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Active time 100%, Avg response time >1000ms, Write speed < 7 MB/s

Thomas :

Aug 17, 2019

When I'm backing up from my internal SSD to the external HDD, things seem to become very slow as soon as the cache is full.

The details:

At the beginning, I have write speeds up to 50 MB/s, which might not be very fast for a disk, but I never saw transfer rates near those in PC magazine tests. 50 MB/s would be fine for me. In that initial situation, active time is about 20% and response time is <40 ms. I consider that as normal for a HDD.

In the "slow" situation, I see memory usage of 12.5 GB and 83.5 GB cached. As soon as all of my 96 GB are eaten up by the disk cache, things become very different: Active time is 100%, response time is >1000 ms (up to 7000 ms) and write speed is <7 MB/s (down to some kbs/s).

In both situations, CPU is ~5%. I am not performing any other work, especially not disk-intense work.

Is that normal? If not, what would I need to change to improve performance?

I have already uninstalled my virus scanner (ESET) and disabled Windows Defender.

Alex Pankratov :

Aug 18, 2019



I can't comment on "active time" and "avg response time" metrics, because I don't know what these are and where they come from exactly.

I can however tell you that the file cache shouldn't be swelling this much, nor is it a normal behavior. My guess would be that it's due to overly aggressive write caching in the driver that is specific to your device. It can also be some sort of quirk with the target drive enclosure - taking up a lot requests for processing, but being very slow with acknowledging them... or something similar.

In terms of the write speed - it depends. You can have USB3 drives that are actually running at USB2 speeds. You can have USB2 flash sticks that will give posted speeds for the first gig and then slide down to kbps range afterwards. It all really depends.

You can try and run your external drive through https://ccsiobench.com tests, see what you get in terms of peak bulk performance. Make sure to customize the test profile though and set "max run time" to 2-3 seconds.

Thomas :

Aug 22, 2019

Yeah, it seems I missed to post some information. Might have been too obvious for me :-)

a) All numbers are from Windows 10 Task Manager. It has a quite nice performance tab in the meanwhile.
b) I'm using eSATA to connect the external drive. Not sure if the USB issues apply there as well.

I ran the benchmark. The destination drive had a read rate of 25 MB/s to 135 MB/s. Similar for the write rate.

You'll find that Windows always uses the whole RAM (almost) as disk cache, if not needed by programs. And IMHO that's a good idea - otherwise the RAM would be unused and disk wear is increased.
On this 16 GB machine for example, cache is already at 5 GB, even I did nothing else but use Outlook and Firefox yet this morning.

Alex Pankratov :

Aug 23, 2019

You'll find that Windows always uses the whole RAM (almost) as disk cache, if not needed by programs.


All modern OSes are this way, however from what I understand the issue is that in your case it is being overly aggressive with caching data that's being _written_.

In terms of an advice - I really don't have anything constructive to say. Something somewhere is acting funny and bvckup2 in its stock config manages to trigger that. I haven't seen anything like this before, so your guess is as good as mine here :-/

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