[rhelv6-beta-list] Can RHEL6 installer align the disk partition with 4k-sector or raid stripe size?

[Pasi Kärkkäinen]

On Mon, Jun 07, 2010 at 07:37:28PM -0400, Nico Kadel-Garcia wrote:
> On Sun, Jun 6, 2010 at 11:24 PM,  <caiqian at redhat.com> wrote:
> >
> >> I assume that the original poster mant "4k block size". 4k
> >> sectors.... doesn't make much sense.
> >>
> >> So far as I can tell, this only works with hard drives which are
> >> detected as 4K block size. According to my preliminary tests, it does
> >> *NOT* work with VMware, Xen, or KVM disk images. This is an important
> >> feature for certain attached storage devices, such as NetApps, which
> >> behave very badly if the partitions of virtual disk images are not 4K
> >> aligned. It should, ideally, be a simple setting for disk
> >> configuration in Anaconda and especially in a virtual machine setup
> >> tool, but even fdisk mishandles this: at least up through RHEL 5.
> >>
> >> You see, you can set the block size with fdisk inside a '%pre'
> >> statement. But if you don't have a partition set, fdisk automagically
> >> reverts the block size to 512 for you. This means that in order to
> >> preserve the block size information for Anaconda's disk configuration
> >> tools, which have *no* options to select specific sectors to start or
> >> end partitions, or to do this 4K block size, you have to set aside a
> >> partition that anaconda will not touch and that will preserve the4096
> >> block size setting. That's.... fairly awkward to allocate and recover
> >> later.
> >>
> >> I've also found no way to tell Anaconda "I've built the partitions for
> >> you in %pre: use *THESE* partitions for *THOSE* mount points, and
> >> don't try to repartiton them." If someone knows a way, I'd love to
> >> hear it.
> >
> > Have you seen this in the RHEL6 Beta manual?
> > http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6-Beta/html/Storage_Administration_Guide/newstorage-iolimits.html#iolimits
> >
> > "As of this release, boot support for 4K sector devices is planned, but not yet supported."
> >
> > C.
> 
> This is thoughtful, but it's relevant only to physical media, not
> virtual images. Until virtualization "disks" are configurable as "4096
> byte block devices", the emulated disks have a behavioral issue that
> the guest OS's cannot see.
> 

at least Fedora 13 installer aligns partitions to 2 MB starting offset (iirc),
even when the drive has 512 bytes per sector. I assume RHEL6 installer does the same.

No mis-alignment there, like RHEL5 installer has (the famous offset 63 issue).

-- Pasi

> The related issues with fdisk discarding information about block
> sizes, and anaconda's incapacity to accept such settings nor to accept
> pre-set partitions as "this partition or LVM goes with *THIS*
> filesystem, do not try to reset this!!!!!" merely means that the next
> set of clean ways to work around the problem are impossible. NetApp
> publishes a tool to "disk align" such images for VMWare use, but it's
> pretty nasty: it builds a larger disk image, and uses something like
> "dd" to image the existing volumes into new partitions, which start on
> 4096 byte block boundaries but don't necessarily end on them. Coupled
> with this, they ignore cylinders, so "fdisk" and tools like it
> complain forever more about cylinder misalignment.
> 
> It is, unfortunately, a classic case of a set of small deficits in
> individual components adding up to a nightmare of dealing with a new
> situation.
> 
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