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Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

From: Avi Kivity
Subject: Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Date: Tue, 11 May 2010 22:11:46 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

On 05/11/2010 06:40 PM, Anthony Liguori wrote:

But if the goal is to make sure that fsync's don't result in data actually being on disk, there are many other ways to accomplish this. First, for the vast majority of users, this is already the case because ext3 defaults to disabling barriers. Alex is running into this issue only because SuSE enables barriers by default for ext3 and fsync()'s are horribly slow on ext3. The fact that this is measurable is purely a statement of ext3 suckage and a conscious decision by the SuSE team to live with that suckage. It shouldn't be nearly as bad on ext4.

There is a huge difference between disabling barriers and cache=volatile.

With barrier=0, data is still forced out of host pagecache and into disk cache; it's simply not forced from disk cache to the platter. But since the disk cache is very limited, you'll still be running at disk speed.

With cache=volatile and enough memory you'll never wait for the disk to write data.

On the other hand, the cost of adding another caching option is pretty significant. Very few users really understand what caching options they should use under what circumstance. Adding another option will just make that worse. The fact that this new option can result in data corruption that won't occur under normal circumstances is troubling.

I don't think it's a real problem with proper naming, but we can always hide the option behind a ./configure --enable-developer-options.

Mounting a filesystem with barrier=0 is not a good answer because it's a
global setting. While my qemu VM may be disposable, it's unlikely that the
same is true of the rest of my machine.

You can have multiple mounts. In fact, you can just make a loopback mount within your existing file system which means that you don't even have to muck with your disk.

If your VM is disposable, then shouldn't you be using -snapshot?

For my use case (autotest) the VM is not disposable (it's reused between tests) but I don't care about the data in case of a host crash.

   By your argument linux shouldn't be
allowing me to do that in the first place because a dumb sysadmin could use that option on the filesystem containing the mail store. In fact with the average user that's *likely* to happen because they'll only have a single
partition on their machine.

We aren't preventing sophisticated users from doing sophisticated things. But why should we simplify something that most people don't need and if they accidentally use it, bad things will happen?

We don't have a real alternative.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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