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

[Simon Hobson]

Jay Foster <jayf0ster at roadrunner.com> wrote:

> The statement, "drives are rated at 6 Gb/sec", depends on the drive and the system.  SATA III drives are (when used with a SATA III controller), but SATA II drives or IDE drives, or other drives are not.

And that is only the interface transfer rate which is typically significantly higher than the transfer rate on/off the disk itself.
A quick look (at Seagate) suggests their drives average around the 125 to 150Mbyte/second real transfer rate - so that's around 1 to 1.2Gbit/s, far less than the 1.5/3.0/6.0 Gbps of the interface.

The same look also suggests that modern drives have seek times in the order of around 10ms. Given that each write requires a minimum of two seeks, and some writing in between, that means something in the order of (say) 25ms min to write a stream. On that basis, seeks alone shouldn't be an issue until there are a fair number of streams being recorded - 10 simultaneous streams would allow an average of 100ms/stream for a write each second.
In addition, the write processes will wait (and buffer more data) - so the "write once a second" is really "write at most once/second".

So no, it shouldn't really be a problem.

Without looking, I believe I have the combination option set that balances disk I/O but favours the filesystem with the most free space. Ie one recording will go on the filesystem with the most free space, two will go across the two filesystems I have. IMO this makes the best use of the available disks I/O, while also reasonably favouring best use of available disk space.