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Re: [Qemu-devel] [PATCH] qcow2: Metadata preallocation

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH] qcow2: Metadata preallocation
Date: Sun, 16 Aug 2009 14:58:03 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Thunderbird/3.0b3

On 08/14/2009 06:00 PM, Kevin Wolf wrote:
This introduces a qemu-img create option for qcow2 which allows the metadata to
be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
tables, but no data is written to them. Metadata is quite small, so this
happens in almost no time.

Especially with qcow2 on virtio this helps to gain a bit of performance during
the initial writes. However, as soon as create a snapshot, we're back to the
normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
while we're still having trouble with qcow2 on virtio.

Note that the option is disabled by default and needs to be specified
explicitly using qemu-img create -f qcow2 -o preallocation=metadata.

Can't say I'm thrilled with this. I'd prefer coalescing metadata updates on parallel writes. I don't object to this though.

+    /*
+     * It is expected that the image file is large enough to actually contain
+     * all of the allocated clusters (otherwise we get failing reads after
+     * EOF). So just write some zeros to the last sector.
+     */
+    if (cluster_offset != 0) {
+        uint8_t buf[512];
+        memset(buf, 0, 512);
+        bdrv_write(s->hd, (cluster_offset>>  9) + num - 1, buf, 1);
+    }

Older versions of Windows don't support sparse files, and newer ones need a flag. It's a good idea to set this flag when opening on Windows.

error compiling committee.c: too many arguments to function

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