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Re: [Qemu-devel] Is cache=writeback safe yet?

From: Virtbie
Subject: Re: [Qemu-devel] Is cache=writeback safe yet?
Date: Mon, 20 Feb 2012 16:43:18 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111124 Thunderbird/8.0

On 02/20/12 16:06, Anthony Liguori wrote:
On 02/20/2012 08:18 AM, Virtbie wrote:
Dear qemuers,
thanks for your exellent software.

I would like to use cache=writeback, but I still can't understand if this is
safe or not in case of power loss.

"Safe" is too simplistic of a view. The documentation in qemu-options.hx probably needs to be revisited.

cache=writeback emulates a large disk cache much the same as every modern hard drive has a builtin cache.

The only real difference is that the host cache is very, very large. Some modern file systems did not take the necessary steps to ensure consistency when a volatile write cache is present (ext3 up until very recently when barrier=1 became default).

In practice, this didn't create a huge issue because disk write caches are flushed often. Ted T'so has written a lot about the practical bits here.

But since the host cache is very large, and may not be flushed for many minutes after the initial write, this can exacerbate the problem.

In short, if you're using a recent kernel with ext3 or ext4, cache=writeback is absolutely safe. If you're using an older version of ext3, cache=writeback is still safe but ext3 itself isn't. cache=writeback can make the situation worse.

cache=writethrough presents a non-volatile cache which makes even older ext3 filesystems safe.


Anthony Liguori

Great explanation Anthony,

may I still ask:

Is WCE + volatile flag exposed to the guest, by all three virtual devices:
- virtio
- scsi
- ide
(if not, I still don't understand how this works)

2) Is there a minimum guest kernel and a minimum viostor Windows driver version, to see such WCE+volatile flag in a virtio disk, so that the guest OS can actually see the cache?

Because I can't seem to find such flag for a virtio disk on a linux 2.6.38 guest and that seems serious to me.
Is it visible somewhere in /sys hierarchy? Or is 2.6.38 too old?

Thank you

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