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Noldor
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New External HDD very fast and unusual speed

Tue May 01, 2018 12:26 pm

Hello, here, I have a question for the storage experts here. I just got myself a new 2 TB external drive, type WD Elements SE, and there is something which puzzles me: this is the first time I use this type and its reading speed is significantly faster than any previous models. As I always do with new drives, I ran a surface scan of the drive and it took off with speed of 150-170 MB/s (previous WD models I possess start at about 115 MB/s and Seagate at almost 140 MB/s): but, what is most surprising is that it never slowed down towards the end of the platter, as all others did. As a result, it ended the scan also with around 150 MB/s, in just 3 hours 30 min, while other drives of comparable sizes took at least 5 hours.
The seller was one of the two biggest electronics retailers in my country so a scam (at least a deliberate one) is impossible. Could anyone tell me how come this drive never slows down towards the end of the platter? Did Western Digital implemented a way to avoid this in their SE models?
 
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 1:11 pm

Have you tried a straight-up file copy of a single large uncompressible file (movies work well)?

I suspect that *IF* the drive inside is a WD Blue - WD20SPZX with a relatively large 128MB of cache, perhaps the cache isn't getting exhausted by your "surface scan" and that's allowing it to maintain those high speeds.
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Waco
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 1:15 pm

Modern HDDs can run at upwards of 250 MB/s depending on the model, so the increase in speed isn't terribly unsurprising.

I'd be surprised if the speeds really didn't fall off over the breadth of the platter, though, since that's just a fundamental design limitation. Can you run something like HDTune that scans the entire drive and post the performance graph? The only way performance won't drop off by nearly 50% at the end of the drive is if it's short-stroked (but no manufacturer sells consumer drives configured that way).
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Noldor
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 1:28 pm

Waco wrote:
Modern HDDs can run at upwards of 250 MB/s depending on the model, so the increase in speed isn't terribly unsurprising.

I'd be surprised if the speeds really didn't fall off over the breadth of the platter, though, since that's just a fundamental design limitation. Can you run something like HDTune that scans the entire drive and post the performance graph? The only way performance won't drop off by nearly 50% at the end of the drive is if it's short-stroked (but no manufacturer sells consumer drives configured that way).


The software I used for that initial surface scan was HDTune, actually. I can't post the performance graph (at least not right now) because I did not thought of taking a screenshot, so you'll have to take my word for it: the graph was very uniform, speeds from 140-170 MB all across (on older drives, the speeds curve was very steady, but on the newer ones I own, there are small fluctuations - this last drive included).

DPete27 wrote:
Have you tried a straight-up file copy of a single large uncompressible file (movies work well)?

I suspect that *IF* the drive inside is a WD Blue - WD20SPZX with a relatively large 128MB of cache, perhaps the cache isn't getting exhausted by your "surface scan" and that's allowing it to maintain those high speeds.


Yes, I did. I copied around 120 GB on data on it and everything seems in order (meaning that I got no errors on it).
But I can't check the speeds that way because:
1. None of my other drives can transfer data faster than 130 MB/s;
2. It's the fact that the speeds don't drop at the end which is most weird, but the files I would be transferring now would be placed at the beginning of the platter. I would need to fill up the drive to check if the speed stays constant throughout.
 
Waco
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 1:38 pm

Ah, I wonder if you're interface limited in the external enclosure - that would certainly cover this type of behavior. It wouldn't be the first time the enclosure itself was limited to some speed that was significantly lower than the drive itself was capable of.

Does the random-access test in HDTune look like the curve you expect as the LBAs increase?
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Noldor
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 1:47 pm

Waco wrote:
Ah, I wonder if you're interface limited in the external enclosure - that would certainly cover this type of behavior. It wouldn't be the first time the enclosure itself was limited to some speed that was significantly lower than the drive itself was capable of.

Does the random-access test in HDTune look like the curve you expect as the LBAs increase?


I didn't do a random access test, though, just a full surface scan to check if it didn't come with any bad blocks (I had once a drive which came like that, 7 years ago, and since then I always scan them immediately after purchase). Should I perform that random-access test?
 
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 2:25 pm

Yeah, sounds like something else may be capping the speed of the drive.

Another (less likely) possibility is that this particular model has been "short-stroked". This is where the manufacturer intentionally disables the inner tracks on a drive and sells it as a lower capacity drive. Short-stroking results in higher average sustained transfer speeds and lower average seek times compared to a similar capacity drive that uses the entire platter surface.

Edit: I just did a quick test of STR on a 6TB drive I had handy. 230 MB/sec on the outer tracks, 145 MB/sec on the inner tracks. Granted this is a bit of an apples-oranges comparison since it is a higher capacity drive, but drives of similar vintage (this is a relatively new drive) will tend to have similar linear bit densities (and therefore similar STRs), with the higher capacity drives using more platters to get the extra capacity. So this is consistent with your drive being capable of >200 MB/sec, but something (either the USB bridge chip in your drive's enclosure, or some hardware/software component of your test environment) capping throughput at ~170 MB/sec.
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 2:41 pm

I came here to say 'short-stroked' too.

It's not cost-effective for WD to produce platters at varying areal densities so most of WD's non-helium, non-shingled drives have been at 1.2TB/platter since 2014. The 6TB model gets 5 platters at 1.2TB each, fully-stroked and anything larger than that uses helium tech for more platters and higher density.

The smaller sizes don't fit neatly into multiples of 1.2GB so they are short-stroked to meet standard market capacities, and that's probably why they've dropped the 5TB model (because it costs them as much to make as the 6TB one when using 1.2TB platters).

It's pretty likely that a 2TB drive made after 2014 actually has a couple of 1.2TB platters, but the firmware and controller are not designed to expose the last 400GB of capacity. As such, the last 17% of the drive's innermost tracks aren't used, so you don't see that precipitous drop in transfer rates like you would with a fully-stroked disk.

I don't think it's an interface issue keeping the transfer rates nearly constant, because 170MB/s isn't a rate that matches any bandwidth limits of USB, Firewire or eSATA. USB 2.0 transfers at around 38MB/s USB 3.0 is fast enough to hit around 440MB/s and Firewire800 gets around 65MB/s in my experience. 170MB/s is exactly where a lower-capacity mechanical drive ought to perform, IMO.
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 3:29 pm

Short-stroking by 10% isn't enough to account for how flat the STR is though. You'd still see significant degradation. I think you'd need to short-stroke by around 50% to get that sort of behavior.
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 3:41 pm

Chrispy_ wrote:
I came here to say 'short-stroked' too.

Even in a 50% short-stroke design, he'd see more falloff on the inner tracks. It's likely the controller in the enclosure that's rate-limiting him.

A quick random access test (the one that sweeps the LBAs in HDTune) will prove this out both in the increase in latency across the drive and the overall latencies that it reports.
Last edited by Waco on Tue May 01, 2018 4:02 pm, edited 1 time in total.
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 3:47 pm

Well, he says 150-170 at the start and 150 at the end, so I'm assuming there was some slowdown.
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Waco
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 4:02 pm

just brew it! wrote:
Well, he says 150-170 at the start and 150 at the end, so I'm assuming there was some slowdown.

Sure, but HDDs (especially small modern ones) don't have crazy short-stroked designs. If I were a betting man I'd say it's the enclosure, and the drive is capable of a lot more (like 200+) on the outside tracks.
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Re: New External HDD very fast and unusual speed

Tue May 01, 2018 4:44 pm

Waco wrote:
just brew it! wrote:
Well, he says 150-170 at the start and 150 at the end, so I'm assuming there was some slowdown.

Sure, but HDDs (especially small modern ones) don't have crazy short-stroked designs. If I were a betting man I'd say it's the enclosure, and the drive is capable of a lot more (like 200+) on the outside tracks.

I did say (in my previous post) that I thought short-stroking was a less likely explanation. :wink:
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