A lifetime ago I had to run cables for DEC VS100s, which were graphical displays that connected to VAX 750s. (these were the ones the X windows system was developed on, but I disclaim all responsibility for that...) The machine room was separated from the terminal location by about 100 feet of fairly full conduit, and I swear the cables were coated in the sort of sticky rubber that they use on rock climbing shoes. I think there was a lot of Kevlar or something on the inside of the cable, because we never broke one.
10 posts • joined 19 Dec 2014
There ain't no problem that can't be solved with the help of American horsepower – even yanking on a coax cable
Western Digital shingled out in lawsuit for sneaking RAID-unfriendly tech into drives for RAID arrays
Re: Storm in a teacup?
I'm curious as to why the rebuild actually fails.
Although contrary to popular belief drive-managed SMR[*] isn't great for most sequential write patterns, if you stream enough sequential data (100-300MB minimum, I would guess, depending on the drive generation) the ones I've played with will eventually go into a streaming mode where they're as fast as normal drives.
I should go and look at the original reports - I'm going to guess that the rebuild was interrupting its sequential writes, and then wasn't able to handle the multi-second write latencies that were the eventual result.
[* the other SMRs are host-managed - they report errors if you try to perform non-sequential writes. They don't sell those through normal channels, probably because they figure we'd RMA them for working as advertised]
Re: Storm in a teacup?
In my mind, the issue is that they branded them as NAS drives, so people went and put them into RAID arrays. RAID5 is just about the worst possible thing I can think of that you can do with drive-managed SMR.
For that matter, last week I just gave up on a DM-SMR drive I was using for my time machine backup because I was getting annoyed at how slow and noisy the hourly incremental backups were getting. Not gonna ID the vendor because it was a repurposed engineering sample, and the final shipped version might not be quite as crappy...
The code of conduct that Rafael has so much trouble with (https://llvm.org/docs/CodeOfConduct.html) basically requires professional conduct in official LLVM forums, although it's phrased in slightly touchy-feely language.
Basically it equals "don't be an asshole", and requiring that of someone in a professional context is evidently considered "restricting their freedom of speech" nowadays.
Re: Looked at the other way
Is someone who works part-time at Dunkin Donuts and part-time at Starbucks an employee of both? Of course.
Is someone who works full-time for a company and skips out during the day to work a part-time job somewhere else an employee at both places? Yes, the fact that he's skipping out is an issue between employer 1 and their employee.
Is someone who decides on a ride-by-ride basis whether to take an Uber passenger or a Lift passenger an employee of both? I think the appropriate answer is "yes, and if Uber and Lyft want to avoid both being on the hook for paying minimum wage, they'd better figure out a legal way to talk to each other about it."
HDD/Flash and technological evolution
"Will NAND flash replace disk?" isn't really a valid question - the question is "(when) will NAND flash replace disk for application X?", and there are hundreds of applications out there with different answers.
If disks weren't increasing their capacity/price ratio as fast as flash, the answer would be simpler - given enough time, flash would be better for everything. But they're increasing at about the same speed, so the most storage-hungry applications are going to use disk far into the future, while others tip over to flash sooner. In particular, for apps with bounded requirements, once flash becomes cost-effective then it's all over for disk, because you only get the cost/GB that disk offers if you need an entire disk.
For iPods the answer was 2005, which curiously happens to be the year that 2GB of flash became as cheap as a micro disk drive. Every year after, instead of increasing the capacity of their base model as disks grew, they could keep it the same and pay less for flash.
For a wider market the transition is less abrupt - e.g. what's been happening with laptops in the last 9 years since Apple introduced their 2nd-gen Macbook Air. (about 45% of laptops are expected to ship with SSDs this year; the rest are evidently sold to 4chan trolls who need the extra storage for all the porn they download...)
For just storing lots and lots of data (like that Microsoft OneDrive account you haven't touched in a couple of years, or the Dropbox folder from a project you finished last year) hard drives are going to remain king for years and years, possibly becoming weird and specialized in the process because they don't have to remain plug-compatible with your old machine.
Finally, flash has had the speed advantage since it was introduced; if you have a small amount of data and you need it to be fast, you're stupid to put it on a disk. However as the size of that data grows it starts becoming cost-effective to play all sorts of caching and tiering games, so that you can use disk for capacity and flash for performance.
RTFWP (Read The Fine White Paper)
George Tyndall, “Why Specify Workload?,” WD Technical Report 2579-772003-A00
Basically the heads are so close to the platter that they touch occasionally. To keep the drives from dying too quickly, the heads are retracted a tiny bit when not reading or writing. From the article:
"As mentioned above, much of the aerial density gain in HDDs has been achieved by reducing the mean head-disk clearance. To maintain a comparable HDI [head-disk interface] failure rate, this has required that the standard deviation in clearance drop proportionately. Given that today’s mean clearances are on the order of 1 – 2nm, it follows that the standard deviation in clearance must be on the order of 0.3nm – 0.6nm. To put this into perspective, the Van der Waals diameter of a carbon atom is 0.34nm. Controlling the clearance to atomic dimensions is a major technological challenge that will require continued improvements in the design of the head, media and drive features to achieve the necessary reliability.
In order to improve the robustness of the HDI, all HDD manufactures have in the recent past implemented a technology that lessens the stress at this interface. Without delving into the details of this technology, the basic concept is to limit the time in which the magnetic elements of the head are in close proximity to the disk. In previous products, the head-disk clearance was held constant during the entire HDD power-on time. With this new technology, however, the head operates at a clearance of >10nm during seek and idle, and is only reduced to the requisite clearance of 1 – 2nm during the reading or writing of data. Since the number of head- disk interactions becomes vanishingly small at a spacing of 10nm, the probability of experiencing an HDI-related failure will be proportional to the time spent reading or writing at the lower clearance level. The fact that all power-on-time should not be treated equivalently has not been previously discussed in the context of HDD reliability modeling."
Re: Change in Flash technology to eliminate finite write lifetime?
Until recently the *only* goal of flash designers was to deliver more bits for less money - until maybe 3 years ago maybe 95% of the flash that was produced went into iPods, cell phones, thumb drives, and SD cards, with SSDs accounting for a total of 3% of flash production. In the 10 years before that point, flash performance went down by nearly a factor of 10, and lifespan of low-end flash went down by far more.
A lot can change in 3 years, though, and there is now enough demand for flash in performance-critical roles to at least halt this decline. (whether enough customers will actually pony up the money for higher-performance chips to make them profitable is debatable, though.)
The reason we don't see flash devices failing left and right due to wear-out is because they've been getting bigger more rapidly than they've been getting faster. (or more accurately, than computer I/O workloads have been getting faster) The internal wear-leveling algorithms distribute writes evenly over the chips, so if you build an SSD out of consumer-grade chips with a 3000-write lifetime, you have to over-write the *entire* device 3000 times before the first chips start failing. (Sort of. A small number will die earlier, but there's spare capacity that can deal with that.) For a 500GB laptop drive, that's 1.5PB of writes, or 35GB/hr, 24/7, for 5 years. For a 1TB enterprise drive full of 30,000-cycle eMLC chips, you'd have to run it at 700GB/hr (200MB/s) for those 5 years to wear it out.
And wear-out isn't just a problem for flash anymore - the newest high-capacity drives are only rated for a certain total volume of reads and writes, due to head wear effects, which works out to a shorter lifespan than flash. (and reads count, too, while they're free on flash) See http://www.wdc.com/wdproducts/library/other/2579-772003.pdf for the gory details...
Of course they're complicated
Unlike ancient drives with fixed numbers of sectors per track, and thus varying areal density, current drives with ZCAV (google it) try to achieve constant areal density by varying the number of bits on each track. Since bits come in 4KB sectors you're never going to achieve true constant areal density, although at 1-2MB per track, +/- 4KB isn't a big difference.
However it's quite possible that what was being referred to is the difference in areal density between *heads* (or equivalently, surfaces) in a modern drive. Different heads in the same device can have wildly different (e.g. 25%) areal densities - google "disks are like snowflakes" for a very interesting article from Greg Ganger and the CMU PDL lab on why this is the case. A simple test in fio can show this behavior - e.g. latency of 64KB sequential reads across the raw disk LBA space. You'll see the disk serpentine pattern, with different speeds for each surface.