[mythtv-users] Howto prevent MythTV flushing/Syncing disk writes (LiveTV/Recordings)

[Albert Graham]

Frosty wrote:
> Roger Heflin wrote:
>   
>> Albert Graham wrote:
>>   
>>     
>>> Hello,
>>>
>>> Does anyone know how to prevent MythTV from flushing/syncing every single write request when watching LiveTV or 
>>> recording something.
>>>
>>> My machine has lots of memory and very large fast disks, so I would prefer writes to occur in much larger chunks and a 
>>> lot less often, rather than every 3 or so MB.
>>>
>>>
>>> I've set my /proc/sys/vm as follows:
>>>
>>> dirty_ratio = 90
>>> dirty_background_ratio = 90
>>> dirty_expire_centisecs = 10000
>>> dirty_writeback_centisecs = 0
>>> vfs_cache_pressure = 100
>>> swappiness = 60
>>>
>>> But even after droping the caches, (so all memory is free) with drop_caches = 3, it seems to bypass the Linux caching.
>>>
>>> I'm using XFS over an 8 disk (7TB) hardware raid5 array (3ware 9650SE) and my write speeds are >625MB/s - uncached, so I 
>>> would really like to see only a major write every 10/15 minutes, rather than the disks hammering away all day long.
>>>
>>> Any help would be very very much appreciated.
>>>
>>> Albert.
>>>     
>>>       
>> Even if there are no sync calls in the application, Linux will write the data as 
>> soon as is reasonable, it won't hold off a huge amount of data for a really long 
>> time unless you force it to by having the input IO rate exceed the disk IO rate, 
>> and then it will always be behind, and always write as much data as it can.
>>
>> For mythtv to do this, it would have to buffer things up itself and only write 
>> every so often, and the disadvantages of this are it takes a lot of memory, and 
>> more data would be lost on a unexpected crash and it would take longer to 
>> shutdown things (the application buffers would have to be flushed--or lost), and 
>> for most people which less capable disk subsystems it would be rather useless.
>>
>> You could probably make a patch to define a configurable disk buffer setup to do 
>> this, but it is not going to help anything.
>>
>> What do you thing you are going to gain by only writing every 10-15 minutes?
>>
>>   
>>     
>
> To have 8 drives continuously writing many times a second puts avoidable 
> wear on the system.
This was also a concern to me too, however, my drives (Seagate do have a 
5 year warranty - so this was low on my list of prioities.

>  I have a similar setup although not for MythTV and 
> having just 6 drives continuously patter away is distracting. And when 
> you know the drives are capable of doing these tiny 1ms bursts in a 
> single 1/2 second burst once every few mins, you can't help but think 
> that your drives will die before their time. Adding a buffer isn't 
> something I can help with but its a good idea for those of us that have 
> lots of RAM, a stable system and wish to reduce constant use of the drives.
>                          Roger
>   

See my thread same subject with [SOLVED] in the title, the latter part 
may be of interest to you.



Albert.