Re: Dirty deleted files cause pointless I/O storms (unless truncatedfirst)

From: Dave Chinner
Date: Mon Jan 20 2014 - 23:47:11 EST

On Mon, Jan 20, 2014 at 04:59:23PM -0800, Andy Lutomirski wrote:
> The code below runs quickly for a few iterations, and then it slows
> down and the whole system becomes laggy for far too long.
> Removing the sync_file_range call results in no I/O being performed at
> all (which means that the kernel isn't totally screwing this up), and
> changing "4096" to SIZE causes lots of I/O but without
> the going-out-to-lunch bit (unsurprisingly).

More details please. hardware, storage, kernel version, etc.

I can't reproduce any slowdown with the code as posted on a VM
running 3.31-rc5 with 16GB RAM and an SSD w/ ext4 or XFS. The
workload is only generating about 80 IOPS on ext4 so even a slow
spindle should be able handle this without problems...

> Surprisingly, uncommenting the ftruncate call seems to fix the
> problem. This suggests that all the necessary infrastructure to avoid
> wasting time writing to deleted files is there but that it's not
> getting used.

Not surprising at all - if it's stuck in a writeback loop somewhere,
truncating the file will terminate writeback because it end up being
past EOF and so stops immediately...


Dave Chinner
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