This is interesting- in particular dkctl. I'd suggest that you could also add
functions going the *Other* way and add 'write check' functionality as well.



On Sun, 2 Sep 2001, Jason R Thorpe wrote:

> Folks...
> 
> I've been thinking of ways we can improve I/O performance, and one
> obvious way is to provide a mechanism to explicitly manipulate the
> read and write caches on disks.
> 
> The first step in this is to provide an abstract mechanism to query,
> enable/disable, and flush the caches on disks.  I.e. you don't want
> to litter knowledge of SCSI throughout the tree, and e.g. RAID controllers
> which have caches are likely to have different mechanisms for cache
> manipulation.
> 
> So, what I'm proposing are 3 new ioctls:
> 
> 	DIOCGCACHE	get cache enable bits
> 	DIOCSCACHE	set cache enable bits
> 	DIOCSYNCCCACHE	synchronize the cache
> 
> DIOC{G,S}CACHE use an `int' argument that has the following bits:
> 
> 	DKCACHE_READ	read cache enabled
> 	DKCACHE_WRITE	write(-back) cache enabled
> 
> DIOCCACHESYNC uses an `int' argument, treated as a boolean, indicating
> whether or not to force the cache flush.  This allows e.g. battery-backed
> RAID caches to skip the cache flush unless you tell it to REALLY REALLY
> flush it (e.g. you're going to swap out the disk).
> 
> I've written a cheezy little dkctl(8) program that uses these ioctls
> to do the obvious thing.  This program will need a little work before
> it goes into the tree.
> 
> But the eventual goal is for the file system to be able to explicitly
> use the cache flush operation as a barrier, which would let disks
> run with the write-back cache enabled in a safe fashion.  Something like
> this:
> 
> 	write out data -> sync cache -> write out metadata -> sync cache
> 
> Side note -- I'm considering a bit in `struct buf', maybe B_WRITETHROUGH,
> which would make it look more like this:
> 
> 	write out data -> sync cache -> write-through out metadata
> 
> ..saving a SCSI command (SCSI has a bit in the CDB that can force
> write-through on a per-write basis).
> 
> Attached are diffs that implement the above ioctls in the SCSI disk
> driver.  If we decide these are a good idea, then I'll add stubs in
> the other disk drivers as well (at least for the SYNCCACHE ioctl).
> 
> -- 
>         -- Jason R. Thorpe <thorpej@wasabisystems.com>
>