On Tuesday 04 December 2007 03:29:40 Nick Piggin wrote: > On Tue, Dec 04, 2007 at 01:55:17AM -0600, Rob Landley wrote: > > On Monday 03 December 2007 22:26:28 Nick Piggin wrote: > > > There is one slight downside -- direct block device access and > > > filesystem metadata access goes through an extra copy and gets stored > > > in RAM twice. However, this downside is only slight, because the real > > > buffercache of the device is now reclaimable (because we're not playing > > > crazy games with it), so under memory intensive situations, footprint > > > should effectively be the same -- maybe even a slight advantage to the > > > new driver because it can also reclaim buffer heads. > > > > For the embedded world, initramfs has pretty much rendered initrd > > obsolete, and that was the biggest user of the ramdisk code I know of. > > Beyond that, loopback mounts give you more flexible transient block > > devices than ramdisks do. (In fact, ramdisks are such an amazing pain to > > use/size/free that if I really needed something like that I'd just make a > > loopback mount in a ramfs instance.) > > They are, although we could easily hook up a couple more ioctls for them > now (if anybody is interested). >From a usability perspective, an ioctl is no substitute for being able to resize a device with "dd". (Or for that matter, make it sparse.) > The rd driver can potentially be a _lot_ more scalable than the loop > driver. It's completely lockless in the fastpath and doesn't even dirty any > cachelines except for the actual destination memory. OK, this is only > really important for testing purposes (eg. testing scalability of a > filesystem etc.) But it is one reason why I (personally) want brd... I wouldn't say not important: The "database in RAM" people will probably love you, at least if they insist on using Oracle. (Filesystem schmilesystem, gimme a raw block device and let me implement my own filesystem from userspace...) But I'm not personally likely to care. :) > > Embedded users who still want a block interface for memory are generally > > trying to use a cramfs or squashfs image out of ROM or flash, although > > there are flash-specific filesystems for this and I dunno if they're > > actually mounting /dev/mem at an offset or something (md? losetup -o? > > Beats me, I haven't tried that myself yet...) > > OK, it would be interesting to hear from anyone using rd for things like > cramfs. I'd be interested in that too. I've heard it proposed a lot but not actually seen anyone bother to implement it yet... Rob -- "One of my most productive days was throwing away 1000 lines of code." - Ken Thompson. - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
