On Monday 16 March 2009 17:38:30 Theodore Tso wrote: > Dave, > > It wasn't my intention to say that XFS was bad; in fact, I thought I > was actually complementing XFS by talking about some of the advanced > features that XFS had (many of which I have always said that ext3 has, > and some of which ext4 still does not have, and probably never will > have). I stand corrected on some of the details that I got wrong. > What I was trying to say was that *if* (and perhaps I'm > misunderstanding fsblock) that fsblock is requiring that as soon as a > page is dirty, fsblock requests the filesystem to assign a block > allocation to the buffers attached to the dirty page, that this would > spike out delayed allocation, which would be unfortunate for *both* > ext4 and XFS. > > But maybe I'm misunderstanding what fsblock is doing, and there isn't > a problem here. Yeah, Dave's understanding of fsblock is correct. I might have stated things confusingly... fsblock allocates the in-memory fsblock metadata structure (~= struct buffer_head) at the time of block dirtying. It also asks the filesystem to respond to the dirtying event appropriately. In the case of say ext2, this means allocating a block on disk. Wheras XFS does the delalloc/reserve thing (yes, XFS appears to be working with fsblock well enough to get this far). fsblock really isn't too much different to buffer_head from an abstract capability / functionality point of view except that it is often more strict with things where I feel it makes sense. So for this particular example; in buffer.c, buffers do tend to be allocated when a page is dirtied, but not always, and even when they are, they can get reclaimed while the page is still dirty. fsblock tigtens this up. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
