[Qemu-devel] Re: Problems KVM-84

[Uri Lublin <uril@redhat.com>]

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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 <uril@redhat.com>
>>    Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>>
>> 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.

[-- Attachment #2: 0001-block-pass-BDRV_BACKING-flag-to-backing-file-open.patch --]
[-- Type: text/x-patch, Size: 1782 bytes --]

>From 1ccf1940e0b45a9001b916bc6160c03a098a5d1d Mon Sep 17 00:00:00 2001
From: Uri Lublin <uril@redhat.com>
Date: Wed, 11 Mar 2009 20:16:23 +0200
Subject: [PATCH] block: pass BDRV_BACKING flag to backing-file open

With this information, open can behave differently for the image
filename and its backing files.

Signed-off-by: Uri Lublin <uril@redhat.com>
---
 qemu/block.c |    3 ++-
 qemu/block.h |    2 ++
 2 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/qemu/block.c b/qemu/block.c
index 4c556ec..45fa5f4 100644
--- a/qemu/block.c
+++ b/qemu/block.c
@@ -397,7 +397,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
     /* Note: for compatibility, we open disk image files as RDWR, and
        RDONLY as fallback */
     if (!(flags & BDRV_O_FILE))
-        open_flags = BDRV_O_RDWR | (flags & BDRV_O_CACHE_MASK);
+        open_flags = BDRV_O_RDWR | (flags & (BDRV_O_CACHE_MASK | BDRV_O_BACKING));
     else
         open_flags = flags & ~(BDRV_O_FILE | BDRV_O_SNAPSHOT);
     ret = drv->bdrv_open(bs, filename, open_flags);
@@ -429,6 +429,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         }
         path_combine(backing_filename, sizeof(backing_filename),
                      filename, bs->backing_file);
+        open_flags |= BDRV_O_BACKING;
         if (bdrv_open(bs->backing_hd, backing_filename, open_flags) < 0)
             goto fail;
     }
diff --git a/qemu/block.h b/qemu/block.h
index e1927dd..ff70d64 100644
--- a/qemu/block.h
+++ b/qemu/block.h
@@ -56,6 +56,8 @@ typedef struct QEMUSnapshotInfo {
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_CACHE_DEF)
 
+#define BDRV_O_BACKING     0x0100
+
 void bdrv_info(void);
 void bdrv_info_stats(void);
 
-- 
1.6.0.6


[-- Attachment #3: 0002-block-qcow2-do-not-scan-refcounts-when-opening-a-ba.patch --]
[-- Type: text/x-patch, Size: 930 bytes --]

>From c618b293a64d1690105505d7cb28ba1ca8dd33c7 Mon Sep 17 00:00:00 2001
From: Uri Lublin <uril@redhat.com>
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 <uril@redhat.com>
---
 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