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

[Jake Anderson]

>>> 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.
>>
>>     
>
> Even with generic drives, you can get better overall bandwidth and  a 
> higher number of I/O operations per second with a group of disks than 
> with one drive.
>   
That's true, but if you write 2 streams to 2 disks individually vs 2 
streams to the same disks with RAID, the seek load on the 2 disks 2 
streams is going to be lower.

essentially it all boils down to this
Seek time is not improved with RAID.*

*The max transfer rate, and max IO operations cannot both happen at the 
same time. Xfering one file can happen at say 70MB/sec, if you copy 2 
files your going to see probably 25MB/sec per file, ie a total of 50MB/sec.
Scale it to RAID and storage groups.
The RAID gets 100MB/sec because even though its peak transfer is faster 
(140mb/sec for one stream) the seek load brings its performance down to 
just 2x the single disk. Seek time is not improved with RAID.
The storage pool style can xfer 140MB/sec because it has no(practically) 
seeking to worry about.


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