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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<> 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.


    Anthony Liguori

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