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[Qemu-devel] [QEMU] VVFAT help required


From: Pintu Kumar
Subject: [Qemu-devel] [QEMU] VVFAT help required
Date: Sat, 15 Oct 2011 15:32:40 +0000 (UTC)
User-agent: Loom/3.14 (http://gmane.org/)

Dear Mr. Johannes Schindelin,

I needed your help in understanding and doing few modifications in VVFAT 
(vvfat.c).

I am working for Samsung india for Linux mobile platform division.
We wanted to experiment on VVFAT and plan to integrate it for one of our Linux 
mobile platform.
But before that we wanted to do some modifications with the current 
implementation on VVFAT.

Our major requirement:
We wanted to share ext4 partition content on Linux mobile to Windows PC (which 
recognises FAT file system only) when connected through USB cable and without 
network support.
We found that VVFAT is best suited for this purpose.

Our current modifications:
We started with latest version of VVFAT and did few modifications for our 
platform:
We divided the VVFAT into 2 part : VVFAT block driver + VVFAT daemon.
VVFAT block driver performs ext4 block layer operation.
VVFAT daemon scans the ext4 directory structure and build the mapping for FAT 
table.
Then we run the deamon by specifying our target directory of ext4 as follows:
# ./vvfatd  /opt/targetdir  &
Then we mount vvfat as follows:
/dev/vvfat              515584        48    515536   0% /mnt/movifs

Here /mnt/movifs is the partition which will be accessible to Windows using USB 
connection.

But we found that if our ext4 targetdir is very huge (which normally is and may 
grow in future) then mounting using vvfat takes long time.
After analysis we found that VVFAT scans and maps all the files directories and 
sub-directories in targetdir in the beginning itself.
Thus mounting all these content take huge time.


We wanted to change this approach.
We wanted to perform only partial scanning in the beginning. That is scanning 
and mapping only the top level files and directory. And do not scan the sub-
directory content except the _dot_ and _dotdot_ entries.
Then we wanted to introduce on-demand scanning and mapping of sub-directories 
whenever that particular sub-directory is accessed by the guest OS.
This will help us to reduce the mount time initially and also scan and create 
mapping only when it is required.

We could able to achieve part 1 that is partial scanning in the beginning.
But we are facing problem during on-demand scanning and mapping.

We wanted to know if this kind of partial scanning and dynamic scanning is 
possible to implement in VVFAT.

Please let me know your thoughts and suggestions to proceed further.

Please let me know if you need more information.

Thank you very much.


Thanks, Regards,
Pintu Kumar





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