From 25b5ec4399eec0a03d91a103658a4fba4263ba4d Mon Sep 17 00:00:00 2001
From: Max Reitz <mreitz@redhat.com>
Date: Mon, 18 Jun 2018 18:04:50 +0200
Subject: [PATCH 073/268] qemu-img: Special post-backing convert handling

RH-Author: Max Reitz <mreitz@redhat.com>
Message-id: <20180618180451.23808-2-mreitz@redhat.com>
Patchwork-id: 80795
O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 1/2] qemu-img: Special post-backing convert handling
Bugzilla: 1527898
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Jeffrey Cody <jcody@redhat.com>

Currently, qemu-img convert writes zeroes when it reads zeroes.
Sometimes it does not because the target is initialized to zeroes
anyway, so we do not need to overwrite (and thus potentially allocate)
it.  This is never the case for targets with backing files, though.  But
even they may have an area that is initialized to zeroes, and that is
the area past the end of the backing file (if that is shorter than the
overlay).

So if the target format's unallocated blocks are zero and there is a gap
between the target's backing file's end and the target's end, we do not
have to explicitly write zeroes there.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180501165750.19242-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 351c8efff9ad809c822d55620df54d575d536f68)
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 qemu-img.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index 76d6e47..e2395b9 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1553,7 +1553,9 @@ typedef struct ImgConvertState {
     BlockBackend *target;
     bool has_zero_init;
     bool compressed;
+    bool unallocated_blocks_are_zero;
     bool target_has_backing;
+    int64_t target_backing_sectors; /* negative if unknown */
     bool wr_in_order;
     int min_sparse;
     size_t cluster_sectors;
@@ -1582,12 +1584,23 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
 {
     int64_t src_cur_offset;
     int ret, n, src_cur;
+    bool post_backing_zero = false;
 
     convert_select_part(s, sector_num, &src_cur, &src_cur_offset);
 
     assert(s->total_sectors > sector_num);
     n = MIN(s->total_sectors - sector_num, BDRV_REQUEST_MAX_SECTORS);
 
+    if (s->target_backing_sectors >= 0) {
+        if (sector_num >= s->target_backing_sectors) {
+            post_backing_zero = s->unallocated_blocks_are_zero;
+        } else if (sector_num + n > s->target_backing_sectors) {
+            /* Split requests around target_backing_sectors (because
+             * starting from there, zeros are handled differently) */
+            n = s->target_backing_sectors - sector_num;
+        }
+    }
+
     if (s->sector_next_status <= sector_num) {
         int64_t count = n * BDRV_SECTOR_SIZE;
 
@@ -1609,7 +1622,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
         n = DIV_ROUND_UP(count, BDRV_SECTOR_SIZE);
 
         if (ret & BDRV_BLOCK_ZERO) {
-            s->status = BLK_ZERO;
+            s->status = post_backing_zero ? BLK_BACKING_FILE : BLK_ZERO;
         } else if (ret & BDRV_BLOCK_DATA) {
             s->status = BLK_DATA;
         } else {
@@ -2330,6 +2343,16 @@ static int img_convert(int argc, char **argv)
         }
     }
 
+    if (s.target_has_backing) {
+        /* Errors are treated as "backing length unknown" (which means
+         * s.target_backing_sectors has to be negative, which it will
+         * be automatically).  The backing file length is used only
+         * for optimizations, so such a case is not fatal. */
+        s.target_backing_sectors = bdrv_nb_sectors(out_bs->backing->bs);
+    } else {
+        s.target_backing_sectors = -1;
+    }
+
     ret = bdrv_get_info(out_bs, &bdi);
     if (ret < 0) {
         if (s.compressed) {
@@ -2339,6 +2362,7 @@ static int img_convert(int argc, char **argv)
     } else {
         s.compressed = s.compressed || bdi.needs_compressed_writes;
         s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
+        s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero;
     }
 
     ret = convert_do_copy(&s);
-- 
1.8.3.1