Hi Adam, On Wed, Feb 01, 2012 at 03:11:54PM -0800, Adam Berkan wrote: > When we make-bcache on a drive we need to replace the filesytem > superblock with a bcache superblock so the kernel knows to load the > drive through bcache, but this destroys the filesystem. We've talked well, I guess it will destroy the md superblock 1.1 too, how about LVM metadata? I think the mismatch is with /dev/bcacheX device. The first implementation, as far as I remember, was simply telling the caching device (using UUID) which was the backing device, i.e. it was registering the backing to the caching. Then, still if I got it right, the bcache was caching the backing device directly, without any need of a third device (/dev/bcacheX). I understand that the actual implementation is easier and, maybe, simpler, since a completely new device is added, which will have the new caching "features", while the old one (backing device) is just a further layer. This is similar to LVM over md over /dev/sdX. Nevertheless, my opinioni is, while still considering bcache a great project, that it should work on already existing devices, without touching them. Anyway, thanks a lot for the chat, bye, pg > about hacky ways to hide the bcache superblock somewhere else, but > it's very dangerous stuff that's likely to fail and we don't want to > support it. > > Adam > > > > On Wed, Feb 1, 2012 at 1:44 PM, Piergiorgio Sartor > <piergiorgio.sartor@xxxxxxxx> wrote: > > Hi Adam, > > > > On Wed, Feb 01, 2012 at 01:38:12PM -0800, Adam Berkan wrote: > >> Oh, sorry I misunderstood. > >> > >> You have to run make-bcache once to add a bcache superblock to the drive. > >> After that the drive contents are destroyed and it needs to be formatted > >> with a filesystem. > > > > ah! That's not good... > > > > Is there any plan to have the caching device attachable > > and detachable from *any* backing device without prior > > "formatting" of this second one? > > > > I think bcache is a very interesting and promising > > project, but formatting the backing device is > > something, I think, that should be avoided. > > > > bye, > > > > pg > > > >> At that point you can attach or detach the drive while it is in use. > >> > >> Adam > >> > >> On Wed, Feb 1, 2012 at 12:54 PM, Piergiorgio Sartor < > >> piergiorgio.sartor@xxxxxxxx> wrote: > >> > >> > Hi Adam, > >> > > >> > thanks for the answer, see below. > >> > > >> > On Wed, Feb 01, 2012 at 11:04:59AM -0800, Adam Berkan wrote: > >> > > You can attach bcache to a drive with an existing file system, and it > >> > will > >> > > continue as normal. If you connect to a drive without a file system, > >> > then > >> > > it will continue to not have a file system, but you can format it while > >> > > attached. > >> > > >> > Maybe I misused the term "format". > >> > > >> > I did not mean filesystem format, but bcache format. > >> > > >> > What I understood, maybe I'm wrong, is that the backing > >> > device, before being used, must be "initialized" with > >> > the bcache tool. > >> > > >> > From the docs: > >> > > >> > Getting started: > >> > You'll need make-bcache from the bcache-tools repository. Both the cache > >> > device > >> > and backing device must be formatted before use. > >> > make-bcache -B /dev/sdb > >> > make-bcache -C -w2k -b1M -j64 /dev/sdc > >> > > >> > I understand this as the backing device gets something > >> > on written on it (note the term "formatted"). > >> > > >> > Am I wrong? I hope so... > >> > > >> > Thanks again, > >> > > >> > bye, > >> > > >> > pg > >> > > >> > > Attach/detach should work while the device is in use. This isn't the > >> > most > >> > > tested code path, especially with writeback on, but it's supposed to > >> > work. > >> > > Detaching while the cache is dirty requires flushing all that data so > >> > > performance will be bad until the detach completes. > >> > > > >> > > Let us know if you find any bugs. > >> > > Adam > >> > > > >> > > On Wed, Feb 1, 2012 at 2:10 AM, Piergiorgio Sartor < > >> > > piergiorgio.sartor@xxxxxxxx> wrote: > >> > > > >> > > > Hi all, > >> > > > > >> > > > first of all I would like to congratulate for this > >> > > > project, I think it is one of the most promising > >> > > > feature the Linux kernel can have. > >> > > > > >> > > > Wrote that, I've a question about the concept of > >> > > > formatting the backing device. > >> > > > > >> > > > As far as I understood, the first concept of bcache > >> > > > was to simply "register" or "attach" a cache to a > >> > > > backing device, that is, the backing device had not > >> > > > to be formatted. > >> > > > > >> > > > Lately, still if I understood it correctly, this > >> > > > behaviour was changed and, now, the backing device > >> > > > needs to be formatted. > >> > > > > >> > > > So, the question is: > >> > > > > >> > > > How about an already running device? Is it still > >> > > > possible to attach a cache under such situation? > >> > > > > >> > > > In general, would it be possible to attach/detach > >> > > > a cache to any already available device (in the > >> > > > future)? Or the caching/backing setup must be planned > >> > > > before the HW is available, so to speak? > >> > > > > >> > > > It would be useful (and cool too), to have the > >> > > > possibility to attach/detach the SSD cache, on > >> > > > the fly (at run-time) to any device it needs it. > >> > > > > >> > > > I hope the question(s) are clear, if not please > >> > > > let me know. > >> > > > > >> > > > Thanks a lot in advance, > >> > > > > >> > > > bye, > >> > > > > >> > > > -- > >> > > > > >> > > > piergiorgio > >> > > > -- > >> > > > To unsubscribe from this list: send the line "unsubscribe > >> > linux-bcache" in > >> > > > the body of a message to majordomo@xxxxxxxxxxxxxxx > >> > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > >> > > > > >> > > >> > -- > >> > > >> > piergiorgio > >> > > > > > -- > > > > piergiorgio -- piergiorgio -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
