On Tue, Aug 28, 2007 at 12:21:03PM +0200, Goswin von Brederlow wrote:
> Raid 1:
> There is a "bug" in recent linux kernels with raid1 regarding read
> scheduling. Before when multiple read requests where made to a raid1
> device the kernel would distribute them across both disks giving you
> double the read speed to a single request. But something is broken
> and that no longer happens. Read and write from7to raid1 is now both
> the same (slow) speed.
If you mean the md driver (there is also RAID1 based on dm), then it
never did really distribute the reads. Continuous read requests are
always served from the same disk. For parallel requests, the md driver
maintains the sector count of the last disk access position and issues
the request to the disk where the head is closer. Of course this simple
heuristics works only if the disks has just a single RAID1 partition; if
there are more RAID partitions per device then this optimization is
worthless.
> Raid 5:
> At home something is broken there too. I get less performance from
> my raid5 than form any one disk in the raid5. With older kernel and
> different controlers I remember getting far more speed. I'm not sure
> if it is the controler or if this is releated to the raid1 "bug".
> Hopefully this is specific to my setup or temporary.
If you have numbers I suggest sending them to the linux-raid mailing
list, just to make sure that the problem is known to the developers.
Gabor
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MTA SZTAKI Computer and Automation Research Institute
Hungarian Academy of Sciences
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