Bug 216556 - Virtual disk size wraps around at 2 TB mark
Summary: Virtual disk size wraps around at 2 TB mark
Alias: None
Product: Red Hat Enterprise Linux 5
Classification: Red Hat
Component: kernel-xen   
(Show other bugs)
Version: 5.0
Hardware: i686 Linux
Target Milestone: ---
: ---
Assignee: Rik van Riel
QA Contact:
Depends On: 216555
Blocks: 217580
TreeView+ depends on / blocked
Reported: 2006-11-21 01:34 UTC by Daniel Berrange
Modified: 2007-11-30 22:07 UTC (History)
1 user (show)

Fixed In Version: 5.0.0
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
Last Closed: 2007-01-26 20:45:38 UTC
Type: ---
Regression: ---
Mount Type: ---
Documentation: ---
Verified Versions:
Category: ---
oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

Attachments (Terms of Use)
Script to create huge block devices with device mapper trickery (1.51 KB, text/plain)
2006-11-21 01:39 UTC, Daniel Berrange
no flags Details
Patch to fix large blktap disks (3.13 KB, patch)
2006-11-21 22:46 UTC, Daniel Berrange
no flags Details | Diff
Patch to fix blkback driver handling of > 2tb disk (3.11 KB, patch)
2006-12-01 15:02 UTC, Daniel Berrange
no flags Details | Diff

Description Daniel Berrange 2006-11-21 01:34:58 UTC
+++ This bug was initially created as a clone of Bug #216555 +++

Description of problem:
If I create a virtual disk (either a file, or block device backed) in the dom0
host with a size of 5 TB, and export this to a guest DomU, the domu will only
see it as being 1 TB in size. It looks as if the Xen virtual disk driver is
wrapping around at the 2 TB mark.

Version-Release number of selected component (if applicable):

How reproducible:
Always (on 32-bit)

Steps to Reproduce:
1. Create an block device 7 TB in size
2. Export this to the DomU in the guest config file
3. Look at /proc/partitions inside the guest kernel
4. Look at /proc/partitions in the dom0 host kernel
Actual results:
The #blocks column for the disk is different in DomU & Dom0

Expected results:
The #blocks seen in DomU matches #blocks seen in Dom0

Additional info:
The same problem occurs for file backed virtual disks on 32-bit host/guest.

There is no problem on 64-bit kernels - I have exported disks as large as 15 PB
 to the guest.

This is all testing with paravirt guests. Not verified if fully virt guests have
any issues yet.

Post from Ian upstream suggests 32-bit wraparound in blkfront driver


Comment 1 Daniel Berrange 2006-11-21 01:39:34 UTC
Created attachment 141715 [details]
Script to create huge block devices with device mapper trickery

The attached script is useful for reproducing the problems.

First create a data file to store the actual data, say 10 GB in size

  * dd if=/dev/null of=/xen/scratch.img bs=1 count=0 seek=10GB

Setup as loop device

  * losetup /dev/loop0 /xen/scratch.img

Now, create  a huge (5 TB) device backed by this small file

  * sparse_create xenhuge  /dev/loop0 $(echo 2000*1000*1000*5 | bc)

Then, add


to the disk section of an existing guest.

Finally boot the guest & compare /proc/partitions for this device in the Dom0 &

Comment 2 RHEL Product and Program Management 2006-11-21 02:00:58 UTC
This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release.  Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release.  This request is not yet committed for

Comment 3 Daniel Berrange 2006-11-21 12:40:36 UTC
Patch added in xen-unstable


Comment 4 Daniel Berrange 2006-11-21 22:45:06 UTC
I have tested the patch listed in comment #3 and confirmed it fixes operation of
file: and phy: backed disks > 2 TB in size. It does not, however, fix tap:aio:
backed disks.

Comment 5 Daniel Berrange 2006-11-21 22:46:25 UTC
Created attachment 141846 [details]
Patch to fix large blktap disks

This additional patch fixes blktap based disks which are > 2TB

Comment 6 Brian Stein 2006-12-01 03:16:22 UTC
Please post for ACKs

Comment 7 Jay Turner 2006-12-01 12:58:43 UTC
QE ack for RHEL5.

Comment 8 Daniel Berrange 2006-12-01 15:02:52 UTC
Created attachment 142572 [details]
Patch to fix  blkback driver handling of > 2tb disk

Comment 9 Daniel Berrange 2006-12-01 15:04:50 UTC
The equivalent userspace blktap patches are tracked in a separate bug 217580

Comment 10 Don Zickus 2006-12-05 19:52:51 UTC
in 2.6.18-1.2817.el5

Comment 11 Jay Turner 2007-01-26 20:45:38 UTC
2.6.18-7.el5 included in 20070125.0.

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