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SubjectRe: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter
On 03/17/2010 10:14 AM, Chris Webb wrote:
> Anthony Liguori<anthony@codemonkey.ws> writes:
>
>
>> This really gets down to your definition of "safe" behaviour. As it
>> stands, if you suffer a power outage, it may lead to guest
>> corruption.
>>
>> While we are correct in advertising a write-cache, write-caches are
>> volatile and should a drive lose power, it could lead to data
>> corruption. Enterprise disks tend to have battery backed write
>> caches to prevent this.
>>
>> In the set up you're emulating, the host is acting as a giant write
>> cache. Should your host fail, you can get data corruption.
>>
> Hi Anthony. I suspected my post might spark an interesting discussion!
>
> Before considering anything like this, we did quite a bit of testing with
> OSes in qemu-kvm guests running filesystem-intensive work, using an ipmitool
> power off to kill the host. I didn't manage to corrupt any ext3, ext4 or
> NTFS filesystems despite these efforts.
>
> Is your claim here that:-
>
> (a) qemu doesn't emulate a disk write cache correctly; or
>
> (b) operating systems are inherently unsafe running on top of a disk with
> a write-cache; or
>
> (c) installations that are already broken and lose data with a physical
> drive with a write-cache can lose much more in this case because the
> write cache is much bigger?
>

This is the closest to the most accurate.

It basically boils down to this: most enterprises use a disks with
battery backed write caches. Having the host act as a giant write cache
means that you can lose data.

I agree that a well behaved file system will not become corrupt, but my
contention is that for many types of applications, data lose ==
corruption and not all file systems are well behaved. And it's
certainly valid to argue about whether common filesystems are "broken"
but from a purely pragmatic perspective, this is going to be the case.
Regards,

Anthony Liguori


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