[Xen-users] RE: raid10 + lvm + domU NoGo?

["Ross S. W. Walker" <RWalker@xxxxxxxxxxxxx>]

henry ritzlmayr wrote:
> Am Montag, den 08.09.2008, 10:09 -0400 schrieb Ross S. W. Walker:
> > 
> > The kernel md raid10 driver is a little off.
> > 
> > You could try striping LVs across md RAID1 PVs.
> > 
> > Say you have 6 drives, create 3 MD RAID1s, convert them to PVs
> > (whole disk is fine no need to partition) add them to a volume
> > group then create the LVs with lvcreate -i 3 ... which will
> > cause them to stripe across all 3 PVs.
> > 
> > This will provide identical performance to the MD RAID10 and
> > should work fine with Xen.
> > 
> > -Ross
> 
> Hi Ross, 
> 
> thanks for the reply, I will give this a shot. Do you have any
> information (link) on why the md raid10 driver is "a little off" or
> any information if there are any plans to change this. I guess this 
> is probably an upstream issue, but on some test systems I only have
> three disks so the above solution works only for a subset of my
> machines.

Nothing besides what has been reported in the bug databases.

This one in particular:

https://bugzilla.redhat.com/show_bug.cgi?id=223947

And comment #6 about an outstanding dm patch for max_hw_sectors
not being preserved sounds like it MIGHT fit the bill here.

If the upper stackable storage drivers don't get the correct
max_hw_sectors they can in fact create bios bigger then the
underlying storage system can handle which can cause the
type of errors that are reported.

The unconfirmed workaround of using tap:aio also seems to fit
this too as this will force io to go through the VFS layer
which means it will be memory mapped and limited to PAGE_SIZE
io requests which will always fit into the max_hw_sectors, but
tt also means that throughput will be choked at PAGE_SIZE/sec.

You can test this with iSCSI Enterprise Target, a blockio lun
on a raid10 partition (with or without LVM) and an initiator
that pumps io requests larger then the max_hw_sectors of
your SATA/SAS controller (dd bs=X, X>max_hw_sectors).

I know for a fact that the blockio storage module in IET
uses max_hw_sectors for determining the largest bio size
that can be sent to the hardware, cause I wrote that part of
the module.

Having said all this though, if all you want is to get your
storage up and running without debugging the kernel, then
I advise you avoid the dm raid10 driver for now.

-Ross

PS You can also create raid10 by creating a bunch of dm
raid1s then creating a dm raid0 out of them as well. I
have always felt the dm raid10 module wasn't actually
a raid10 module, but a completely new raid level that
should have it's own identifier (actually wikipedia
feels the same way too).

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