[mythtv-users] Planning Tuners and Disk I/O for a MythTVBackend System
mikep at randomtraveller.org.uk
Mon Feb 8 12:34:14 UTC 2010
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.
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