On 01/12/2010 12:59 PM, Ric Wheeler wrote:
On 01/12/2010 05:54 AM, Michal Novotny wrote:
On 01/11/2010 09:06 PM, Ric Wheeler wrote:
On 01/08/2010 04:36 AM, Michal Novotny wrote:
This patch extends functionality of e{2|4}fsprogs to add
EXT2_FLAG_DIRECT flag to be passed to ext2fs_open2() function. This
internally calls open() function with O_DIRECT and handles the memory
alignment for both read and write operations.
In some cases direct access to devices is necessary and that was the
main reason for this patch to be done.
The main reason why this was done is that pygrub (used by xen
virtualization user-space package, it's a python version of grub for
paravirtualized guests) sometimes uses outdated version of grub.conf
file. Modifications to xen package were *not* enough because
e2fsprogs doesn't open the files directly. That's why I added
EXT2_FLAG_DIRECT support to make read/write operations work directly
when passed. It's been tested with pygrub like mentioned above for
read operation and it's working fine.
Signed-off-by: Michal Novotny <minovotn@xxxxxxxxxx>
Can you add to this some kind of data flow overview? Seems like a
really odd way to update this file system...
ric
What do you mean by adding some kind of data flow overview? I am not
working on file systems but I needed option open files directly to be
added to e2fsprogs to make pygrub working right with not using
cached/outdated data so this was the first patch I did for file system
so I don't realy know what do you mean by data flow overview ...
Thanks,
Michal
Hi Michael,
What would be useful is where the data is written (guest/host) and the
state of the file system (mounted/unmounted). Which app it is that
writes the data that gets cached, who needs to read it and misses that
cached data.
From the above, it is really not clear that you should not be able to
use existing interfaces to flush any cached data to disk....
ric
Hi Ric,
I don't really know if I see your point but the thing here is that there
was no way to open a file directly (ie. using O_DIRECT). The direct
write support has been added only to make it possible to use both read
and write directly. The main reason to create this patch was to add
direct read support and flush capability won't help me at all. I am
working in Red Hat, Virtualization team on Xen so I am really not that
much familiar with file systems but what I needed was an option to read
the data directly (using O_DIRECT) in e2fsprogs. One bug was about
pygrub (Python version of GRUB of Xen PV guests that is internally using
e2fsprogs functionality to access data on ext2/3/4 partition to boot the
PV guests) uses outdated/cached data so some modifications were
necessary to open everything directly... It was done to Xen by a
collegue but there was some open() call with no O_DIRECT set so I
investigated this further and it turned out that this call was coming
from e2fsprogs library. That was the reason why I needed to add O_DIRECT
read support and O_DIRECT write support was added just to make it for
both read and write. For write, yes, there may be the option to flush
the disk but not for read. To workaround this but dropping caches is a
really ugly idea so finally patching e2fsprogs too (along with Xen)
turned out to the best idea. Once again, this was done to support
O_DIRECT for read function only - it is *not* used for write function
and it was added just to make it complete for both read and write
functions. Regarding the functionality of read function, this is done in
the host machine before the PV guest boots. It reads grub.conf, then it
reads initrd and kernel and boots from it... Some incidents of booting
outdated grub.conf/initrd and kernel has been reported to us and this
patch is to make it don't use caches so outdated data can't be used...
Is the scenario clear or do you need some other information about that?
Thanks,
Michal
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