[mythtv-users] Planning Tuners and Disk I/O for a MythTVBackend System

[Mike Perkins]

Raymond Wagner wrote:
> On 2/7/2010 22:37, MythTV wrote:
>> Recording speed with 12 tuners requires 12 concurrent streams to be
>> written to the disk.  The
>> bottleneck becomes the seek times and rotational latency.
>>    
> 
> The innermost track of a 1.5TB 5400RPM drive is going to push 50MB/s 
> sequentially.  With some safe estimates of 30 seeks/sec, and a 15ms seek 
> latency, almost half the time would be spent seeking, dropping 
> throughput to 25MB/s (ignoring onboard cache and NCQ).  Those twelve 
> recordings each at a respectable 16mbps will use 24MB/s.  That puts it 
> in the realm of possible, but probably not without problems.  Of course 
> if you own 12 tuners, you should have no problem getting multiple hard 
> drives to record to.
> 
> 
> On 2/7/2010 23:00, Ben Kamen wrote:
>> Could always start striping drives in a raid 0 kinda deal. :D
> 
> No. Striping causes the same exact problem.  You still have all the 
> disks seeking in unison, you just end up with significantly higher 
> throughput.  If you've got proper hardware RAID with write-back cache, 
> it may be able to re-order the writes to be more sequential.  The whole 
> purpose of Storage Groups added in 0.21 was to allow you to add multiple 
> independent drives to MythTV, and allow it to properly load balance 
> between them.
> 
Don't forget that along with the 12 different streams you're writing to those 
disks, you're *also* writing seek information for 12 different recordings to the 
database - on another disk, one assumes. Consider therefore the requirements 
needed for database buffering, throughput, etc.

-- 

Mike Perkins