Chris Mason wrote:
On Mon, 2009-04-06 at 17:01 -0500, Steven Pratt wrote:
I am continuing to do runs to provide more data on the random write
issues with btrfs. I have just posted 2 sets of runs here:
http://btrfs.boxacle.net/repository/raid/longrun/
these are on a pull of the btrfs-unstable experimental branch from 4/3.
These are 100 minute runs of the 128 thread random write workload on the
raid system (1 for btrfs and 1 for ext3). Included in these runs are
graphs of all the iostat, sar and mpstat data (see analysis directories).
A couple of interesting things. First, we see the choppiness of the IO
in btrfs compared to ext3.
http://btrfs.boxacle.net/repository/raid/longrun/btrfs-longrun/btrfs1.ffsb.random_writes__threads_0128.09-04-06_10.25.03/analysis/iostat-processed.001/chart.html
http://btrfs.boxacle.net/repository/raid/longrun/ext3-longrun/btrfs1.ffsb.random_writes__threads_0128.09-04-06_13.44.49/analysis/iostat-processed.001/chart.html
In particular look at graphs 7 and 11 which show write iops and
throughput. Ext3 is nice and smooth, while btrfs has a repeating
pattern of dips and spikes, with IO going to 0 on a regular basis.
The dips and spikes may be from the allocator. Basically what happens
is after each commit we end up with a bunch of small blocks available
for filling again. Could you please try with -o ssd?
Will give it a shot.
Another interesting observation is what looks a lot like a memory leak.
Looking at chart 6 Memory at :
http://btrfs.boxacle.net/repository/raid/longrun/btrfs-longrun/btrfs1.ffsb.random_writes__threads_0128.09-04-06_10.25.03/analysis/sar-processed.001/chart.html
we see that the amount of page cache drops slowly throughout the entire
run. Starting up around 3.5GB and dropping to about 2.3GB by the end of
the run. The memory seems to have moved to the slab which grew to
1.5GB. Doing a repeat of the run while watching slabtop, we see that
size-2048 is responsible for the majority of the slab usage (over 1GB).
size-2048? That's probably the csums. I'll give it a shot when I get
back next week
Ok.
One other thing I noticed that is really bad. For ext3, we see
115MB/sec both from the benchmark reporting and from iostat write
throughput. However, for btrfs, we see a benchmark throughput of
2.5MB/sec while iostat shows a whopping 35MB/sec of writes. That to me
implies that btrfs is doing an additional 32-33MB/sec of metadata or
journal writes. More than 10x the amount of actual data being written.
Can that be right?
Steve
.
-chris
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