Re: Writes doubled by NILFS2

[Ryusuke Konishi <ryusuke-sG5X7nlA6pw@public.gmane.org>]

Hi,
On Tue, 20 Apr 2010 17:39:13 +0900, "Yongkun Wang" <yongkun-b/Nwfc8pyCekRP3n4FtbsiGn5s73+xxf@public.gmane.org> wrote:
> Hey, guys,
> 
> We have a database system, the data is stored on the disk formatted with
> NILFS2 (nilfs-2.0.15, kmod-nilfs-2.0.5-1.2.6.18_92.1.22.el5.x86_64).
> 
> I have run a trace at the system call level and the block IO level, that is,
> tracing the requests before processed by NILFS2 and after processed by
> NILFS2.
> 
> We use synchronous IO. So the amount of writes at the two trace points
> should be equal. 
> It is true when we use EXT2 file system.
> 
> However, for NILFS2, we found that the writes have been doubled, that is,
> the amount of writes is doubled after processed by NILFS2. The amount of
> writes at the system call level is equal between EXT2 and NILFS2. 

Interesting results.  What kind of synchronous write did you use in
the measurement ?  fsync? or O_SYNC writes ?
 
> Since all the address are log-structured, it is hard to know what are the
> additional writes.
>
> Can you provide some hints on the additional writes? Is it caused by some
> special functions such as snapshot?

You can look into the logs with dumpseg(8) command:

 # dumpseg <segment number>

This shows summary of blocks written in the specified segment. lssu(1)
command would be of help for finding a log head.


In the dump log, files with inode number 3,4,5,6 are metadata.  The
log format is depicted in the page 10 of the following slides:

  http://www.nilfs.org/papers/jls2009-nilfs.pdf


In general, copy-on-write filesystems including lfs are said to incur
overheads by metadata writes especially for synchronous writes.

I guess small-sized fsyncs or O_SYNC writes are causing the overhead.

Thanks,
Ryusuke
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