Date: Tue, 23 Jan 2007 08:03:07 -0600 From: Eric Anderson <anderson@freebsd.org> To: Oliver Fromme <olli@lurza.secnetix.de> Cc: vd@freebsd.org, silby@silby.com, xride@x12.dk, freebsd-geom@freebsd.org Subject: Re: Where to start? Message-ID: <45B6159B.8050703@freebsd.org> In-Reply-To: <200701231010.l0NAAcp6017303@lurza.secnetix.de> References: <200701231010.l0NAAcp6017303@lurza.secnetix.de>
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On 01/23/07 04:10, Oliver Fromme wrote: > Vasil Dimov wrote: > > [discussion moved from freebsd-hackers@ to freebsd-geom@] > > Oliver Fromme wrote: > > > Vasil Dimov wrote: > > > > Mike Silbersack wrote: > > > > > Soeren Straarup wrote: > > > > > [...] > > > > > > I'm looking for a project. > > > > > [...] > > > > > I'd like to see the ability to run gjournal without reformatting. > > > > > If you could create a dummy file inside the filesystem, then use > > > > > that area for the journal, it might be possible. I'm sure that > > > > > would let a lot more people see if journalling is right for them. > > > > > > > > I am not sure about gjournal internals but what if a system crash > > > > occurs in the middle of a transaction and the fs gets corrupted and > > > > the data, necessary to fix it is in the journal, but you cannot > > > > access the journal because the file, which contains the journal, > > > > is on a corrupted fs? > > > > > > I think you should still be able to mount the file system > > > read-only, even if it's not "clean", so there's no problem > > > locating the journal file. Particularly, note that the > > > journal file should probably be located in the root of the > > > file system, and it will have a constant size and should > > > be allocated from the start (i.e. it never grows nor > > > changes allocation), which means there is no way that its > > > meta data could be damaged. > > [...] > > > > With gjournal not only the metadata is journaled but also the actual > > data (e.g. files' contents). So writes to the journal file (which is a > > "regular file" on the FS) should bypass the journaling stuff. Otherwise > > they will get journaled themselves and it will end up in an endless > > loop. > > Right. That's a point I forgot. The journal file would > be required to have its data be exempt from being journaled. > That's probably not trivial to implement, I'm afraid (but > I could be wrong). > > > This thing still looks to me like roping your chest to your leg (instead > > of to an unmovable object) in order to avoid falling, but I might be > > wrong... > > True, it's certainly not a clean nor efficient solution. > But Mike has a valid point that it would enable people to > turn on journaling on existing file systems, without the > need for repartitioning or adding a disk. It would be a > nice way to _quickly_ set up journaling, for testing > purposes, or simply for curiosity. Why not disable swap, use the swap partition as the new journaling device, and then enable vn-backed swap for the system? > BTW, I think in Solaris you can also add journaling to an > existing UFS partition on the fly, without the need for > newfs or adding space. (Provided that there is enough > free space inside the existing file system, of course.) Sure - many journaling fs have that ability. There's been several attempts in the past to add journaling to our UFS2, without completion. Eric
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