[Libvir] Re; virDomainBlockStats

[Daniel P. Berrange]

On Sat, Jan 26, 2008 at 11:29:54PM +0000, Gareth Bult wrote:
> Is there any documentation to the effect that "file" is bad??
> The official XEN documentation lists "file" as the standard and makes no mention of "aio".

The upstream documentation leaves alot to be desired. First of all I
should say, there is a difference between file: with PV vs file: with
HVM. file: with HVM has no problem, because all IO is handled by QEMU.
It is only file: with PV guests that is flawed. This is because it
uses loopback devices to acess the file. Loop devices cache data writes
in memory for an undefined amount of time, even if the guest requests
a sycn to disk. The result is that if the host OS crashes your guest
disks can loss huge amounts of data that they thought were already synced
to disk. Not even a journalling FS helps, because there's no write ordering
guarentees.

tap:aio: addresses all these issues by doing Direct IO + Async I/O
to allow you to have multiple outstanding I/O operations, to avoid the
host OS buffer cache, and to provide firm guarentee that the data is on
disk when the guest asks for it to be.


> On Sat, Jan 19, 2008 at 05:18:58PM +0000, Gareth Bult wrote:
> > Mmm,
> > 
> > Interesting ..
> > 
> > First off, xentop doesn't display block device stats for tap:aio based systems and it does for file.
> > Second, tap:aio generated kernel Ooops's when you shutdown a DomU.
> > 
> > Not exactly what I'd call mainstream (!)
> 
> That must be a flaw in Ubuntu's kernels. tap:aio works flawlessly
> in Fedora / RHEL and is the only supported option, because file:
> has catastrophic data loss issues during host crashes.


Dan.
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