[Qemu-devel] [PATCH 01/15] block: add -drive copy-on-read=on|off

[Stefan Hajnoczi]

This patch adds the -drive copy-on-read=on|off command-line option:

  copy-on-read=on|off
  copy-on-read is "on" or "off" and enables whether to copy read backing
  file sectors into the image file.  Copy-on-read avoids accessing the
  same backing file sectors repeatedly and is useful when the backing file
  is over a slow network.  By default copy-on-read is off.

The new BlockDriverState.copy_on_read field indicates whether
copy-on-read is enabled.  Block drivers can use this as a hint to copy
sectors read from the backing file into the image file.  The point of
copy-on-read is to avoid accessing the backing file again in the future.

Block drivers that do not honor the copy-on-read hint simply read data
without populating the image file.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 block.c         |    5 +++++
 block.h         |    1 +
 block_int.h     |    1 +
 blockdev.c      |    6 ++++++
 hmp-commands.hx |    5 +++--
 qemu-config.c   |    4 ++++
 qemu-options.hx |   10 +++++++++-
 7 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 9549b9e..3d074af 100644
--- a/block.c
+++ b/block.c
@@ -430,6 +430,11 @@ static int bdrv_open_common(BlockDriverState *bs, const 
char *filename,
     /* buffer_alignment defaulted to 512, drivers can change this value */
     bs->buffer_alignment = 512;
 
+    bs->copy_on_read = 0;
+    if (flags & BDRV_O_RDWR) {
+        bs->copy_on_read = !!(flags & BDRV_O_COPY_ON_READ);
+    }
+
     pstrcpy(bs->filename, sizeof(bs->filename), filename);
 
     if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv)) {
diff --git a/block.h b/block.h
index 59cc410..f6ffa93 100644
--- a/block.h
+++ b/block.h
@@ -34,6 +34,7 @@ typedef struct QEMUSnapshotInfo {
 #define BDRV_O_NATIVE_AIO  0x0080 /* use native AIO instead of the thread pool 
*/
 #define BDRV_O_NO_BACKING  0x0100 /* don't open the backing file */
 #define BDRV_O_NO_FLUSH    0x0200 /* disable flushing on this disk */
+#define BDRV_O_COPY_ON_READ 0x0400 /* copy read backing sectors into image */
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH)
 
diff --git a/block_int.h b/block_int.h
index efb6803..69dd5a7 100644
--- a/block_int.h
+++ b/block_int.h
@@ -154,6 +154,7 @@ struct BlockDriverState {
     int encrypted; /* if true, the media is encrypted */
     int valid_key; /* if true, a valid encryption key has been set */
     int sg;        /* if true, the device is a /dev/sg* */
+    int copy_on_read; /* if true, copy read backing sectors into image */
     /* event callback when inserting/removing */
     void (*change_cb)(void *opaque, int reason);
     void *change_opaque;
diff --git a/blockdev.c b/blockdev.c
index 0b8d3a4..b337732 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -237,6 +237,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
     const char *devaddr;
     DriveInfo *dinfo;
     int snapshot = 0;
+    int copy_on_read;
     int ret;
 
     translation = BIOS_ATA_TRANSLATION_AUTO;
@@ -253,6 +254,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
 
     snapshot = qemu_opt_get_bool(opts, "snapshot", 0);
     ro = qemu_opt_get_bool(opts, "readonly", 0);
+    copy_on_read = qemu_opt_get_bool(opts, "copy-on-read", 0);
 
     file = qemu_opt_get(opts, "file");
     serial = qemu_opt_get(opts, "serial");
@@ -517,6 +519,10 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi)
         bdrv_flags |= (BDRV_O_SNAPSHOT|BDRV_O_CACHE_WB|BDRV_O_NO_FLUSH);
     }
 
+    if (copy_on_read) {
+        bdrv_flags |= BDRV_O_COPY_ON_READ;
+    }
+
     if (media == MEDIA_CDROM) {
         /* CDROM is fine for any interface, don't check.  */
         ro = 1;
diff --git a/hmp-commands.hx b/hmp-commands.hx
index c857827..cbaa9a0 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -863,9 +863,10 @@ ETEXI
         .args_type  = "pci_addr:s,opts:s",
         .params     = "[[<domain>:]<bus>:]<slot>\n"
                       "[file=file][,if=type][,bus=n]\n"
-                      "[,unit=m][,media=d][index=i]\n"
+                      "[,unit=m][,media=d][,index=i]\n"
                       "[,cyls=c,heads=h,secs=s[,trans=t]]\n"
-                      "[snapshot=on|off][,cache=on|off]",
+                      "[,snapshot=on|off][,cache=on|off]\n"
+                      "[,readonly=on|off][,copy-on-read=on|off]",
         .help       = "add drive to PCI storage controller",
         .mhandler.cmd = drive_hot_add,
     },
diff --git a/qemu-config.c b/qemu-config.c
index b2ec40b..2e5ee3c 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -84,6 +84,10 @@ static QemuOptsList qemu_drive_opts = {
             .name = "readonly",
             .type = QEMU_OPT_BOOL,
             .help = "open drive file as read-only",
+        },{
+            .name = "copy-on-read",
+            .type = QEMU_OPT_BOOL,
+            .help = "copy read data from backing file into image file",
         },
         { /* end of list */ }
     },
diff --git a/qemu-options.hx b/qemu-options.hx
index 1d57f64..b7e52fe 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -135,7 +135,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
     "       [,cache=writethrough|writeback|none|unsafe][,format=f]\n"
     "       [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
-    "       [,readonly=on|off]\n"
+    "       [,readonly=on|off][,copy-on-read=on|off]\n"
     "                use 'file' as a drive image\n", QEMU_ARCH_ALL)
 STEXI
 @item -drive @var{option}[,@var{option}[,@var{option}[,...]]]
@@ -183,6 +183,9 @@ host disk is full; report the error to the guest otherwise).
 The default setting is @option{werror=enospc} and @option{rerror=report}.
 @item readonly
 Open drive @option{file} as read-only. Guest write attempts will fail.
address@hidden address@hidden
address@hidden is "on" or "off" and enables whether to copy read backing
+file sectors into the image file.
 @end table
 
 By default, writethrough caching is used for all block device.  This means that
@@ -210,6 +213,11 @@ like your host losing power, the disk storage getting 
disconnected accidently,
 etc. you're image will most probably be rendered unusable.   When using
 the @option{-snapshot} option, unsafe caching is always used.
 
+Copy-on-read avoids accessing the same backing file sectors repeatedly and is
+useful when the backing file is over a slow network.  By default copy-on-read
+is off.  Note that copy-on-read is a hint and may by ignored by block drivers
+which do not support it.
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom
-- 
1.7.5.4