each head. Drive stores averaged MR Offset and PLO Delay for each head at least for each zone in special tables called Adaptive tables or just Adaptives (adaptives also include some other critical parameters). Adaptives may be stored of the platters in special area called System Area or SA. System Area contains part of driveís firmware. And here is a dilemma: drive basically cannot read and write w/o adaptives but to get adaptives drive supposed to read SA first, what to do? Here is the solution: SA formatted with predetermined TPI and BPI in known place of platters and that allows drive to read SA w/o adaptives. Most of modern drives write SA in the middle of platters, so the drive would have ability to write SA w/o adaptives. But for data area drive has to calc DACs for reading and writing procedures separately, with MR Offset included. If adaptives got lost drive may become totally helpless and data may be lost forever. Some drives put part of adaptives into flash and that makes PCB unique for each drive.
At the end I want to tell you about serpentine reading/writing. Imaging that you have a drive and that drive has 4 heads and you want this drive to do linear reading or writing. Apparently drive has to use all the heads and itís understandable that you would like to get maximum speed from this drive. To minimize heads movements when drive needs to switch a head that would be really logical if drive would read all tracks from one head and then move to another head etc. But itís going to create a problem Ė the very first head most likely will have more loading than the other heads.
To minimize heads movements and equalize each head usage different drives use different clever schemas of reading/writing. Take a look on next picture.
The red line represents path which would make HSA during linear reading or writing. As you can see itís zigzag or serpentine movements. The number of tracks in serpentine could be different for different drives and on modern drives it also could be unique for each head on each drive (because each head can have different TPI and variable serpentine would help to decrease movements of HSA). Serpentine schemas vary from model to model and it also can be backward reading serpentine for odd heads but the idea is the same for any drive.
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