Lustre discuss - Dec 2009 - creating and using loopback device on a file on a lustre filesystem?

Hi,

Would it be safe to create a file on a lustre file system and use it as 
a loop device for mounting?

Sort of doing something like this (creates a 100G file):

mount -t lustre msdserver at o2ib0:/lustre /mnt/lustre
dd if=/dev/zero of=/mnt/lustre/myfsfile bs=1024 count=104857600
mkfs.ext3 /mnt/lustre/myfsfile
mkdir /mnt/myfs
mount -o loop /mnt/lustre/myfsfile /mnt/myfs

On 2009-12-05, at 16:13, Richard Lefebvre wrote:
> Would it be safe to create a file on a lustre file system and use it  
> as
> a loop device for mounting?
>
> Sort of doing something like this (creates a 100G file):
>
> mount -t lustre msdserver at o2ib0:/lustre /mnt/lustre
> dd if=/dev/zero of=/mnt/lustre/myfsfile bs=1024 count=104857600
> mkfs.ext3 /mnt/lustre/myfsfile
> mkdir /mnt/myfs
> mount -o loop /mnt/lustre/myfsfile /mnt/myfs

You mean like sanity.sh test 54c does?  Yes.  However, I''m not totally
sure that ext3 on top of a loop device is "safe", regardless of  
whether this is Lustre or not.

Some users do this (with ext2) to have a node-local mechanism for  
accessing a lot of small files.  It is also possible to use such  
loopback files in read-only mode (again with ext2 only) from many  
nodes at one time.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

Thank you Andreas, the lots of small files scenario what the main reason 
I was looking into the loop device. As for the ext3, it was a quick 
example I had found on the net. I appreciate your advice on going to 
ext2 instead. I don''t want to put my large lustre system into trouble.

One thing I might be looking for in the future is something that does 
the same thing but is encrypted and user level mountable. I might have 
future users that have sensible data and simple UNIX ownership isn''t
enough.

Richard

Andreas Dilger wrote:
> On 2009-12-05, at 16:13, Richard Lefebvre wrote:
>> Would it be safe to create a file on a lustre file system and use it as
>> a loop device for mounting?
>>
>> Sort of doing something like this (creates a 100G file):
>>
>> mount -t lustre msdserver at o2ib0:/lustre /mnt/lustre
>> dd if=/dev/zero of=/mnt/lustre/myfsfile bs=1024 count=104857600
>> mkfs.ext3 /mnt/lustre/myfsfile
>> mkdir /mnt/myfs
>> mount -o loop /mnt/lustre/myfsfile /mnt/myfs
>
>
> You mean like sanity.sh test 54c does?  Yes.  However, I''m not
totally
> sure that ext3 on top of a loop device is "safe", regardless of 
> whether this is Lustre or not.
>
> Some users do this (with ext2) to have a node-local mechanism for 
> accessing a lot of small files.  It is also possible to use such 
> loopback files in read-only mode (again with ext2 only) from many 
> nodes at one time.
>
> Cheers, Andreas
> -- 
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.