[mythtv-users] OT-RAID 5 failure. MDADM experts? Seagate 1.5TB failure

[Jake Anderson]

George Mari wrote:
> For me, it would have been much more time to "format the lot and start 
> over", and restore from my backup, than it would to just take a little 
> time to do some research on how to recover.
>   
If it was a data drive with important stuff on it then yeah i can see 
spending time on it.
Alternately if its just TV... ;->
>   
>> IE the array is in an inconsistent state and theres no real way to 
>> reconcile that.
>>
>>     
>
> With one drive dead and another drive with a different value for 
> 'events' like Mark has, you can force assemble the array with one 
> missing drive, fsck the filesystem to make sure it's ok (JFS has never 
> missed a beat for me) and add in a new drive.
>
> Now, granted - in my situations, I knew that the filesystems that were 
> on these arrays were not being written to at the time of failure, so 
> that made success much more likely.
>
>   
>> Only other thought I seem to remember something about needing to pass 
>> mdadm an option when rebuilding to tell it to make it into a new array 
>> or some such, might only have been when you moved the array to a 
>> different host though.
>>
>>     
>
> There is no such option, to the best of my knowledge.  Have you done 
> this before?  Even explicitly telling mdadm to create a new array, using 
> the same devices/partitions that were in the array before, mdadm will 
> basically try to re-create the old array if it sees there was an array 
> there before.  That's what one of the links I posted previously explain.
>   
I recall needing to give mdam some special loving to get it to work when 
I moved an array from one machine to another, i seem to recall vuagley 
similar errors.
I am wondering if the OP might be facing the same problem.
>   
>> When you sort it all out, use storage groups rather than raid, you will 
>> get much better performance and probably disk life, the seek load is 
>> going to be drastically reduced.
>> _______________________________________________
>>     
>
> To each their own.  I have nothing against storage groups, and haven't 
> tried them yet, as RAID5 has been wonderful for me, but that's an 
> awfully general statement regarding "much better performance".  Got any 
> hard data comparing performance of RAID5 and storage groups?  Yeah, 
> yeah, I know - the infamous RAID5 write performance penalty.  Sure, it's 
> there - but for mythtv, or for storing large amounts of media files, who 
> cares?  Write performance is more than good enough for those use case 
> scenarios.
>   
For me its a few personal experiences.
I have a raid 0 setup (started before storage groups were available) and 
if I watch something whilst something else is being recorded and 
commflagged the little blue light is almost solid on. I had to add more 
ram to the machine to let it record 2 shows and watch a 3rd. I needed to 
go to 2gb of ram from 1gb, adding a further 2gb helped some more too.
My fathers myth box which is a P4 3ghz (vs my Q6600) was doing about the 
same job (without the commercial flagging, but as that is in realtime on 
my machine it shouldn't affect disk IO much at all) will record 2 shows 
and watch a 3rd with 512mb of ram and the disk light flashes at about 
1-2 Hz.

Sure you can tweak things to try and work around the seek time issue 
while your on raid but with storage groups so handy just use them. It 
seems to let everything run much easier. Worst case a drive failure 
takes out some hours of TV, you should have better things to do anyway ;->

What I plan to do for the next round is to partition some large drives 
up 80/20 for storage groups and raid 5.
put bulk TV on the storage group, then anything "important" gets moved 
into the raid 5 storage area.

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