On Wed, 21 February 2007 19:31:40 +0100, Juan Piernas Canovas wrote: > > I do not understand. Do you mean that if I have 10 segments, 5 busy and 5 > free, after cleaning I could need 6 segments? How? Where the extra blocks > come from?
This is a fairly complicated subject and I have trouble explaining it to people - even though I hope that maybe one or two dozen understand it by now. So let me try to give you an example: In LogFS, inodes are stored in an inode file. There are no B-Trees yet, so the regular unix indirect blocks are used. My example will be writing to a directory, so that should only involve metadata by your definition and be a valid example for DualFS as well. If it is not, please tell me where the difference lies. The directory is large, so appending to it involves writing a datablock (D0), and indirect block (D1) and a doubly indirect block (D2). Before: Segment 1: [some data] [ D1 ] [more data] Segment 2: [some data] [ D0 ] [more data] Segment 3: [some data] [ D2 ] [more data] Segment 4: [ empty ] ... After: Segment 1: [some data] [garbage] [more data] Segment 2: [some data] [garbage] [more data] Segment 3: [some data] [garbage] [more data] Segment 4: [D0][D1][D2][ empty ] ... Ok. After this, the position of D2 on the medium has changed. So we need to update the inode and write that as well. If the inode number for this directory is high, we will need to write the inode (I0), an indirect block (I1) and a doubly indirect block (I2). The picture becomes a bit more complicates. Before: Segment 1: [some data] [ D1 ] [more data] Segment 2: [some data] [ D0 ] [more data] Segment 3: [some data] [ D2 ] [more data] Segment 4: [ empty ] Segment 5: [some data] [ I1 ] [more data] Segment 6: [some data] [ I0 ] [more data] Segment 7: [some data] [ I2 ] [more data] ... After: Segment 1: [some data] [garbage] [more data] Segment 2: [some data] [garbage] [more data] Segment 3: [some data] [garbage] [more data] Segment 4: [D0][D1][D2][I0][I1][I2][ empty ] Segment 5: [some data] [garbage] [more data] Segment 6: [some data] [garbage] [more data] Segment 7: [some data] [garbage] [more data] ... So what has just happened? The user did a single "touch foo" in a large directory and has caused six objects to move. Unless some of those objects were in the same segment before, we now have six segments containing a tiny amount of garbage. And there is almost no way how you can squeeze that garbage back out. The cleaner will fundamentally do the same thing as a regular write - it will move objects. So if you want to clean a segment containing the block of a different directory, you may again have to move five additional objects, the indirect blocks, inode and ifile indirect blocks. At this point, your cleaner is becoming a threat. There is a real danger that it will create more garbage in unrelated segments than it frees up. I claim that you cannot keep 50% clean segments, unless you move away from the simplistic cleaner I described above. Jörn -- If you're willing to restrict the flexibility of your approach, you can almost always do something better. -- John Carmack - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
