Re: Disabling block layer

[Jens Axboe <jens.axboe@oracle.com>]

On Mon, Mar 26 2007, Mark Lobo wrote:
> Hello!
> 
> I had a question about disabling the block layer for SCSI devices. We
> have an embedded device, and it runs 2.4.30. We need to be able to
> support a lot of SCSI devices (in the thousands) for our device, and we
> talk to the devices via SG. We are facing a memory allocation problem
> after discovering a few thousand devices. For every device,  there
> seems to be a lot of memory allocated in the block layer. This memory
> includes cache memory (which IIRC is reclaimable by the kernel memory
> subsystem when it needs it) and also pages that are used for the
> alloc_pages pool.

A much easier approach would be to limit the memory used for each device
in the block layer. Since SCSI uses the block layer as a transport for
commands, you cannot disable the block layer in any easy manner.

But your memory is likely being eated by the queue freelist. So edit
drivers/block/ll_rw_blk.c and hardcode nr_requests to a low number (like
2).

-- 
Jens Axboe