Re: RAID-5 streaming read performance

[Ming Zhang <mingz@ele.uri.edu>]

On Wed, 2005-07-13 at 22:18 +0100, David Greaves wrote:
> Ming Zhang wrote:
> 
> >>component partitions, e.g. /dev/sda7: 39MB/s
> >>raid device /dev/md2:                 31MB/s
> >>lvm device /dev/main/media:           53MB/s
> >>
> >>(oldish system - but note that lvm device is *much* faster)
> >>    
> >>
> >
> >this is so interesting to see! seems that some read ahead parameters
> >have negative impact.
> >  
> >
> I guess each raw device does some readahead, then the md0 does some
> readahead and then the lvm does some readahead. Theoretically the md0
> and lvm should overlap - but I guess that much of the raw device level
> readahead is discarded.
> 
> David
> 
for a streaming read, what you readahead now will always be used exact
once in near future. at least i think raw device readahead can be turned
on at the same time with one of OS components, raid or lvm, readahead
being turned on. but in your case, u get best result when turn only one
on.

ming