On Aug 3, 2010, at 10:57 PM, Richard Elling wrote:
Unfortunately, zpool iostat is completely useless at describing
performance.
The only thing it can do is show device bandwidth, and everyone here
knows
that bandwidth is not performance, right? Nod along, thank you.
I totally understand that, I only used the output to show the space
utilization per raidz1 volume.
Yes, and you also notice that the writes are biased towards the
raidz1 sets
that are less full. This is exactly what you want :-) Eventually,
when the less
empty sets become more empty, the writes will rebalance.
Actually, if we are going to consider the values from zpool iostats,
they are just slightly biased towards the volumes I would want -- for
example, on the first post I've made, the volume with less free space
had 845GB free.. that same volume now has 833GB -- I really would like
to just stop writing to that volume at this point as I've experience
very bad performance in the past when a volume gets nearly full.
As a reference, here's the information I posted less than 12 hours ago:
# zpool iostat -v | grep -v c4
capacity operations bandwidth
pool used avail read write read write
------------ ----- ----- ----- ----- ----- -----
backup 35.2T 15.3T 602 272 15.3M 11.1M
raidz1 11.6T 1.06T 138 49 2.99M 2.33M
raidz1 11.8T 845G 163 54 3.82M 2.57M
raidz1 6.00T 6.62T 161 84 4.50M 3.16M
raidz1 5.88T 6.75T 139 83 4.01M 3.09M
------------ ----- ----- ----- ----- ----- -----
And here's the info from the same system, as I write now:
# zpool iostat -v | grep -v c4
capacity operations bandwidth
pool used avail read write read write
------------ ----- ----- ----- ----- ----- -----
backup 35.3T 15.2T 541 208 9.90M 6.45M
raidz1 11.6T 1.06T 116 38 2.16M 1.41M
raidz1 11.8T 833G 122 39 2.28M 1.49M
raidz1 6.02T 6.61T 152 64 2.72M 1.78M
raidz1 5.89T 6.73T 149 66 2.73M 1.77M
------------ ----- ----- ----- ----- ----- -----
As you can see, the second raidz1 volume is not being spared and has
been providing with almost as much space as the others (and even more
compared to the first volume).
I have the impression I'm getting degradation in performance due to
the limited space in the first two volumes, specially the second,
which has only 845GB free.
Impressions work well for dating, but not so well for performance.
Does your application run faster or slower?
You're a funny guy. :)
Let me re-phrase it: I'm sure I'm getting degradation in performance
as my applications are waiting more on I/O now than they used to do
(based on CPU utilization graphs I have). The impression part, is that
the reason is the limited space in those two volumes -- as I said, I
already experienced bad performance on zfs systems running nearly out
of space before.
Is there any way to re-stripe the pool, so I can take advantage of
all spindles across the raidz1 volumes? Right now it looks like the
newer volumes are doing the heavy while the other two just hold old
data.
Yes, of course. But it requires copying the data, which probably
isn't feasible.
I'm willing to copy data around to get this accomplish, I'm really
just looking for the best method -- I have more than 10TB free, so I
have some space to play with if I have to duplicate some data and
erase the old copy, for example.
Thanks,
Eduardo Bragatto
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