mark - good point about the controller cache and the 1-bye/block, but remember my files in this case are only 1 block.
dave - a long time ago I found/reported a bug in the way linux was doing their block stats. They were basically incrementing the byte counts as block were written to cache and the reported numbers were ridiculously high. I had wondered if something like this could be going on, but also remember the numbers being reported by xfs are much too high. I did 10K 1K writes, which I do understand are really 1-4k page each, but that's still only 40MB. If I add up all the 500MB/sec bytes xfs logging is reporting (even one of which is over 10 times larger), I see something on the order of of 10GB. But again this IS with the older kernel and so may not be worth worrying about.
-mark
On Mon, Jun 17, 2013 at 8:15 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
Right, and the XFS log is circular and about 1GB in size on theOn Mon, Jun 17, 2013 at 10:28:19PM +0200, Stefan Ring wrote:
> > I asked around and people believe the cache is on the order of a few GB and
> > the test ran I was intentionally large enough to overshadow any cache
> > effects, running for about a minute and doing 100K 1K file creates. The
> > disk write data was close to a sustained 475MB/sec and would have easily
> > filled the cache in the first handful of seconds which would have produced
> > enough backpressure to slow down the write rate which it never did.
>
> If a limited number of blocks gets written over and over, you won't
> ever fill the cache.
filesystem under test, so should fit completely in cache. Hence
speeds faster than a physical disk are acheivable if the log stays
resident in the cache....
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