[mythtv-users] Question re: available SATA ports and linux software RAID

[Simon Hobson]

John Drescher wrote:

>  >> The problem is that you are using the same spindle for your OS (and
>>>  presumably your DB) and recordings.  That configuration can cause
>>>  performance issues.
>>
>>  I agree that with only one spindle this can be an issue. But with 8? ;)
>
>With 8 drives and modern hardware you should have plenty of 
>bandwidth to spare.

Raw bandwidth isn't the issue - even a single drive can sustain a 
transfer rate in excess of (I'd think) pretty well anyone's Myth 
needs.

The problem is that RAID does not eliminate the requirement to seek 
back and forth. So you are recording a stream or two to the 
recordings storage area, and then do a DB update. It matters not 
whether you are running a RAID array or a single disk - the disk or 
disks will have to seek to the DB storage area and then seek back to 
the recordings storage area, repeatedly. The difference is that with 
RAID, you have a group of disks all seeking back and forth vs only 
one drive doing it.

If you have plenty or RAM and buffer writes then you probably needn't 
lose any data from your recordings*, but playback would stutter.

* Did I read somewhere that Myth syncs data as it writes, so doesn't 
make effective use of OS level write caching ? Or am I just imagining 
things ? (say) a gig of RAM would make seek performance of the drives 
pretty well moot for writes.
-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.