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

George Mari george_mythusers at mari1938.org
Mon Dec 29 14:42:11 UTC 2008

Jake Anderson wrote:
> 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. 

I'm assuming you're talking about writing 2 data streams to a non-RAID5 
volume, i.e., a single disk?  If so, I would think that whether you see 
a "greatly degraded" speed or not depends on whether you exceed either 
the rated badwidth of the drive, or the rated number of IO operations 
per second.  Once you exceed either of these for any type of hard drive, 
the drive controller or OS has to queue data, and things slow down.

This cost of drive IO you mention (seeking) usually correlates with the 
rated # of IO operations per second.  If a drive has a fast seek time, 
it can obviously do more IO operations per second.

I submit that most common single drives can easily handle 2 simultaneous 
data streams with no noticeable degradation of performance.

> 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.

What if the filesystem is fragmented, and there is not a big enough 
chunk of space to write the next part of your data stream in the same 
part of the disk?

>> 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.

More information about the mythtv-users mailing list