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

[Raymond Wagner]

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.