[vdr] mdadm software raid5 arrays?

[Udo Richter]

On 18.11.2009 18:28, H. Langos wrote:
> I/O-load can have some nasty effects. E.g. if your heads have to jump
> back and forth between an area from where you are reading and an area
> to which you are recording.

I remember reading some tests about file system write strategies that 
showed major differences between file systems when writing several file 
streams in parallel. IIRC the old EXT2/3 was way at the lower end, while 
XFS scored more at the upper end.

One major point here is to avoid heavy seeking, by massive use of write 
caching and read ahead caching. Another one is a smart allocation 
strategy so that the files don't get interleaved too much, and so that 
metadata doesn't have to be read/written too often. (-> extents)

> In a raid1 setup you have two sets of heads that you can work with.
> (Or more if you are willing to put in more disks.)

In theory yes, but I would really like to know whether raid systems are 
actually smart enough to split their read operations between the heads 
in an efficient way. For example, while reading two data streams, its 
probably the best to use one head for each stream. Unless one of the 
streams needs a higher bandwidth, in which case it would be more wise to 
use one head exclusively, and let the other jump between the streams. 
And what if there are several small reads in parallel? Which head should 
be interrupted?

In the end you can probably put a lot of strategy fine tuning into this, 
and there will still be situations where a different strategy would 
still improve performance in some scenarios - or in others not.


Cheers,

Udo