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

Simon Hobson linux at thehobsons.co.uk
Sun Apr 10 21:40:14 UTC 2011

Bobby Gill wrote:

>I suppose the last hurdle is to determine whether or not to build 
>one big array or perhaps 2 or more separate ones. I am set on a 
>minimum of 6 drives, likely 8, x 2 TB, and RAID 6. I'm currently 
>simulating rrays in Virtualbox to fiddle around and get some 
>experience doing it myself first.
>What are key considerations if for example I have 6 x 2 TB, in doing 
>a 6x2TB array vs. say, 2 x 3x2TB? I don't want to overcomplicate 
>things so if one big array is a fine path I'll do it, but this 
>thread has shown me how frakkin much "fine print" there is with the 
>whole RAID deal :)

OK, for Myth storage there is a school of thought that says not to 
build an array at all - just create multiple storage directories and 
that will allow Myth to spread the recordings (and load) between them.

In terms of capacity, it depends on the level of raid.

If striping, then there is no difference in capacity between 2x3 and 
1x6 in the array - both will give you a total of 3 drives worth of 
However, for RAID 5 then 1x6 will give you 5 drives worth of storage, 
while 2x3 will give you 4 drives worth (2x2). RAID 6 would give 4 
drives worth for 1x6, but would not be worthwhile for 2x3 since 3 
drives would only give a single data drive for RAID 6 !

For performance, it is very heavily dependent on the type of 
workload. For the sort of large sequential reads & writes typical of 
Myth recordings, then 2 arrays may give better performance - but that 
assumes multiple recordings will be split across the arrays (which is 
the norm) and that playback will be similarly split (which may or may 
not be the case). In practice, a single large array should be quite 
adequate for any reasonable setup.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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