[mythtv-users] Wiki: Balanced Disk I/O

[Michael T. Dean]

On 08/11/2015 03:57 PM, Peter Bennett (cats22) wrote:
> On 08/11/2015 01:44 PM, Michael T. Dean wrote:
>>
>> a) it creates files with the least amount of fragmentation (this is 
>> the primary benefit)***
> A good point I had not thought of.
>
>> b) when hard drives fill up, it doesn't favor a single hard drive for 
>> every single recording, leading to all new recordings going to the 
>> same drive and all expired recordings coming from that same drive 
>> (meaning that before long, the system is expiring new recordings when 
>> there are tons of older--possibly Deleted and/or Watched--recordings 
>> that could be expired instead if the system would just choose another 
>> disk). If using Balanced Free Space or Balanced Percent Free Space, 
>> this is exactly what happens when disks fill up and autoexpire kicks 
>> in.  See 
>> http://www.gossamer-threads.com/lists/mythtv/users/556505#556505 
>> (note, too, that neither Combination, nor Balanced Disk I/O will 
>> avert this issue if the user never records multiple shows at the same 
>> time--in which case, the user /must/ rebalance his/her recordings)****.
>
> This is I assume when you set "Deleted Max Age" to -1 so that it keeps 
> all of your files around for as long as possible (or set auto-expire 
> in your recording settings and never delete anything). I gave up on 
> this after I got burned when the expiring of old files did not keep up 
> with recording of new ones and recordings failed because of running 
> out of disk space. It seems to me that this is not the best idea, 
> having your disk always at 99% full will, I think, cause fragmentation.

Yes, it will, so that makes it even more important to try to write only 
one new recording at a time to a given file system.


> So now I set it to 7 so that deleted recordings are removed after 7 
> days. Another problem with setting "Deleted Max Age" to -1 is that it 
> is difficult to know how much space you have available for recordings 
> since your disk is always 99% full.

Right, I hope to make it easy to see in the UI sometime.  Currently, 
it's only available on MythWeb and only as a total (not per file 
system).  When I do, it will also

> Regarding re-balancing your storage groups and using Archive storage 
> groups, doing this requires some technical finesse to find the file 
> names of your recordings and move them around. This is not the easiest 
> thing for a newcomer.

Agreed, if they get very specific about trying to ensure a proper 
representative sample.  I do it by assuming that if I get some random 
recordings spread out through channels and time, I'll probably have a 
pretty good mix.  That's what I'd recommend in general.  Again, 
eventually more of this information will be available to the users and, 
in fact, and ability to manually and/or automatically rebalance the file 
systems (where mythbackend would be able to just move recordings around 
when it's not busy).

> I can certainly document how and why to do it for those wanting to get 
> into it.
>
> Thanks for all the information, I will digest it and try to explain it 
> in the wiki.

Thank you.  We all (users and devs) appreciate the work you're putting 
into the wiki and the documentation.  It's a huge job you've taken on, 
but one that will pay off big.  I hope, too, we can find you some other 
people to help with the effort so you don't get overwhelmed and/or burn out.

Mike