xfs FAQ update for write cache

xfs FAQ update for write cache

[Timothy Shimmin]

Modid:  current:xfs-website:212892a
faq.html - 1.88 - changed
	- Update info about write cache.
	  Mention persistent write cache, external logs,
	  checking it was actually enabled in xfs msgs.

--Tim

Re: xfs FAQ update for write cache

[Martin Steigerwald]

Am Donnerstag 20 Juli 2006 08:02 schrieb Timothy Shimmin:
> Modid:  current:xfs-website:212892a
> faq.html - 1.88 - changed
> 	- Update info about write cache.
> 	  Mention persistent write cache, external logs,
> 	  checking it was actually enabled in xfs msgs.

Hello Thimothy,

thanks a lot... thats awesome... I have that directory corruption problem 
also mentioned in the FAQ on a workstation at work. When you have a new 
xfs_check available I can test it. (I know how to compile it under 
Knoppix 5 ;-).

A little feedback: lines are not wrapped in either Firefox or Konqueror 
(http://oss.sgi.com/projects/xfs/faq.html). This is for the complete FAQ. 
It makes reading it difficult.

It might make sense to include the log messages when barriers are disabled 
at the approbiate places of the FAQ:

root@deepdance:/usr/src/linux/fs/xfs -> grep -ir "barrier" *
linux-2.6/xfs_super.c:xfs_mountfs_check_barriers(xfs_mount_t *mp)
linux-2.6/xfs_super.c:            "Disabling barriers, not supported with 
external log device");
linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;
linux-2.6/xfs_super.c:            "Disabling barriers, not supported by 
the underlying device");
linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;
linux-2.6/xfs_super.c:  error = xfs_barrier_test(mp);
linux-2.6/xfs_super.c:            "Disabling barriers, trial barrier write 
failed");
linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;

While they are quite self-explanatory it might still help to make it 
absolutely clear what each log message mean.

Is the last one  "Disabling barriers, trial barrier write failed" issued 
when the underlying device does not support write barriers?

It would be good to know which drivers do and which don't but thats more 
of a kernel FAQ regarding write barrier support. Its probably best to 
test for oneself and use the logs ;-)

Regards,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7

Re: xfs FAQ update for write cache

[Timothy Shimmin]

Hi Martin,

Martin Steigerwald wrote:
> Am Donnerstag 20 Juli 2006 08:02 schrieb Timothy Shimmin:
>> Modid:  current:xfs-website:212892a
>> faq.html - 1.88 - changed
>> 	- Update info about write cache.
>> 	  Mention persistent write cache, external logs,
>> 	  checking it was actually enabled in xfs msgs.
> 
> Hello Timothy,
> 
> thanks a lot... thats awesome... I have that directory corruption problem 
> also mentioned in the FAQ on a workstation at work. When you have a new 
> xfs_check available I can test it. (I know how to compile it under 
> Knoppix 5 ;-).
> 
> A little feedback: lines are not wrapped in either Firefox or Konqueror 
> (http://oss.sgi.com/projects/xfs/faq.html). This is for the complete FAQ. 
> It makes reading it difficult.
I believe Nathan has fixed this now.

> 
> It might make sense to include the log messages when barriers are disabled 
> at the approbiate places of the FAQ:
> 
Yeah, I was thinking of that - like Dave (dgc) did in his reply.
If you think that is helpful.

> root@deepdance:/usr/src/linux/fs/xfs -> grep -ir "barrier" *
> linux-2.6/xfs_super.c:xfs_mountfs_check_barriers(xfs_mount_t *mp)
> linux-2.6/xfs_super.c:            "Disabling barriers, not supported with 
> external log device");
> linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;
> linux-2.6/xfs_super.c:            "Disabling barriers, not supported by 
> the underlying device");
> linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;
> linux-2.6/xfs_super.c:  error = xfs_barrier_test(mp);
> linux-2.6/xfs_super.c:            "Disabling barriers, trial barrier write 
> failed");
> linux-2.6/xfs_super.c:          mp->m_flags &= ~XFS_MOUNT_BARRIER;
> 
> While they are quite self-explanatory it might still help to make it 
> absolutely clear what each log message mean.
Ok.

> 
> Is the last one  "Disabling barriers, trial barrier write failed" issued 
> when the underlying device does not support write barriers?

You mean is it the same as the msg:
"Disabling barriers, not supported by the underlying device" ;-)
I guess the end result is pretty much the same but the test is
different. I didn't write the code (Christoph could provide more
info here) but looking at it now...

The "trial barrier write" actually tries a barrier write on the
superblock and sees if we get an error back.

Whereas the "not supported by" case, we use the information set by
the driver for the block layer - it sets one of the ordered modes
as described in the doc you quoted Documentation/block/barrier.txt.
We test for QUEUE_ORDERED_NONE to give this message.
The doc says:
   QUEUE_ORDERED_NONE
       I/O barriers are not needed and/or supported.
Looking at IDE case, it would set ordered mode to NONE if
write cache was on and not supported by drive.
But I guess perhaps the only definitive way to be sure is to
try it out and hence the trial write.

And as for the external log case, we _may_ end up changing the
code to flush on the data device and log device and so not
need this message and action. We'll see.

Cheers,
Tim.

Re: xfs FAQ update for write cache

[Martin Steigerwald]

Am Freitag 21 Juli 2006 08:17 schrieb Timothy Shimmin:

> > A little feedback: lines are not wrapped in either Firefox or
> > Konqueror (http://oss.sgi.com/projects/xfs/faq.html). This is for the
> > complete FAQ. It makes reading it difficult.
>
> I believe Nathan has fixed this now.

Hello Timothy,

yes. And it makes a huge difference in readability.

> > While they are quite self-explanatory it might still help to make it
> > absolutely clear what each log message mean.
>
> Ok.

Thanks for updating the FAQ. The XFS FAQ has the best and the only 
coverage on write caching and write barrier! No mention of it in ext3, 
Reiser(fs) and JFS FAQs.

BTW I can do a review of the complete FAQ. I have written lots of 
documentation and articles. Contact me when you are interested.

But first I continue writing my article ;)

Regards,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7