Michal Novotny wrote:
On 01/12/2010 02:29 PM, Ric Wheeler wrote:
On 01/12/2010 08:23 AM, Michal Novotny wrote:
On 01/12/2010 02:12 PM, Michal Novotny wrote:
On 01/12/2010 02:04 PM, Ric Wheeler wrote:
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
That's right. I am going write an e-mail regarding this information to
the reproducer if this bug and tell him that I need more information
about what's happening at the customer side.
One more thing to point out, let's have a look at:
https://bugzilla.redhat.com/show_bug.cgi?id=466681#c15 .This is about
workaround to drop caches to be added to pygrub in the host machine
using this command:
echo 1> /proc/sys/vm/drop_caches
So this really looks like the caching issue if it's working fine after
dropping the caches. That may be the reason why this could be fine with
this patch present in e2fsprogs.
Michal
That BZ has a pretty long and twisted history, but after a quick
read, I still don't see why a cleanly shutdown guest would have
issues with caching that using O_DIRECT on read would help.
We will need to dig into a bit more...
ric
I am not saying we don't need to dig a little bit more, we surely do
but unfortunately I am waiting for information from reporter. But I am
also thinking that this O_DIRECT functionality support to bypass
caches could be useful...
Thanks,
Michal
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I can not see where the cache could cause this problem but is it
possible that it is in the Host file system rather than than the guest
where it is causing a problem; If a guest shuts down it writes to it's
file system and all is good only it's file system is a file on another
file system. So the cache looking after those writes as managed by the
hyper visor or whatever could hold that data un-flushed for whatever reason.
But when the guest starts up it should get the cached or most recent
version, this should not be stale data unless more than one guest is
using this file system and they are overwriting each others files, then
a cached version might be newer and different from what is expected and
the on disk version might be the old and expected version.
End user needs to provide more information.
Chris.
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