Write Speed on External USB C SSD is Slower Than Read Speed

ondert

macrumors 6502
Original poster
Aug 11, 2017
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On my wife's iMac (2017 4K w/ Radeon Pro 555) we're using Western Digital My Passport USB C 256gb external ssd as a boot drive since the internal 1tb hdd is ridiculously slow.
However, on Blackmagic Disk Speed Test I've seen the write speed is quite low compared to the read speed.
The ssd uses USB 3.1 port at 10Gbit/s. Is it related to TRIM? Is there anything I can do?
Thanks

Ekran Resmi 2020-02-16 12.13.24.png
 

vertical smile

macrumors 601
Sep 23, 2014
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TRIM is the first thing that comes to mind.

How long did it take to have the slow write speeds since setting it up? Do you have access to any older Macs?

I have a theory on something to do to correct it, but I haven't had any of my USB SSDs' write speeds slow down yet to test it.

Older versions of Disk Utility had a lot more utility to them before they were nerfed, I think starting with El Capitan. The older version had many useful features that were removed.

One feature that could potentially be useful for the situation that you are is "Erase Free Space".

I don't know if it would work, but I thought that using this feature could be like a manual TRIM.

Even if it did work, you wouldn't be able to simply boot up into an old enough OS version that had this feature due to your iMac being relatively new.

If you had an older Mac, you could try it by putting your iMac in Target Disk Mode and connecting the two Macs.


If it is TRIM related, another fix would be to put your SSD into a Thunderbolt enclosure (or get a Thunderbolt SSD). You can enable TRIM with Thunderbolt, but not USB.
 

Fishrrman

macrumors Core
Feb 20, 2009
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Re "TRIM"...

I doubt it has ANYTHING to do with the speeds you're seeing at all.

The read speeds are EXACTLY where they should be with a USB3 external SSD.
The write speeds on an external USB3 SSD often vary with the drive's manufacturer, model number, and SIZE.
Smaller-capacity drives seem to be slower.

My guess is that the write speeds are about "all you're gonna get" from that particular drive. A DIFFERENT drive may yield better write speeds -- but remember, the reads aren't going to be any better than what you're seeing now.

In the real world (bootup, app loading, etc.) it's the read speeds that count. The exception would be tasks that maintain huge "working files" that require disk in/out activity. What does your wife USE the iMac for?

Questions:
If you ignore the test software, how does the computer RUN in day-to-day usage?
Is your wife complaining about it?

A 2017 iMac should have USBc/tbolt3, so if you want "faster", you could try:
- USB3.1 gen2 drive in the USBc port, which will give reads in the 960MBps range and writes over 800
- tbolt3 drive which will give reads/writes up towards the 2,000MBps range.
 

vertical smile

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Sep 23, 2014
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Of course this may have nothing to do with TRIM, but I am thinking it might.

While there could be variations in manufacturers and sizes, I will post the disk test screen shots I have from two different SSD brands and sizes, and two HDDs in a SW RAID0 for comparisons to the OP's results.

All these drives are SATAIII drives, the SSDs are using a USB3 to SATA adapter, and the HDDs are internally from a MP which has SATAII.

Note: I didn't have any relatively new, single HDDs to test, so that is why I am using the RAID0 for comparison.


500GB Samsung 860EVO:
USB 3.0 500GB SSD Samsung 860EVO.png

120GB ADATA SU650:
USB 3.0 120GB SSD ADATA SU650.png


8TB WD Red HDDs in a SW RAID0 using SATAII:
SATAII 8TB HDD RAID0 WD Red.png
If you just halve the write speed, you get about 167MBps for write speed for a single HDD. Just slightly slower than the OP's USB3.1 SSD.

The SSDs are pretty consistent. The Samsung is well used, but spent most of its life internally with TRIM Enabled. The ADATA is pretty new and barely used. Also keep in mind that the ADATA is considered a "budget" brand.

The OP's write speeds are really low imo. It is barely faster than my HDD. I am not sure if that is age, brand, or TRIM related. Or maybe some combination of above.

Just looking up reviews on the drive the OP has, typical write speeds are much higher than what the OP is getting.


But, @Fishrrman is correct by asking if your wife notices the slower write speed and what she does with the computer. If she isn't complaining, I wouldn't worry about write speed. Read speeds are much more noticeable.

If your wife uses the computer for simple tasks, email, web, word processing, I wouldn't worry about the write speeds at all.

Just remember to have a back up.
 

theluggage

macrumors 601
Jul 29, 2011
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However, on Blackmagic Disk Speed Test I've seen the write speed is quite low compared to the read speed
Write speeds on flash SSDs are slower than read speeds - it is a fundamental feature of the way SSDs read, write and (can't) overwrite data.

Also, write speeds will get worse as the disc fills up - read speeds not so much.

TRIM is part of the solution, as is SSD garbage collection (on some SSDs you need to leave them powered on but not being used to let garbage collection happen). Also, avoid letting them get anywhere near 100% full.
 

vertical smile

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Sep 23, 2014
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Also, write speeds will get worse as the disc fills up - read speeds not so much.
While I had my 120 ADATA SSD still connected to my iMac, I decided to test this.

I copied 118GB over to it leaving to just 1.23GB free, just enough to do the 1GB stress test on BlackMagic. During the test, there was about 160MB of free space on the drive, so about 99.9% of the space was used while testing.

I tested it about 30 sec after I copied all the data, and it did have a huge effect on write speeds, almost as bad as what the OP posted. Also while testing, the write speed varied quite a bit, like 180MBps - 300MBps, not so steady like before.

Read speeds were slightly lower than when I tested it earlier, and slightly more varied, but nothing like the write speeds.

I was about to post my results on this thread, but got distracted with something, leaving my computer for about an hour. I decided to test the ADATA drive again when I got back, and the speeds improved a lot.

Almost back to normal, except the write speed isn't as steady as it was with it nearly empty. It varies a lot. Some writes might be lower than 200MBps, while most is above 300MBps.

I ran the BMDT for ten minutes without a break, and my write speeds would slow. If I would let it sit for a while, it seemed to improve the speeds for a while, but then the speeds would decrease again.

I have since deleted the stored data, and things haven't really improved much.

I would say that from my experiment, the speed of the SSD was hardly impacted by the disk being full, but impacted greatly from constantly using the disk.

This was only with one cheap SSD though....

read speeds not so much.
This was pretty much the case with my experiment.. While the read speeds did vary, it was just a relatively small difference.
 

ondert

macrumors 6502
Original poster
Aug 11, 2017
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Sorry for the late response, the issue still persists. My wife doesn't complain about the speed, I tried it and actually performs quite well.
Since it is usb ssd, it doesn't support TRIM as I know. Correct me if I'm wrong.
On my iMac, I use Jeyi TB3 nvme external closure and Sabret Rocket 1tb ssd. It supports TRIM since it uses TB3 interface.
Btw, the disk is not filled. More than half of it is empty.

I ran the software for a few times again, the read speed seems nice but the write speed is both slow and not persistent.
 

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Fishrrman

macrumors Core
Feb 20, 2009
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OP:

To reiterate (hadn't realized I had posted earlier):
There's probably nothing you can do to change this
That's "just the way it is".
The write speeds may be a factor of what kind of memory the drive is using, etc.
I doubt "TRIM" has much to do with this. It's just "slower memory cells".

Having said that, your read speeds are EXACTLY where they should be.
430MBps is about "the max" you can get out of USB3 and UASP (USB attached SCSI protocol).

So long as your wife isn't complaining, I wouldn't worry about it.
 

chrfr

macrumors G3
Jul 11, 2009
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OP:

To reiterate (hadn't realized I had posted earlier):
There's probably nothing you can do to change this
That's "just the way it is".
The write speeds may be a factor of what kind of memory the drive is using, etc.
I doubt "TRIM" has much to do with this. It's just "slower memory cells".

Having said that, your read speeds are EXACTLY where they should be.
430MBps is about "the max" you can get out of USB3 and UASP (USB attached SCSI protocol).

So long as your wife isn't complaining, I wouldn't worry about it.
USB3 drives can perform at well over 430MBps on a 10Gbps connection, like the OP's 2017 iMac has. The Samsung T7, for instance, benchmarks at over 700MBps for writes and over 900MBps in read performance. It accomplishes this by using an NVMe drive connected to a USB bridge. SATA based drives like the T5 will be slower.
 

Fishrrman

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Feb 20, 2009
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Yes, chrfr, you are correct.
But to do faster than 430mbps, I think he's going to need something like:
- an nvme blade drive
and
- a USB3.1 gen2 enclosure.

That combination will easily yield reads around 960mbps and writes in the 850mbps range.
That's what I get with the one I put together.
 

davidlv

macrumors 65816
Apr 5, 2009
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Kyoto, Japan
Yes, chrfr, you are correct.
But to do faster than 430mbps, I think he's going to need something like:
- an nvme blade drive and
- a USB3.1 gen2 enclosure.
That combination will easily yield reads around 960mbps and writes in the 850mbps range.
That's what I get with the one I put together.
Could you post some details as to the enclosure you mentioned and the NVME blade?
I am thinking of adding an external thunderbolt drive to my 2012 iMac which has Thunderbolt 1 ports, doable?
I remember reading a prior post of yours saying that Apple's Thunderbolt 3 to Thunderbolt 2 adapter won't let the drive get bus power. What the latest info on that?
 

Fishrrman

macrumors Core
Feb 20, 2009
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"I remember reading a prior post of yours saying that Apple's Thunderbolt 3 to Thunderbolt 2 adapter won't let the drive get bus power. What the latest info on that?"

Nothing's changed. I doubt it ever will.

You need USBc that is also capable of handling the tbolt3 protocol.
This means a 2017 or 2019 iMac. No adapter needed on these, just plug the tbolt3 drive in.
Earlier ones won't do.

davidlv --
If you have a 2012 iMac that has USB3, the best solution is a USB3 SSD.
Not much more to say about that.
 
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davidlv

macrumors 65816
Apr 5, 2009
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Kyoto, Japan
"I remember reading a prior post of yours saying that Apple's Thunderbolt 3 to Thunderbolt 2 adapter won't let the drive get bus power. What the latest info on that?"

Nothing's changed. I doubt it ever will.

You need USBc that is also capable of handling the tbolt3 protocol.
This means a 2017 or 2019 iMac. No adapter needed on these, just plug the tbolt3 drive in.
Earlier ones won't do.

davidlv --
If you have a 2012 iMac that has USB3, the best solution is a USB3 SSD.
Not much more to say about that.
Thanks. I didn't realize that the USB C ports on the later models were different. Good to know.
I have a Crucial MX500 512GB in a Transcend USB3 case. Using it as the boot disk, Speeds range frond 325-350MBps Writes to 425-450MBps Reads, which seems fast enough.
I have had trouble twice coming out of sleep, so currently I am booting from the 128GB Apple blade, which gets slower Writes at about 250MBps but good Reads at about 405MBps considering that the internal drive has trim, I have settled down to using that. Thanks again.
 
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