Problems KVM-84

Commit Message

Uri Lublin March 11, 2009, 6:37 p.m. UTC

Anthony Liguori wrote:
> Avi Kivity wrote:
>> Jay Mann wrote:
>>> Hi,
>>>
>>> I just downloaded built and installed kvm-84 on ubuntu Hardy x64  
>>> 2.6.24-23-
>>> server and I’m getting the following 2 problems that did not exists 
>>> in kvm-83.
>>>
>>> 1.    The qemu emulater (bios screen) takes a long time to start (~10 
>>> seconds), and subsequently Libvirt times out when I try to start a 
>>> guest VM from virsh.    
>>
>> This is caused by qemu r6404:
>>
>> commit 5d4cbd78aa33f6d034a62207c99ad0b64af44621
>> Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>
>> Date:   Thu Jan 22 18:57:22 2009 +0000
>>
>>    block-qcow2: keep highest allocated byte (Uri Lublin)
>>      We want to know the highest written offset for qcow2 images.
>>    This gives a pretty good (and easy to calculate) estimation to how
>>    much more allocation can be done for the block device.
>>      It can be usefull for allocating more diskspace for that image
>>    (if possible, e.g. lvm) before we run out-of-disk-space
>>      Signed-off-by: Uri Lublin <[e-mail]>
>>    Signed-off-by: Anthony Liguori <[e-mail]>
>>
>> Scanning the file at startup is slow.  We need to find a better way.
> 
> Any quick ideas?  Seems like this is broken by design.  Unless we can 
> find a quick fix, I'm going to revert this in the stable tree.
> 
> Regards,
> 
> Anthony Liguori
> 

We only need to scan the given filename (no backing files). We can pass a flag 
to qcow_open (bdrv_open of bs->backing_hd) specifying it's a backing file.
(attached 2 patches). That makes things better for small images with big backing 
files. It does not fix the problem for very large images.

A better solution may we to allocate a qcow2-extension to keep highest-alloc and 
num-free-bytes so we don't have to scan refcount table of the image upon open.
With that solution qcow_create initializes them, qcow_open reads them and 
bdrv_close updates them. We can also add a qemu-img command to scan and update 
those values.

Regards,
     Uri.

Patch

From c618b293a64d1690105505d7cb28ba1ca8dd33c7 Mon Sep 17 00:00:00 2001
From: Uri Lublin <[e-mail]>
Date: Wed, 11 Mar 2009 20:18:23 +0200
Subject: [PATCH] block-qcow2: do not scan refcounts when opening a backing file

It takes too long and is not needed.

Signed-off-by: Uri Lublin <[e-mail]>
---
 qemu/block-qcow2.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/qemu/block-qcow2.c b/qemu/block-qcow2.c
index 465dcd6..41cdbe9 100644
--- a/qemu/block-qcow2.c
+++ b/qemu/block-qcow2.c
@@ -276,7 +276,8 @@  static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
     if (refcount_init(bs) < 0)
         goto fail;
 
-    scan_refcount(bs, &s->highest_alloc, &s->nc_free);
+    if ((flags & BDRV_O_BACKING) == 0)
+        scan_refcount(bs, &s->highest_alloc, &s->nc_free);
 
     /* read the backing file name */
     if (header.backing_file_offset != 0) {
-- 
1.6.0.6