Re: [Qemu-devel] [PATCH] use bdrv_flush to provide barrier semantic in block/vdi.c for metadata updates

[Max Reitz]

@subject: It's a bit long, and it's missing a prefix telling what this patch is about. I would have probably used "block/vdi: Use bdrv_flush after metadata updates" or something like that.

On 01.05.2015 17:03, phoeagon wrote:

Looks like VDI is the only writable image format that does not use write-with-barrier(sync) when updating the metadata. A sequence of commits b0ad5a455d~078a458e077d6b0db2 fixes this for QCOW/COW/QCOW2/VPC/VMDK, but the VDI does not issue a barrier by sync after updating the metadata.

This commit adds a `bdrv_flush` after updating block map.

Signed-off-by: Zhe Qiu <address@hidden>

Hm, this doesn't look quite right. :-)

---------------

These should be only "---", I guess, so the block below is omitted from the commit message.

From 2ea36d9a0e676b534483dc54c191f421f9889dc6 Mon Sep 17 00:00:00 2001

From: phoeagon <address@hidden>

Date: Fri, 1 May 2015 19:00:22 +0800

Subject: [PATCH] use bdrv_flush to provide barrier semantic in block/vdi.c

In reference to b0ad5a455d7e5352d4c86ba945112011dbeadfb8~078a458e077d6b0db262c4b05fee51d01de2d1d2, metadata writes to qcow2/cow/qcow/vpc/vmdk are all synced prior to succeeding writes.

---

 block/vdi.c | 1 +

 1 file changed, 1 insertion(+)

diff --git a/block/vdi.c b/block/vdi.c

index 7642ef3..5d09b36 100644

--- a/block/vdi.c

+++ b/block/vdi.c

@@ -713,6 +713,7 @@ static int vdi_co_write(BlockDriverState *bs,

         logout("will write %u block map sectors starting from entry %u\n",

                n_sectors, bmap_first);

         ret = bdrv_write(bs->file, offset, base, n_sectors);

+        ret = bdrv_flush(bs->file);

This overwrites the return value from bdrv_write(), which I don't think is right. We could either ignore bdrv_flush()'s return value, or make it something like "if (ret < 0) { bdrv_flush(bs->file); } else { ret = bdrv_flush(bs->file); }" or "ret_flush = bdrv_flush(bs->file); if (!(ret < 0)) { ret = ret_flush; }". Or skip the flush in case bdrv_write() failed ("if (ret < 0) { return ret; } ret = bdrv_flush(bs->file);"), like bdrv_pwrite_sync() does.

The idea of the change (adding the flush) looks good, though.

Max

     }

 

     return ret;

-- 

2.3.7