[lustre-devel] lustre and loopback device

[NeilBrown]

On Mon, Apr 02 2018, Jinshan Xiong wrote:

> Hi Neil,
>
> Sure. Patches are attached for your reference.
>
> The first patch is to bring llite_lloop driver back; the 2nd fixes some
> bugs and the 3rd one adds async I/O. The patches are based on 2.7.21, but I
> don't think it would be difficult to port them to master. Anyway, it's just
> for your reference.
>
> This is a piece of work in progress, please don't use it for production.

Thanks,
just one quick comment at this stage:


>  .PP
> +.SS Virtual Block Device Operation
> +Lustre is able to emulate a virtual block device upon regular file. It is necessary to be used when you are trying to setup a swap space via file.

We should fix this properly.  Creating a loop device just to provide
swap is not the best approach.
The preferred approach for swapping to a networked filesystem can be
seen by examining the swap_activate address_space_operation in nfs.
If a file passed to swap_on has a swap_activate operation, it will be
called and then ->readpage will be used to read from swap, and
->direct_IO will be used to write.

swap_activate needs to ensure that the direct_IO calls will never block
waiting for memory allocation.
For NFS, all that it does is calls sk_set_memalloc() on all network
sockets that might be used.  This allows TCP etc to use the reserve
memory pools.
Lustre might need to pre-allocate other things, or make use PF_MEMALLOC
in other contexts, I don't know.

Thanks,
NeilBrown
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