Re: BTRFS && SSD

[Yuehai Xu <yuehaixu@gmail.com>]

On Wed, Sep 29, 2010 at 3:59 PM, Sean Bartell <wingedtachikoma@gmail.com> 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 <wingedtachikoma@gmail.com> 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