Tag: BPM

Disk drive density multiplying by 6X

Posted on by Ray in Disk storage, Storage, Storage density, Strategic Inflection Points, System effectiveness
Sodium Chloride by amandabhslater (cc) (From Flickr)
Sodium Chloride by amandabhslater (cc) (From Flickr)

In a news story out of Singapore Institute of Materials Research and Engineering (IMRE), Dr. Joel Yang has demonstrated 6X the current density on disk platter media, or up to 3.3 Terabits /square inch (Tb/sqin). And it all happens due to salt (sodium chloride) crystals.

I have previously discussed some of the problems encountered by the disk industry going to the next technology transition trying to continue current density trends.  At the time, the then best solution was to use bit-patterned media (BPM) and shingled writes discussed in my Sequential Only Disk!? and Disk trends, revisited posts.  However, this may have been premature.

Just add salt

It turns out that by adding salt to the lithographic process used to disperse magnetic particles onto disk platters for BPM, the particles are more regularly spaced. In contrast, todays process used in current disk media manufacturing, causes the particles to be randomly spaced.

More regular magnetic particle spacing on media provides two immediate benefits for disk density:

  • More particles can be packed in the same area. With increased magnetic particles located in a square inch of media, more data can be recorded.
  • Bigger particles can be used for recording data. With larger grains, data can be recorded using a single structure rather than using multiple, smaller particles, increasing density yet again.

Combining these two attributes increases disk platter capacities by a factor of 6 without having to alter read-write head technology.  The IMRE team demonstrated 1.9Tb/sqin recording capacity but fabricated media with particles at levels that could provide 3.3Tb/sqin.  Currently, the disk industry is demonstrating 0.5Tb/sqin.

Other changes needed

I suppose other changes will also be needed to accommodate the increased capacity, not the least of which is speeding up the read-write channels to support 6X more bits being accessed per revolution.  Probably other items need to be changed as well,  but these all come with increased disk density.

Before this technique came along the next density levels was turning out to be a significant issue. But now that salt is in use, we can all rest easy knowing that disk capacity trends can continue to increase with todays recording head technology.

Using the recent 4TB 7200RPM hard drives (see my Disk capacity growing out-of-sight post), but moving to salt and BPM, the industry could potentially create a 24TB 7200RPM drive or for the high performance 600GB 15KRPM drives, 3.6TB high performance disks!  Gosh, not to long ago 24TB of storage was a good size storage system for SMB shops, with this technology, it’s just a single disk drive.

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Comments?

Sequential only disk?!

Posted on by Ray in Storage, Storage architecture
St Sadurni d'Anoia - Cordoniu Grid - Shoes on Wires by Shoes on Wires (from flickr) (cc)
St Sadurni d'Anoia - Cordoniu Grid - Shoes on Wires by Shoes on Wires (from flickr) (cc)

Was at a Rocky Mountain Magnetics (IEEE) seminar a couple of weeks ago and a fellow from HDS GST was discussing recent advances in bit patterned media (BPM).  They had shown some success at 45nm by 45 nm bit cells which corresponded to about 380Gb/sqin a little less than current technology is capable of without BPM.  The session was on some of the methodology used to create BPM, some of the magnetic characteristics and parameters that BPM is capable of and some other aspects of the “challenges” inherent in moving to BPM.   I have written before on some of the challenges inherent in the the coming hard drive capacity wall.

But one thing that caught my interest was that even at the 45x45nm spacing, they were forced to use shingled writes to modify the bit cells.  Apparently today’s read-write heads are bigger than 45x45nm in at least width dimension.  Thus, they were forced to write two tracks at a time and then go back and re-write the 2nd (and 3rd) track on the next pass, and then the 3rd and 4th track, etc.  In this fashion they shingle wrote the whole media sample.

This seems to imply to me that the only way BPM can be written with todays head technology is sequentially.  What would this mean to the world of data processing.  There are already other media today that only support sequential, i.e, tape and optical.  And yet one significant advantage of disk at least in the past was that they could support random writes.

Today’s disk, at least SATA, high capacity disk, is already taking over from tape in the first tier backup solution. Any sequential only disk with even higher capacities would be a likely future revision of the current SATA disks in this application.

However there is more to data processing than purely backup.

How would we use a sequential only disk device?

Perhaps this would be an opening to support a hybrid disk like device, one that could support a limited amount of randomly written data while supporting a vast sequential address space.  This sounds like a new device architecture which would take some time to support but it’s not that different from data base and file system structures that exist today.

For file systems, file data is written sequentially through an contiguous sequence of blocks.  File meta-data, e.g., directory entries with file name, date, location, etc. is written randomly.

Database systems are a bit more complex.  Yes there are indexes similar to file meta-data above and tables are typically created sequentially.  But, table data can also be updated randomly.  It might take some effort to change this to be purely sequentially updated but that’s what would be needed to support such a sequential only disk.

Time for hybrid disks to re-appear

A couple of years back, when SSDs were expensive and relatively un-known, there was a version of disks for PCs and laptops that supported a relatively small amount of SSD and a large disk in a single 3.5″ form factor.  This was known as a hybrid disk and had some of the performance of pure SSD with the economics of disk.

Now BPM combined with SSD’s could be configured as a similar device with the SSDs portion supporting the random written data with the BPM disk supporting the sequentially written data.  But one difference between the old hybrid disk and this one is that the random data would only (maybe) exist in the SSD storage alone.

However, with BPM its possible that a portion (maybe a zone or two) of the disk surface could be BPM and a portion using (non-BPM) current technology.  This could also be done on a platter surface by surface basis if doing so on the same platter was too complex.  But such a device would also support hybrid random and sequential write operations without the need for NAND flash.

In any event, this is all relatively new and depends on the relative sizes of write heads and BPM bit cells.  But in order to get to 4Tb/sqin or higher technologists are talking BPM bit cells of 12nm by 12nm.  At that size shingled writes with todays head size would write span 8 or 9 tracks at a time.  Even taking current write head dimensions down by a factor of 5 would still leave one with a dual track width head.

One technique to reduce write size is to use thermally assisted recording (TAR) heads. This involves using a focused laser to heat up a single bit cell for writing.  The laser beam can be focused much smaller than the write head and could be used to isolate the writing to a single track.  Of course TAR heads are yet another new technology that would then have to be integrated into the new disk package.  But maybe this is the way to get back to a truly randomly written disk device.

Who knows this is all new technology and what’s published may not be a true representation of what’s available in the labs. But to get beyond todays capacity limitations there may be a new storage technology architecture on our horizon…