Re: extremely slow nfs when sync enabled

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, May 07, 2012 at 01:59:42PM +0000, Daniel Pocock wrote:
> 
> 
> On 07/05/12 09:19, Daniel Pocock wrote:
> > 
> >>> Ok, so the combination of:
> >>>
> >>> - enable writeback with hdparm
> >>> - use ext4 (and not ext3)
> >>> - barrier=1 and data=writeback?  or data=?
> >>>
> >>> - is there a particular kernel version (on either client or server side)
> >>> that will offer more stability using this combination of features?
> >>
> >> Not that I'm aware of. As long as you have a kernel > 2.6.29, then LVM
> >> should work correctly. The main problem is that some SATA hardware tends
> >> to be buggy, defeating the methods used by the barrier code to ensure
> >> data is truly on disk. I believe that XFS will therefore actually test
> >> the hardware when you mount with write caching and barriers, and should
> >> report if the test fails in the syslogs.
> >> See http://xfs.org/index.php/XFS_FAQ#Write_barrier_support.
> >>
> >>> I think there are some other variations of my workflow that I can
> >>> attempt too, e.g. I've contemplated compiling C++ code onto a RAM disk
> >>> because I don't need to keep the hundreds of object files.
> >>
> >> You might also consider using something like ccache and set the
> >> CCACHE_DIR to a local disk if you have one.
> >>
> > 
> > 
> > Thanks for the feedback about these options, I am going to look at these
> > strategies more closely
> > 
> 
> 
> I decided to try and take md and LVM out of the picture, I tried two
> variations:
> 
> a) the boot partitions are not mirrored, so I reformatted one of them as
> ext4,
> - enabled write-cache for the whole of sdb,
> - mounted ext4, barrier=1,data=ordered
> - and exported this volume over NFS
> 
> unpacking a large source tarball on this volume, iostat reports write
> speeds that are even slower, barely 300kBytes/sec

How many file creates per second?

--b.

> 
> b) I took an external USB HDD,
> - created two 20GB partitions sdc1 and sdc2
> - formatted sdc1 as btrfs
> - formatted sdc2 as ext4
> - mounted sdc2 the same as sdb1 in test (a),
>       ext4, barrier=1,data=ordered
> - exported both volumes over NFS
> 
> unpacking a large source tarball on these two volumes, iostat reports
> write speeds that are around 5MB/sec - much faster than the original
> problem I was having
> 
> Bottom line, this leaves me with the impression that either
> - the server's SATA controller or disks need a firmware upgrade,
> - or there is some issue with the kernel barriers and/or cache flushing
> on this specific SATA hardware.
> 
> I think it is fair to say that the NFS client is not at fault, however,
> I can imagine many people would be tempted to just use `async' when
> faced with a problem like this, given that async makes everything just
> run fast.
> 
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