QEMU online guest disk resize wrt host block devices

I see two likely approaches:

1. Add a parameter to the existing 'block_resize' command 'refreshonly=true|false'

2. Add a separate command 'block_refresh'

We want to support online resize of guest disks in the libvirt API for QEMU, and had intended to use the fairly recently added 'block_resize' command:

commit db97ee6a976bacbb0d18818e951cfc41b39269a7
Author: Christoph Hellwig <[email protected]>
Date: Mon Jan 24 13:32:41 2011 +0100

block: tell drivers about an image resize

commit 6d4a2b3a47959f02e7f307f50396e70e8464f95e
Author: Christoph Hellwig <[email protected]>
Date: Mon Jan 24 13:32:33 2011 +0100

block: add block_resize monitor command

There is unfortunately a problem with the design of this monitor command because it has tied the task of resizing the host backing file for the virtual disk, together with the task of notifying the guest device of the block resize. This means that we can only use 'block_resize' for disks that are backed by image files (raw, qcow2, etc) that QEMU can actually resize.

Quoted 6 lines · Daniel P. Berrange

I see two likely approaches:

1. Add a parameter to the existing 'block_resize' command 'refreshonly=true|false'

2. Add a separate command 'block_refresh'

I think all that you need is a to implement a bdrv_truncate method for host devices that simply checks if the new size smaller or equal to the device size, and return 0 in that case or an error otherwise.

One other question too, when creating a qcow2 image via 'qemu-img create' you can specify a 'prealloc' option to require metadata to be allocated at time of creation.

Should we have the same control at time of resize too. If the app had originally created the qcow2 image with preallocated metadata, then I'd expect they want to pre-allocate metadata when extending it too, or is there no additional metadata allocation required when extending an image ?

Daniel

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Please try the attached (untested) patch:

Index: qemu/block/raw-posix.c
===================================================================

block/raw-posix.c +20 -3

@@ -645,10 +645,23 @@ static void raw_close(BlockDriverState *
 static int raw_truncate(BlockDriverState *bs, int64_t offset)
 {
     BDRVRawState *s = bs->opaque;
-    if (s->type != FTYPE_FILE)
-        return -ENOTSUP;
-    if (ftruncate(s->fd, offset) < 0)
+    struct stat st;
+
+    if (fstat(s->fd, &st))
         return -errno;
+
+    if (S_ISREG(st.st_mode)) {
+        if (ftruncate(s->fd, offset) < 0)
+            return -errno;
+    } else if (S_ISCHR(st.st_mode) || S_ISBLK(st.st_mode)) {
+       if (offset > raw_getlength(bs)) {
+           return -EINVAL;
+       }
+       return 0;
+
+    } else {
+        return -ENOTSUP;
+    }
     return 0;
 }
 

@@ -1167,6 +1180,7 @@ static BlockDriver bdrv_host_device = {
 
     .bdrv_read          = raw_read,
     .bdrv_write         = raw_write,
+    .bdrv_truncate = raw_truncate,
     .bdrv_getlength	= raw_getlength,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,

@@ -1288,6 +1302,7 @@ static BlockDriver bdrv_host_floppy = {
 
     .bdrv_read          = raw_read,
     .bdrv_write         = raw_write,
+    .bdrv_truncate = raw_truncate,
     .bdrv_getlength	= raw_getlength,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,

@@ -1389,6 +1404,7 @@ static BlockDriver bdrv_host_cdrom = {
 
     .bdrv_read          = raw_read,
     .bdrv_write         = raw_write,
+    .bdrv_truncate = raw_truncate,
     .bdrv_getlength     = raw_getlength,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,

@@ -1510,6 +1526,7 @@ static BlockDriver bdrv_host_cdrom = {
 
     .bdrv_read          = raw_read,
     .bdrv_write         = raw_write,
+    .bdrv_truncate = raw_truncate,
     .bdrv_getlength     = raw_getlength,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,

Quoted 4 lines

One other question too, when creating a qcow2 image via 'qemu-img create' you can specify a 'prealloc' option to require metadata to be allocated at time of creation.

Quoted 5 lines

Should we have the same control at time of resize too. If the app had originally created the qcow2 image with preallocated metadata, then I'd expect they want to pre-allocate metadata when extending it too, or is there no additional metadata allocation required when extending an image ?

Sounds reasonable. Keving, is there a sane way to implement this?

Allow to resize images that reside on host devices up to the available space. This allows to grow images after resizing the device manually or vice versa.

Reviewed-by Christoph Hellwig <[email protected]>

Index: qemu/block/raw-posix.c
===================================================================