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Support for shingled magnetic recording devices

Support for shingled magnetic recording devices

Posted Mar 27, 2014 23:05 UTC (Thu) by giraffedata (subscriber, #1954)
In reply to: Support for shingled magnetic recording devices by dlang
Parent article: Support for shingled magnetic recording devices

shingled drives have the same speed random reading as non-shingled drives

Right, so if you can find an application where you have large sequential writing, but fast random reading, that might justify shingled disks. If instead, you have random writing, you'll want a non-shingled disk, and if instead you have large sequential reading, you'll want tape.


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Support for shingled magnetic recording devices

Posted Mar 28, 2014 5:12 UTC (Fri) by Cyberax (✭ supporter ✭, #52523) [Link]

Tapes are waaaay too impractical. Besides, their density is not that great - the best models are just breaking 2Tb barrier per tape.

And you'd be amazed by the number of applications where you need fast access to immutable (or slowly changing) data. Even better, it's possible to use faster hard drives (or even SSDs) as a frontend for the slow shingled disks.

Support for shingled magnetic recording devices

Posted Mar 28, 2014 23:07 UTC (Fri) by giraffedata (subscriber, #1954) [Link]

There are many applications for which tape storage is practical. We know this because people are doing it.

Many of the applications that could tolerate shingled disk could also tolerate tape.

Density per tape cartridge isn't really the point. Cost is the point, and tape is cheaper per terabyte than shingled disk. Part of the reason for that is the storage density per tape drive is far, far greater than for disk drives. Data rate per drive is much greater too.

I don't think I would be amazed at the number of applications that are appropriate for shingled disk, but I also know that there are a lot of applications that aren't, and there are significant storage management costs in using different kinds of disk drives for different kinds of data. I suspect one would need more than a 20% differential in per-terabyte cost to justify that.

Even better, it's possible to use faster hard drives (or even SSDs) as a frontend for the slow shingled disks.

That's the bending over backwards I was talking about that I doubt is worth it for a 20% improvement. Have we ever seen people make such a disruptive transition for 20%? Would people have gone from floppy disks to CD-ROM for 20%? Or CD-ROM to DVD? Would people even have gzipped tar files for only 20%?

Support for shingled magnetic recording devices

Posted Mar 29, 2014 11:25 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link]

Well, tape is actually NOT cheaper unless you do it in a REALLY big way with tape libraries and robots. Even then it's only marginally cheaper than HDs.

And shingled disks are not that bad, it's not a completely disruptive transition. Sure, it'll require some additional engineering for the front-end write-through caches but it's not a big deal compared to tape.

So think about it - would you build a tape library with expensive robots and lots of tape or would you just prefer to buy somewhat slower hard drives?

Support for shingled magnetic recording devices

Posted Mar 29, 2014 19:15 UTC (Sat) by giraffedata (subscriber, #1954) [Link]

Well, tape is actually NOT cheaper unless you do it in a REALLY big way with tape libraries and robots.

Yes, that's what I was talking about. When I compare the economics of storage technologies, I think of large scale storage. With tape, there are thousands of cartridges and plenty of robots.

Even then it's only marginally cheaper than HDs.
The last figures I saw were 5x cheaper. That's total cost (not just e.g. purchase price of some box), and assuming a tape-friendly application. That seems perfectly believable to me, but if you know of a study showing otherwise, do tell.

So think about it - would you build a tape library with expensive robots and lots of tape or would you just prefer to buy somewhat slower hard drives?

I'm not sure what you're comparing here. Shingled disks aren't somewhat slower. Used right, they're the same speed as regular drives; used wrong, they're unusably slow. Since tape applications also work on shingled drives, the question would be, would you use something with 2 minute access time or just use slightly more expensive disk drives. Only since I'm claiming shingled drives are 4X more expensive than tape, that question is moot.

By the way, some of my data is on tape. My company backs up its general purpose filesystem to tape. It takes me 4 minutes to recover a lost file - 2 minutes to go through the interactive dialog and 2 minutes for the robots and tape drives to do their thing. Shingled disk would cut that to 2 minutes total. I can't imagine my company switching unless there is virtually no difference in the storage cost.

Support for shingled magnetic recording devices

Posted Mar 29, 2014 20:32 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link]

> The last figures I saw were 5x cheaper. That's total cost (not just e.g. purchase price of some box), and assuming a tape-friendly application. That seems perfectly believable to me, but if you know of a study showing otherwise, do tell.
No way. The last time we checked (in 2013) the price difference was about 2x. The major advantage of tape was reliability - tape cartridges themselves contain no sensitive mechanic parts, not price.

LTO6 tapes are about $40 per tape (2.5Tb) when bought in bulk. Maybe $30 if you are really big. Hard drives are around $50 per 2Tb in bulk.

And that's without considering the cost of streamers (multiple $$$$), tape robots ($$$$$) and the storage software solution (shockingly, there are no good OpenSource hierarchical storage managers).

Of course, HDDs need some kind of SAN, but they are cheap these days. AoE/iSCSI solution for 1000 drives capable of 100Gb throughput can be bought for just under $30k and doesn't need any fancy software.

Support for shingled magnetic recording devices

Posted Mar 30, 2014 10:22 UTC (Sun) by khim (subscriber, #9252) [Link]

Shingled disks aren't somewhat slower. Used right, they're the same speed as regular drives; used wrong, they're unusably slow.

And when used with GFS or HDFS which need random read access to it's 64 megabytechunks and kinda streamlined write access to these chunks (GFS gives you the ability to append-write files, HDFS does not even offer that today) they are fast.

Shingled disks are only on horizon today, but API which is basically custom-taylored for their limitations is more than decade old and there are thousands of companies and millions of drives which are used for such kinds of applications. End of story.

Support for shingled magnetic recording devices

Posted Mar 30, 2014 16:34 UTC (Sun) by giraffedata (subscriber, #1954) [Link]

And when used with GFS or HDFS which need random read access to it's 64 megabyte chunks and kinda streamlined write access to these chunks (GFS gives you the ability to append-write files, HDFS does not even offer that today) they are fast.

I think those are two examples of things that would require re-engineering to work with shingled disks, because neither writes in log-structured fashion today. The only reason I can think of that an existing disk drive application would write in log-structured fashion (fill the drive, or a large segment of it, from beginning to end) is to maximize write speed. But GFS and HDFS assume there is little writing. HDFS is specifically aimed at fast sequential reading of large files, which means it needs to keep files contiguous on disk, which is not possible with a log structured file system.

Support for shingled magnetic recording devices

Posted Mar 31, 2014 10:36 UTC (Mon) by dlang (subscriber, #313) [Link]

I think that you wildly underestimate how expensive random writes are today, and how much software works to avoid them

I've seen a number of packages that have the pattern of having large chunks of data, but new data is not written directly to those large chunks, instead new data is written sequentially to an 'updates' file, and periodically some other job comes along and re-writes the large chunks to include the changes from the updates files, and then deletes the updates files.

such systems would be perfect for shingled drives, they would just get their chunk sizes and alignments adjusted to match.

Support for shingled magnetic recording devices

Posted Apr 1, 2014 11:10 UTC (Tue) by ricwheeler (subscriber, #4980) [Link]

Not sure where you got the 20% number from, but the better way to think of this is that each kind of disk technology hits a plateau at some point. Further investment in that will not bring improvements in density.

Moving to SMR is effectively moving from curve for our existing technology that is about to plateau onto a new curve. The delta between the current curve and the new one does start small, but over time will take us to a significant density improvement. The slide shown by one vendor showed that eventual difference being closer to 3-4 times the density but that was more of a hand wave I would guess.

The drive vendors shied away from specific numbers, but they all agreed that SMR was a new foundation that other technologies will build on (not something that will be replaced in time).

Reading a crystal ball is hard, but is does seem like a promising technology to invest in :)

Support for shingled magnetic recording devices

Posted Apr 3, 2014 1:56 UTC (Thu) by giraffedata (subscriber, #1954) [Link]

Not sure where you got the 20% number from,

There was a paper from Seagate, which I believe is referenced earlier in this thread, that said the technology provided a 25% improvement in areal density. I read somewhere else that there's a 5% overhead for something else - metadata or guard bands or something, bringing the effective improvement down to 20%.

If that's just a prototype figure and the technology eventually gets to 3-4 times areal density improvement, that's a different story.

Support for shingled magnetic recording devices

Posted Mar 28, 2014 17:52 UTC (Fri) by Creideiki (subscriber, #38747) [Link]

Wouldn't that application be "every log-structured file system ever"? New data and metadata is written to the end of the log, and GC-copied to another zone as necessary. Keep a pointer to the currently valid root block in the random-write zone.

Support for shingled magnetic recording devices

Posted Mar 28, 2014 23:27 UTC (Fri) by giraffedata (subscriber, #1954) [Link]

I don't know what "GC-copied to another zone" is, but ordinary log structured filesystems are ideal applications for shingled disks. An application that works with a log structured filesystem on a traditional disk drive today could presumably use shingled disk instead for a 20% cost saving. Not counting any costs associated with having special disk drives provisioned for that application.

Support for shingled magnetic recording devices

Posted Mar 29, 2014 7:05 UTC (Sat) by Creideiki (subscriber, #38747) [Link]

That's probably my LISP bias showing, assuming everyone is intimately familiar with garbage collection strategies. A copying garbage collector partitions storage into two zones, using only one at a time, and when that one gets full copies only the live data to the other one, leaving the garbage behind to get overwritten in the next cycle.


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