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

[Jake Anderson]

George Mari wrote:
> Jake Anderson wrote:
>
> [deleted]
>
>   
>> 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.
>>
>>     
>
> Well, to be fair, with RAID 0,5 or 6, (and probably 1), your drive light 
> will be on more or at least as much as with a non-raid, single drive 
> solution.  You're just spreading the same number of IO operations 
> (slightly more with RAID5 or 6 because of the parity info) over multiple 
> drives/spindles, so your system as a whole can do more IO.
>   
The thing is though you need to look at the "cost" of the drive IO
The thing that takes a drive the longest is seeking, so if you are 
trying to write 2 data streams simultaneously to a disk you can expect 
to see a greatly degraded speed. Writing one stream per drive means the 
head is going to already be pretty much in the right position when the 
next block of data comes along, IE minimal seek time, vs thrashing the 
head around writing small chunks faster.

Its the same as copying files from one drive to another, its much faster 
(for large files) to copy them sequentially rather than to try and do it 
simultaneously.

Or putting different database tables onto different drives, same/similar 
reason.
> If you have a 1GB system that is a combined BE/FE, that's probably a 
> little tight on RAM, especially when comm flagging, mythfilldatabase and 
>   possible transcode jobs run at the same time, so you may have been 
> cutting into the swap space, and not necessarily hitting the IO limits 
> of your RAID0.
>   
swap wasn't paticularly active, it was running mythbuntu so it's pretty 
light.
> I have a dedicated BE with 576MB RAM and an old SCSI card with 6x180GB 
> drives in a software RAID5.  It's no trouble recording 2 shows, comm 
> flagging them, and watching 3 other shows on my different FEs.  It sits 
> in the basement, and is obviously not suitable for a living room or 
> bedroom, but it's hardware I had lying around or was able to procure 
> inexpensively.
>
> [deleted]
>   
Those are rather high end disks, with low seek times. I was/am using 
generic ATA drives.
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