On Wed, Sep 29, 2010 at 3:59 PM, Sean Bartell <[email protected]> wrote: > On Wed, Sep 29, 2010 at 02:45:29PM -0400, Yuehai Xu wrote: >> On Wed, Sep 29, 2010 at 1:08 PM, Sean Bartell <[email protected]> >> wrote: >> > On Wed, Sep 29, 2010 at 11:30:14AM -0400, Yuehai Xu wrote: >> >> I know BTRFS is a kind of Log-structured File System, which doesn't do >> >> overwrite. Here is my question, suppose file A is overwritten by A', >> >> instead of writing A' to the original place of A, a new place is >> >> selected to store it. However, we know that the address of a file >> >> should be recorded in its inode. In such case, the corresponding part >> >> in inode of A should update from the original place A to the new place >> >> A', is this a kind of overwrite actually? I think no matter what >> >> design it is for Log-Structured FS, a mapping table is always needed, >> >> such as inode map, DAT, etc. When a update operation happens for this >> >> mapping table, is it actually a kind of over-write? If it is, is it a >> >> bottleneck for the performance of write for SSD? >> > >> > In btrfs, this is solved by doing the same thing for the inode--a new >> > place for the leaf holding the inode is chosen. Then the parent of the >> > leaf must point to the new position of the leaf, so the parent is moved, >> > and the parent's parent, etc. This goes all the way up to the >> > superblocks, which are actually overwritten one at a time. >> >> You mean that there is no over-write for inode too, once the inode >> need to be updated, this inode is actually written to a new place >> while the only thing to do is to change the point of its parent to >> this new place. However, for the last parent, or the superblock, does >> it need to be overwritten? > > Yes. The idea of copy-on-write, as used by btrfs, is that whenever > *anything* is changed, it is simply written to a new location. This > applies to data, inodes, and all of the B-trees used by the filesystem. > However, it's necessary to have *something* in a fixed place on disk > pointing to everything else. So the superblocks can't move, and they are > overwritten instead. >
So, is it a bottleneck in the case of SSD since the cost for over write is very high? For every write, I think the superblocks should be overwritten, it might be much more frequent than other common blocks in SSD, even though SSD will do wear leveling inside by its FTL. What I current know is that for Intel x25-V SSD, the write throughput of BTRFS is almost 80% less than the one of EXT3 in the case of PostMark. This really confuses me. Thanks, Yuehai -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
