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Should you use special AV discs for STB's?


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Re: Should you use special AV discs for STB's? #21 Erik Slagter

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Posted 16 August 2019 - 09:44

I don't think the underlying technique is relevant (SMR/CMR) but the actual performance in the end. For multi-job compilation runs I could very well imagine a SMR disk would have a hard time to keep up. But on a STB, where almost all data transfer is sequential and where data can be prefetched (read) and cached (write) I don't think it's that interesting whether a disk uses SMR or CMR. SMR disks are particularly bad at read-modify-write cycles, and recording or playing a stream isn't such a thing.


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Re: Should you use special AV discs for STB's? #22 anudanan

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Posted 16 August 2019 - 10:40

Many users from the vti forum, openatv forum and also I have had problems with SMR HDDs. Sometimes especially during more parallel recordings or other operation on the HDD the SMR technology can produce recording interrupts. If you often record, show and delete movie files than the disadvantage of SMR can produce many additional read and write internal in the SMR HDD and then the HDD is not fast enought to handle the realtime requirement of the incoming DVBS stream..

 

That is true and only with SMR HDDs. CMR HDDs doen´t have that issue. It is not easy to benchmark that issue but it is real and true.

 

SMR HDDs are not recommended für STBs


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Re: Should you use special AV discs for STB's? #23 Erik Slagter

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Posted 16 August 2019 - 10:48

I think there are two separate and isolated issues. Also don't forget that most services only broadcast up to, on average, 8 mbps, which is only 1 mbyte / second. I think there are other issues with this particulair harddisk and I think the users should benchmark them first. Just fire up four instances of dd (or even better: dd_rescue) at the same time and watch what happens. Also keep in mind that the SATA interface of STB'es isn't full-fledged, it will never be as fast as a desktop (or server) PC.


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Re: Should you use special AV discs for STB's? #24 anudanan

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Posted 16 August 2019 - 11:11

I´ve done all these test with a ST2000LM015 and s ST2000LX001, these two are SMR and the disks are ok and first benchmarks look nice. But after some time parallel executing of dd writes  to the disk (to simulate parallel recorings) see the performance lacks and the issue.. If in paralle more dd reads are running than the writes are very slow and stuck somtimes.

 

Many users in the vti forum with the ST2000LM015 or other seagate SMR HDDs have sometime recording interrupts with parallel recordings ond all user don´t have that if the switch from SMR to CMR HDDs.

 

The problems are very big if you make cutting oder moving a movie in parallel to the recorings. With CMR there is no problem, with SMR there are bis problems.

 

 

If I´ve posted my first thesis about these probems  into he vti forum in january 2018 some user haven´t trust me and they said that can´t be true.  But now after many reports of other users they think my thesis is true.

 

 

You can trust me that is really true and not very easy to find it out or benchmark it. SMR HDD sare not suitable for  realtime requirements.


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Re: Should you use special AV discs for STB's? #25 nisseH

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Posted 16 August 2019 - 13:43

It was a very good ide to move the hdd discussion to a new STICKY thread.

 

Thank you so much for the link . It was the first website that have the information about CMR and TMR recordings.

 

 

If a ordinary 2.5" hdd is nearly as good as an 2.5" AV hdd as long as both hdd's using CMR recording, then it could also be a little better choice if you by a 2.5" laptop hdd.

All 2.5 hdd's isn't really made for use in a laptop. A laptop hdd doesn't crash if you for any reason happens to move the box during a recording, but a non laptop hdd can chrash if you move the box.

 

I have learn that the hard way. I bought a 2.5" Seagate Firacuda sshd to my laptop and it didn't last long before the first secors gone bad just bacause I had the laptop in my hands. A laptop hdd don't crash for that reason.


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Re: Should you use special AV discs for STB's? #26 MiLo

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Posted 16 August 2019 - 19:17

SMR disks need "idle" time to be able to do the internal data re-organizing. When recording, the disk isn't really busy, but is also isn't quite idle, so it may be that the housekeeper doesn't get to do its job.

If that's the case, it should be possible to tune Linux's cache strategy to write data in large bursts and stack up loads of dirty data. This is also commonly known as "laptop mode", though a lighter variant is probably going to be good enough.

This can be done runtime. I'd try the following:

# Set data time-out to 5 minutes
echo 30000 > /proc/sys/vm/dirty_expire_centisecs
# Don't bother to write back for 1 minute
echo 6000 > /proc/sys/vm/dirty_writeback_centisecs
These settings last until reboot (nice for testing). If they work as expected, you should cat /proc/meminfo and look at the "Dirty" number. It should steadily go up and up for about a minute, and then there's a quick burst where it's written out to disk.

This should give the disk just under a minute of idle time to do its housekeeping.
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