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Re: [Qemu-devel] [PATCH] use bdrv_flush to provide barrier semantic in b

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] use bdrv_flush to provide barrier semantic in block/vdi.c for metadata updates
Date: Wed, 06 May 2015 19:36:42 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

CC-ing qemu-block and Stefan Weil (maintainer of vdi).

On 06.05.2015 19:23, phoeagon wrote:
Thanks for your input.

So I changed it to:
1. Only call bdrv_flush when bdrv_pwrite was successful
2. Only if bdrv_flush was unsuccessful that the return value of vdi_co_write is updated.

One of both is enough. Both are too much. :-)

It is indeed correct, technically (because ret is 0 before the bdrv_write()), but it's too verbose. (See below)

In this way we try to avoid messing up any potential return value checks possible while still propagating bdrv_flush errors. 
That return value was a catch and I admit I'm no pro with the return value convention in QEMU. bdrv_pwrite doesn't return the same value as bdrv_pwrite_sync I assume (they do return negative values when fail, but different values when successful)

It doesn't really matter, I think. Returning any non-negative value from vdi_co_write() should be enough to signal success.


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

From 19b2fabbe00765b418362d8c1891f266091621f3 Mon Sep 17 00:00:00 2001
From: phoeagon <address-hidden>
Date: Thu, 7 May 2015 01:09:38 +0800
Subject: [PATCH] block/vdi: Use bdrv_flush after metadata updates

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

 block/vdi.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/vdi.c b/block/vdi.c
index 5d09b36..54a5fa8 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -713,7 +713,11 @@ 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);
+        if (!(ret < 0)) {
+            int flush_ret = bdrv_flush(bs->file);
+            if (flush_ret < 0)
+                ret = flush_ret;
+        }

I think bdrv_write() always returns 0 on success. In any case, it's fine for vdi_co_write() to return 0 on success (which is what bdrv_flush() returns), so shorting these four lines to "ret = bdrv_flush(bs->file);" is enough.

The patch is correct, though, so if you want to leave it as it is, all you need to do is bring it into proper form (http://wiki.qemu.org/Contribute/SubmitAPatch).

The previous version was nearly right, except for the things I mentioned: The subject needs to start with the part of qemu the patch is targeting (in this case "block/vdi: " or simply "vdi: "), the Signed-off-by needs to contain your name (or any alias you desire) and your email address, and comments for the patch should be separated from the actual commit message by "---".

Finally, for sending the next version, please change the "[PATCH]" in the subject to "[PATCH v3]" in order to indicate that it will be version 3 of this patch.



     return ret;

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