On Fri, 12 Sep 2008 19:08:52 +1000 "ronnie sahlberg" <ronniesahlberg at gmail.com> wrote: > On Fri, Sep 12, 2008 at 6:33 PM, FUJITA Tomonori > <fujita.tomonori at lab.ntt.co.jp> wrote: > > On Fri, 12 Sep 2008 18:18:34 +1000 > > "ronnie sahlberg" <ronniesahlberg at gmail.com> wrote: > > > >> If in the future STGT gets the ability to snapshot a lun or a set of > >> luns, > > > > You mean adding a snapshot feature to stgt (something like qcow > > file support)? > > I was thinking about it. It would be similar to, but not quite the > same as, qcow. > > > To match enterprise storage systems it is often useful to have > a production device LUN 1 then you associate to this device a "mirror" > device LUN128. > > > This association can be in two states, connected or detached. > > > While in the connected state, the "mirror" LUN 128 is identical to LUN1. > So everytime I/O is performed to LUN1 this is immediately visible in > LUN 128. They are in sync. > In this mode it is similar to a mirror like in normal disk mirroring. > > When it is in detached state it becomes a point in time copy of the > production lun. It is no longer mirroring the production disk and > instead is like a > point in time snapshot of the production LUN. Sounds that you can do this with a writable snapshot feature, which dm-snapshot supports. > The extra stuff now that makes this not just a copy on write > implementation is that once we have split/detached LUN128 so it > becomes a snapshot, > the backend for LUN1 still knows that there is a relation to LUN128 > and thus keeps track (bitmap) of which blocks have been modified > since we split off LUN128. > The purpose of this is to allow you to efficiently re-attach LUN128 > back to LUN1 again so that it once again become a mirror and while > doing so, since LUN1 knows which blocks have changed we can re-sync > the data efficiently (at least more efficiently that copying all > data). I'm not sure re-attach thing. For example, lun1 and lun128 have different data on sector 1 (either (or both) was modified after the detatch). So after the re-attach, if an initiator tries to update sector 1, the new data will be stored to both lun1 and lun128? what can we use this feature for? -- To unsubscribe from this list: send the line "unsubscribe stgt" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html |