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Sheetanshu
1 Copper

Sectors in Zonned Bit Recording

What is the criteria for deciding the no of sectors/track in zonned bit recording ?

or

is it constant ?, then what ?

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19 Replies
Marty_Lonergan
1 Copper

Re: Sectors in Zonned Bit Recording

Hi Sheetanshu,

     I'm sure if you looking for a formula for designing the best layout of a zoned bit disk or a formula for calculating the number of sectors that a ZBR disk has?

If that latter, I believe most drive manufactures will give you a table that lists the number of zones, including the number of sectors each zone has and the transfer rate for that zone.

If the former, I believe it would depend on many factors like the recording bit density, trade-offs between constant linear velocity and constant angular velocity.

Just my 2 cents. I'm curious to hear what others who may have much deeper experience with different recording formats. Maybe someone who has some experience with disk drives directly?

   --Kindly, Marty

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Ev5
2 Bronze

Re: Sectors in Zonned Bit Recording

As far as I can tell there is no standardisation as to how drive manufacturers utilise ZBR or ZCAV. From what I've seen on some of the older drives the number of tracks per zone is the same yet on others its variable and may increase the further away the zones are from the edge of the platter. Typically as far as I can tell the number of zones remains more or less a constant on most drives at in and around 15 zones, but the number of sectors per track will increase incrementally the closer  to the edge of the platter the track is ( in some cases you will see double the amount of sectors on the very outer tracks compared to the inner most tracks). I think its pretty much down to the manufacturer to employ an effecient methodolgy as to how they use ZBR.

The attached document goes into a fair amount of detail on the subject...  some bed time reading for you...

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Marty_Lonergan
1 Copper

Re: Sectors in Zonned Bit Recording

Treating this just as a general discussion about ZBR, I have a couple thoughts:
  • Since the R/W head needs to stay suspended above the platters on a cushion of "ground effect" air, I'm sure that linear velocity at the R/W head is part of any drive's engineering design considerations. My guess is that constant linear velocity must be more complex as the drive motor would need intelligence about where how far out from the spindle the R/W head is as well as accurate velocity across the entire range that the motor is asked to supply. I'm guessing that it's much cheaper to produce a drive that has a constant angular velocity and then combine it with ZBR to not waste sectors (capacity). It can be argued that at least in the consumer or SMB market, that price is EVERYTHING when the end user is attempting to purchase the capacity they need.
  • I understand that for a given magnetic material there is a physical limit to the amount of polar state changes that one can pack into a particular 2 dimensional space. I believe this is where the encoding algorithm comes into play attempting to encode data in the least number of bits or (pole changes). I'm thinking about MFM or RLL as encoding schemes but I'm sure these must be old now and some new higher density scheme is what is used today?
  • I can only imagine that when a drive designer thinks about building a drive they are thinking of maximizing performance with cost. It would seem to me that performance can be impacted by seek times and keeping the number of track seeks to a minimum is a good strategy.
    Also CAV will keep costs and complexity down. I'm guessing that maximizing utilization of those outer high linear velocity tracks is also advantageous and ZBR can help with this.
    I can only imagine that ZBR comes at the cost of more firmware code that needs to track the more complex sector layout.

Thoughts?

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Sheetanshu
1 Copper

Re: Sectors in Zonned Bit Recording

Thanks for valuble replies

What i think , there should be something standard deciding factor , because during logical addressing mapping is performed & for that mapping algorithm we have to provide some standard algo , so there must be some deciding factor for sectors/tracks

?

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Re: Sectors in Zonned Bit Recording

Hello Sheetanshu and Marty (first bullet),

Correct. Each physical disk sector has a unique address (eg CHS), and some allocation for house-keeping. The address of the physical disk sector can be held within the sector itself. This enables the heads to verify their location.

Best regards, Richard.

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Sheetanshu
1 Copper

Re: Sectors in Zonned Bit Recording

Hello , Brother Richard

Yes , i am looking for the relationship between CHS & logical address,

logical address is assigned by the OS , and as far as i know same OS will work for different disk {having different sector/cylinder combination} , so question stands how Os will give the logical no. to a particular sector {having CHS info within the sector} ?

Thanks

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Marty_Lonergan
1 Copper

Re: Sectors in Zonned Bit Recording

Hi Sheetanshu,

    I believe that the abstraction of the CHS tuple to LBA address is something that is done on the SCSI drive controller board.

I belive that all OS SCSI disk drivers would use the same LBA address scheme and simply format the LBAs with their own propriatary volume format (NTFS, EXT3, JFS, ZFS, etc).

--Kindly, Marty

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Ev5
2 Bronze

Re: Sectors in Zonned Bit Recording

Hi Sheetanshu,

I may have mis-interpretted you're question. But just incase I haven't the LBA is defined as follows :

Cylinder x head x sector - 1   = LBA address

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Re: Sectors in Zonned Bit Recording

Hello Sheetanshu,

Good question.

Each physical disk block has a unique physical address - CHS. (Actually, in this scheme, CH identifies a unique track, and so CHS identifies a unique disk sector within that unique track). For an open systems disk, the sector size will be 512B (=1/2 KB).

When the drive is formatted, each disk sector (CHS) is assigned a logical block address. Also, when the drive is formatted, then it is formatted 'down' a cylinder, so logical block 0 -> logical block N-1 all exist within one cylinder (where N is the number of sectors in that cylinder). Logical block N is then located at the start of the next cylinder, and this next cylinder contains disk sectors N -> 2N-1.

When, for example, you build a filesystem in UNIX, the filesystem has a configured filesystem block size (referenced in the filesystem Superblock). Let's imagine that the filesystem block size is 8KB, which is 16 disk sectors. Each  file in the filesystem has an i-node that holds meta-data for that file. The i-node will contain the logical block address of  the start location of the first 10 (say) filesystem blocks for that file. The filesystem driver will request the first filesystem block, which, via the SCSI protocol, and through the SCSI controller/interface on the drive, will request 16 logical blocks from the drive (these 16 logical blocks will be sequentially contiguous disk sectors). The drive can map the logical block address requests to the unique CHS sector address. (For the addresses of the start location of filesystem blocks, for that file, after the 10th (in my example) the i-node points to a disk logical block address of an 'indirect' block, which holds the start location of, say,  the next 256 filesystem blocks making up the file. After this there are double and triple indirect blocks).

Also, to improve performance, when accessing the filesystem via block access, UNIX will utilise a filesystem buffer cache in memory on the file server.

(BTW, the drive is formatted 'down' a cylinder in order to improve the sequential throughput (MB/s) of the drive; the drive reads the sectors in the first cylinder, and this is purely an electro-magnetic operation. We then have the electro-mechanical operation of the moving the heads to the next track, and then we can read the next cylinder.  The random I/O performance of the disk (IOPS) is improved through the use of the command tag queuing technology).

HTH, Richard.

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