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Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

From: Avi Kivity
Subject: Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive
Date: Wed, 26 May 2010 16:06:15 +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 05/17/2010 03:58 PM, Anthony Liguori wrote:
On 05/17/2010 05:14 AM, Alexander Graf wrote:
Usually the guest can tell the host to flush data to disk. In some cases we
don't want to flush though, but try to keep everything in cache.

So let's add a new cache value to -drive that allows us to set the cache
policy to most aggressive, disabling flushes. We call this mode "volatile",
as guest data is not guaranteed to survive host crashes anymore.

This patch also adds a noop function for aio, so we can do nothing in AIO

Signed-off-by: Alexander Graf<address@hidden>

I'd like to see some performance data with at least an ext3 host file system and an ext4 file system.

My concern is that ext3 exaggerates the cost of fsync() which will result in diminishing value over time for this feature as people move to ext4/btrfs.

In fact, btrfs is currently unusable for virt because O_SYNC writes inflate a guest write to a host write. by a huge factor (50x-100x). cache=writethrough is 100% unusable, cache=writeback is barely tolerable. As of 2.6.32, cache=volatile is probably required to get something resembling reasonable performance on btrfs.

Of course, we expect that btrfs will improve in time, but still it doesn't seem to be fsync friendly.

error compiling committee.c: too many arguments to function

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