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

[Tim Draper]

---- On Tue, 11 Aug 2015 16:48:41 +0100 Peter Bennett (cats22)  wrote ---- 
>  Hi MythTV experts
> 
> I am updating the wiki and wondering about the Balanced Disk I/O setting.
> 
> https://www.mythtv.org/wiki/Storage_Groups#Balanced_Disk_I.2FO

> Clearly 50 at a time is theoretical and unlikely because it would be affected by seek times. However HD programs vary from 3 GB to 8 GB per hour which means even more than 50 at the same time.  Also drives are rated at 6 Gb/sec which means 300 HD recordings at the same time(!).


Thats 6Gb/s internal to the drive. Reality from the OS is more around 100MB/s as an average figure across the entire drive.

Raw write speed is only on vector you need to worry about; queue depth/IOPS would likely be the next since your running multiple streams to the disk.
The various Raid array levels get a bit more complex in calculations.

Write speed is also just one side of the coin. If your theoretically writing 50 HD recordings to disk, then i bet you'll also be concurrently watching a handful of these recordings at any one time. they'll half your disk throughput as your both reading&writing at the same time (if you do not cover NCQ technology).
Will you also be running out-of-hours jobs on the recordings - transcoding/advert detection etc as they will all have an affect on things

back to the basics: i agree, 2 recordings seem a very conservative figure for todays drives but thats taking a large pinch of salt for how the mythtv infrastructure is configured