Re: [PATCH v2] nilfs2: improve the performance of fdatasync()

[Ryusuke Konishi <konishi.ryusuke-Zyj7fXuS5i5L9jVzuh4AOg@public.gmane.org>]

On Tue, 23 Sep 2014 10:46:58 +0200, Andreas Rohner wrote:
> Support for fdatasync() has been implemented in NILFS2 for a long time,
> but whenever the corresponding inode is dirty the implementation falls
> back to a full-flegded sync(). Since every write operation has to update
> the modification time of the file, the inode will almost always be dirty
> and fdatasync() will fall back to sync() most of the time. But this
> fallback is only necessary for a change of the file size and not for
> a change of the various timestamps.
> 
> This patch adds a new flag NILFS_I_INODE_SYNC to differentiate between
> those two situations.
> 
>  * If it is set the file size was changed and a full sync is necessary.
>  * If it is not set then only the timestamps were updated and
>    fdatasync() can go ahead.
> 
> There is already a similar flag I_DIRTY_DATASYNC on the VFS layer with
> the exact same semantics. Unfortunately it cannot be used directly,
> because NILFS2 doesn't implement write_inode() and doesn't clear the VFS
> flags when inodes are written out. So the VFS writeback thread can
> clear I_DIRTY_DATASYNC at any time without notifying NILFS2. So
> I_DIRTY_DATASYNC has to be mapped onto NILFS_I_INODE_SYNC in
> nilfs_update_inode().
> 
> Signed-off-by: Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>

I now sent this to Andrew.

The datasync segments that this patch creates more frequently, will
cause rollforward recovery after a crash or a power failure.

So, please test also that the recovery works properly for fdatasync()
and reset.  The situation can be simulated, for example, by using
"reboot -nfh":

 # dd if=/dev/zero of=/nilfs/test bs=4k count=1 seek=9999
 # dd if=/dev/urandom of=/nilfs/test bs=8k count=1 seek=50 conv=fdatasync,notrunc,nocreat
 # reboot -nfh

We can use dumpseg command to confirm that the datasync segment is
actually made or how recovery has done after mount.

Thanks,
Ryusuke Konishi
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