On Wed, Sep 29, 2010 at 07:43:27PM -0400, Christoph Hellwig wrote: > On Wed, Sep 29, 2010 at 10:04:31AM +0200, Kay Sievers wrote: > > On Wed, Sep 29, 2010 at 09:25, Ric Wheeler <rwhee...@redhat.com> wrote: > > > > > Second question is why is checking in /sys a big deal, would ??you prefer > > > an > > > interface like we did for alignment in libblkid? > > > > It's about knowing what's behind the 'nodev' major == 0 of a btrfs > > mount. There is no way to get that from /sys or anywhere else at the > > moment. > > > > Usually filesystems backed by a disk have the dev_t of the device, or > > the fake block devices like md/dm/raid have their own major and the > > slaves/ directory pointing to the devices. > > > > This is not only about readahead, it's every other tool, that needs to > > know what kind of disks are behind a btrfs 'nodev' major == 0 mount. > > Thanks for explaining the problem. It's one that affects everything > with more than one underlying block device, so adding a > filesystem-specific ioctl hack is not a good idea. As mentioned in this > mail we already have a solution for that - the block device slaves > links used for raid and volume managers. The most logical fix is to > re-use that for btrfs as well and stop it from abusing the anonymous > block major that was never intended for block based filesystems (and > already has caused trouble in other areas). One way to to this might > be to allocate a block major for btrfs that only gets used for > representing these links. >
Ok I've spent a few hours on this and I'm hitting a wall. In order to get the sort of /sys/block/btrfs-# sort of thing I have to do 1) register_blkdev to get a major 2) setup a gendisk 3) do a bdget_disk 4) Loop through all of our devices and do a bd_claim_by_disk on each of them This sucks because for step #2 I have to have a request_queue for the disk. It's a bogus disk, and theres no way to not have a request_queue, so I'd have to wire that up and put a bunch of WARN_ON()'s to make sure nobody is trying to write to our special disk (since I assume that if I go through all this crap I'm going to end up with a /dev/btrfs-# that people are going to try to write to). So my question is, is this what we want? Do I just need to quit bitching and make it work? Or am I doing something wrong? This is a completely new area for me so I'm just looking around at what md/dm does and trying to mirror it for my own uses, if thats not what I should be doing please tell me, otherwise this seems like alot of work for a very shitty solution to our problem. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
