On 01/12/2010 08:01 AM, Michal Novotny wrote:
On 01/12/2010 01:46 PM, Christoph Hellwig wrote:
On Tue, Jan 12, 2010 at 01:30:40PM +0100, Michal Novotny wrote:
Not really, pygrub doesn't do any manipulation with file system and
also, it's not working on a life file system. It's called before the
guest boots up to read information about grub.conf/initrd and kernel for
PV guest and after this is read and selected in pygrub then the guest is
booted using the kernel and initrd extracted from the image (after which
the file is closed). Once again, nothing uses write support and it was
added just to make it use O_DIRECT for both read and write operations
but only pygrub uses only read support and O_DIRECT passed here is the
only way to make it use non-cached data.
So what caches get in the way? From the above it seems the situation
is the following:
- filesystem N is a guest filesystem. It's not usually mounted on the
host, except for initial setup long time ago
Yes, it is really a guest file system. This is not mounted in the host
and the reason is to get actual version of grub.conf, initrd and kernel
to be booted...
- before booting a guest your "pygrub" tools needs to read files on
it, and it's doing so using e2fsprogs
Correct.
- once the guest is life it uses the extN kernel driver to access the
filesystem
That's right. So this is no longer pygrub responsibility...
nowhere in this cycle you should have any stale cached data. The kernel
always makes sure to write back data on umount/reboot, as does e2fsprogs
if actually used to write data (which you said is not the case anyway).
In fact I was unable to run into those problems myself but
reporter/customer did.
The only data that may be in the cache are unmodified data from reads
on the block device from either e2fsprogs or a suboptimal virtual block
device implementation, but these can't cause any problems.
Michal
If the guest is the only one (when running) that installs a new grub.conf file
and kernel and it shuts down properly, you should be good. It if does not shut
down cleanly, it could have a stale grub.conf file (or worse, a partially
written one), but using O_DIRECT to bypass the file system cache should not help.
If we cannot reproduce this failure, sounds like we need to go back and get a
better understanding of what the customer saw?
ric
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