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Message #61033
[Bug 361733] Re: dmraid(fakeRAID) raid1 driver doesn't loadbalance reads
it would be great if @lifeless could try attempting dm-devel again. This
is a very important patch that will provide large benefit across a
variety of systems.
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https://bugs.launchpad.net/bugs/361733
Title:
dmraid(fakeRAID) raid1 driver doesn't loadbalance reads
Status in “linux” package in Ubuntu:
Confirmed
Bug description:
This is a kernel bug. Actually, it's intended behavior, but it
shouldn't be.
In theory, fake RAID and software RAID should be the same thing. The
only practical difference is where they get their metadata from. One
from the system BIOS, the other from the disk. However, it turns out
that they take two completely different code paths. The fakeraid
(dmraid) driver reads only from the primary disk, while the software
raid (mdraid) driver implements load balancing for RAID1. Of course,
this difference isn't well-documented, resulting in people sometimes
choosing the wrong one.
(1) There's no good reason why there should be two separate software RAID1 drivers. They should be merged.
(2) In the meanwhile, people should be splashed with loud warnings that if they choose fakeraid, they'll not get the improved read performance that they expect.
Here's the LKML discussion where I asked about this:
http://kerneltrap.org/index.php?q=mailarchive/linux-kernel/2008/7/7/2370344/thread
As far as I know, they haven't addressed this problem since. I'm
reporting this here, because I see Ubuntu as the distro that seems to
take responsibility for fixing a lot of the silliness that others
won't.
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