Messages in this thread | | | Date | Wed, 25 Mar 2009 11:00:41 -0400 | From | Theodore Tso <> | Subject | Re: Linux 2.6.29 |
| |
On Wed, Mar 25, 2009 at 01:37:44PM +0100, Jan Kara wrote: > > Also, we do have to reliably get a lock on the buffer when moving it > > between lists and inspecting its internal state. Otherwise a competing > > read from the underlying block device can trigger an assertion failure, > > and a competing write to the underlying block device can confuse ext3 > > journalling state completely. > > I've looked at this a bit. I suppose you mean the contention arising from > us taking the buffer lock in do_get_write_access()? But it's not obvious > to me why we'd be contending there... We call this function only for > metadata buffers (unless in data=journal mode) so there isn't huge amount > of these blocks.
There isn't a huge number of those blocks, but if inode #1220 was modified in the previous transaction which is now being committed, and we then need to modify and write out inode #1221 in the current contention, and they share the same inode table block, that would cause the contention. That probably doesn't happen that often in a synchronous code path, but it probably happens more often that you're thinking. I still think the fsync() problem is the much bigger deal, and solving the contention problem isn't going to solve the fsync() latency problem with ext3 data=ordered mode.
> Also when I emailed with a few people about these sync problems, they > wrote that switching to data=writeback mode helps considerably so this > would indicate that handling of ordered mode data buffers is causing most > of the slowdown...
Yes, but we need to be clear whether this was an fsync() problem or some other random delay problem. If it's the fsync() problem, obviously data=writeback will solve the fsync() latency delay problem. (As will using delayed allocation in ext4 or XFS.)
- Ted
| |