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 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 -- 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
