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Re: [Qemu-devel] [PATCH] vpc: Implement bdrv_co_get_status()


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] vpc: Implement bdrv_co_get_status()
Date: Wed, 11 Feb 2015 17:51:38 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 2015-02-11 at 11:23, Kevin Wolf wrote:
This implements bdrv_co_get_status() for VHD images. This can
significantly speed up qemu-img convert operation because only with this
function implemented sparseness can be considered. (Before, converting a
1 TB empty image took several minutes for me, now it's instantaneous.)

Signed-off-by: Kevin Wolf <address@hidden>
---
  block/vpc.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
  1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index 46803b1..379ff4a 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -597,6 +597,55 @@ static coroutine_fn int vpc_co_write(BlockDriverState *bs, 
int64_t sector_num,
      return ret;
  }
+static int64_t coroutine_fn vpc_co_get_block_status(BlockDriverState *bs,
+        int64_t sector_num, int nb_sectors, int *pnum)
+{
+    BDRVVPCState *s = bs->opaque;
+    VHDFooter *footer =  (VHDFooter *) s->footer_buf;

Double space before the left parenthesis.

+    int64_t start, offset;
+    bool allocated;
+    int n;
+
+    if (be32_to_cpu(footer->type) == VHD_FIXED) {
+        *pnum = nb_sectors;
+        return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_DATA |
+               (sector_num << BDRV_SECTOR_BITS);
+    }
+
+    offset = get_sector_offset(bs, sector_num, 0);
+
+    *pnum = 1;
+    start = offset;
+    allocated = (offset != -1);
+    nb_sectors--;
+
+    while (nb_sectors > 0) {
+        offset = get_sector_offset(bs, sector_num, 0);

During the first iteration, this will is exactly the function call as the one before this loop.

+
+        if ((offset != -1) != allocated) {
+            break;
+        }
+
+        if (allocated && offset != start + (*pnum * 512)) {

Therefore, if allocated is true, this check will always fail because offset == start.

+            break;
+        }
+
+        /* All sectors in a block are contiguous (without using the bitmap) */
+        n = ROUND_UP(sector_num + 1, s->block_size / 512) - sector_num;
+        n = MIN(n, nb_sectors);

I think you can make use of this assumption even before calling get_sector_offset() in the first iteration.

Also, 512 is shorter than BDRV_SECTOR_SIZE and it's nice to get it on a single line, but I think not introducing another literal 512 in the code would be nicer; the same applies above in "*pnum * 512".

Max

+
+        *pnum += n;
+        sector_num += n;
+        nb_sectors -= n;
+    }
+
+    if (allocated) {
+        return BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start;
+    } else {
+        return 0;
+    }
+}
+
  /*
   * Calculates the number of cylinders, heads and sectors per cylinder
   * based on a given number of sectors. This is the algorithm described
@@ -907,8 +956,9 @@ static BlockDriver bdrv_vpc = {
      .bdrv_reopen_prepare    = vpc_reopen_prepare,
      .bdrv_create            = vpc_create,
- .bdrv_read = vpc_co_read,
-    .bdrv_write             = vpc_co_write,
+    .bdrv_read                  = vpc_co_read,
+    .bdrv_write                 = vpc_co_write,
+    .bdrv_co_get_block_status   = vpc_co_get_block_status,
.bdrv_get_info = vpc_get_info,




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