Author Topic: Write cache overload - the eternal issue  (Read 3768 times)

teomor

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Write cache overload - the eternal issue
« on: December 16, 2016, 08:57:03 PM »
I am downloading on an SD card (Transcend Jet Drive) and download speeds are always up and down. Looking at the statistics, I am seeing lots of write cache overload, precisely at the same times the speeds are going down. I've been fiddling with the cache settings, but I can't seem to get them to have any effect, expect getting worse :) I don't even know if I need to quit and restart qBittorent, of if it's enough just to hit OK..!?!
Any help is much appreciated!

Switeck

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Re: Write cache overload - the eternal issue
« Reply #1 on: December 16, 2016, 11:07:54 PM »
No need to restart qBT if you're changing the cache settings. The very few qBT settings that need a restart say so.

Try setting the cache duration to 600 seconds (from the default of 60 seconds), and set the cache size to 100-500 MB if you can spare that much ram. Above that is probably a waste and above 1000 MB risks a crash caused by qBT using too much ram at once.

You could enable the speed limiter switch at the bottom of qBT's screen when it's overloading so it can recover quicker. Maybe even change the default speeds for that from 10 KB/sec to 100 KB/sec so it's not completely crippled while doing that.

If it's not too much trouble, you might want to set up qBT to download to a temporary folder on the local drive and then auto-move to the SD card when done.

teomor

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Re: Write cache overload - the eternal issue
« Reply #2 on: December 17, 2016, 08:45:18 AM »
OK. Let me see how it goes.

PS: What about OS cache? Enabled or disabled? I never really understood how OS cache can work at the same time as the apps' own cache (i was a long time uTorrent user and never figured it out).

LE: It's still happening. And I noticed that total queued size doesn't go past 1.0-1.3 MB no matter what cache size I use.. That's not supposed to happen, right?
« Last Edit: December 17, 2016, 09:33:02 AM by teomor »

Switeck

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Re: Write cache overload - the eternal issue
« Reply #3 on: December 18, 2016, 12:06:52 AM »
The OS cache tends to double-buffer files and just eat lots of ram -- GB's even! I don't know when it'd ever help, so I tend to disable it except for testing.

The SD card's read/write speeds might be very low when trying to do lots of little I/Os at once.

The queued size should stay small if all you're doing is writing to the SD card, but if you're also uploading to peers it should grow well above 50 MB size.

You may need to use a temp download location on another drive and then have qBT auto-move the torrent/s when completed to the SD card...

teomor

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Re: Write cache overload - the eternal issue
« Reply #4 on: December 18, 2016, 01:10:40 PM »
OK. So the queue isn't actually cache? Whenever the queued size goes up to 1MB, the write cache overload also goes up towards 100%, so naturally, I thought the queued size is actually the cache...

Downloading on a different location isn't really an option for me. I regularly download just parts of a torrent, and never download the other parts, so then the torrent would always be incomplete, and would never move to the final destination..

PS: uTorrent has the same write cache overload issue, but the speed drops much less and the overload goes away much faster.

Switeck

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Re: Write cache overload - the eternal issue
« Reply #5 on: December 18, 2016, 11:02:22 PM »
Where are you seeing the queued size?

teomor

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Re: Write cache overload - the eternal issue
« Reply #6 on: December 19, 2016, 07:23:51 AM »
Statistics window :)

Switeck

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Re: Write cache overload - the eternal issue
« Reply #7 on: December 19, 2016, 10:28:09 PM »
The queued size is probably how much of the cache's allowed memory space qBT is using.
Trying to commit 10+ pieces to 10 locations on a HDD at once is going to cause overloads even at relatively low DL speeds -- like <1 MB/sec!

Sending data to a SD card through a serial interface (1-bit-at-a-time) bottlenecks really badly.