Re: Stalls during writeback for mmaped I/O on XFS in 3.0

[Shawn Bohrer]

Thanks Christoph,

On Thu, Sep 15, 2011 at 10:55:57AM -0400, Christoph Hellwig wrote:
> On Thu, Sep 15, 2011 at 09:47:55AM -0500, Shawn Bohrer wrote:
> > I've got a workload that is latency sensitive that writes data to a
> > memory mapped file on XFS.  With the 3.0 kernel I'm seeing stalls of
> > up to 100ms that occur during writeback that we did not see with older
> > kernels.  I've traced the stalls and it looks like they are blocking
> > on wait_on_page_writeback() introduced in
> > d76ee18a8551e33ad7dbd55cac38bc7b094f3abb "fs: block_page_mkwrite
> > should wait for writeback to finish"
> > 
> > Reading the commit description doesn't really explain to me why this
> > change was needed.
> 
> It it there to avoid pages beeing modified while they are under
> writeback, which defeats various checksumming like DIF/DIX, the iscsi
> CRCs, or even just the RAID parity calculations.  All of these either
> failed before, or had to work around it by copying all data was
> written.

I'm assuming you mean software RAID here?  We do have a hardware RAID
controller.  Also for anything that was working around this issue
before by copying the data, are those workarounds still in place?

> If you don't use any of these you can remove the call and things
> will work like they did before.

I may do this for now.

In the longer term is there any chance this could be made better?  I'm
not an expert here so my suggestions may be naive.  Could a mechanism
be made to check if the page needs to be checksummed and only block in
that case?  Or perhaps some mount option, madvise() flag or other hint
from user-mode to disable this, or hint that I'm going to be touching
that page again soon?

Thanks,
Shawn


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