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So a provider has given us 500 IOPS/TB as their SLA standards for disk performance in a VMWare & RAID5-SAN environment. This is apparently measured with:

  • 16kB average transfer block size
  • 3:1 read:write ratio
  • Multithreaded IO operations
  • 80% random IO modelling
  • Read cache hit of 20%

What I want to do is determine whether any particular Linux VM is getting that performance, and then run the same benchmark with other providers so I can compare.

From looking around, it seems like fio is the most configurable to measure the above. The config I've got so far is:

[global]
blocksize=16k
rwmixread=75     # 3:1 read:write ratio
ramp_time=30
runtime=600
time_based
buffered=1
# size = free-ram * 80% / 5
# so we get a ~20% cache hit across the 5x processes
# this is for an 8GB ram host with 7.3GB free after buffers/cache
size=1180m

# create a mix to get to 80% random reads
# also means we'll be doing at least 5x IO operations in parallel
[sla-0]
readwrite=randrw:2

[sla-1]
readwrite=randrw:2

[sla-2]
readwrite=randrw

[sla-3]
readwrite=randrw

[sla-4]
readwrite=randrw

Suggestions for improvements? Is using buffered and the default ioengine the best way to go?

If I run this on an otherwise-idle 4x virtual-core machine with 8GB of RAM and 470GB of allocated storage, I'd expect to get 235 IOPS via the above (500 * 0.47). The results I get are:

sla-0: (g=0): rw=randrw, bs=16K-16K/16K-16K, ioengine=sync, iodepth=2
sla-1: (g=0): rw=randrw, bs=16K-16K/16K-16K, ioengine=sync, iodepth=2
sla-2: (g=0): rw=randrw, bs=16K-16K/16K-16K, ioengine=sync, iodepth=2
sla-3: (g=0): rw=randrw, bs=16K-16K/16K-16K, ioengine=sync, iodepth=2
sla-4: (g=0): rw=randrw, bs=16K-16K/16K-16K, ioengine=sync, iodepth=2
Starting 5 processes
sla-0: Laying out IO file(s) (1 file(s) / 1180MB)
sla-1: Laying out IO file(s) (1 file(s) / 1180MB)
sla-2: Laying out IO file(s) (1 file(s) / 1180MB)
sla-3: Laying out IO file(s) (1 file(s) / 1180MB)
sla-4: Laying out IO file(s) (1 file(s) / 1180MB)
Jobs: 5 (f=5): [mmmmm] [100.0% done] [5931K/1966K /s] [362/120 iops] [eta 00m:00s] 
sla-0: (groupid=0, jobs=1): err= 0: pid=16701
  read : io=1086MB, bw=1853KB/s, iops=115, runt=600003msec
    clat (usec): min=4, max=1771K, avg=8607.53, stdev=22114.44
    bw (KB/s) : min=    0, max= 4087, per=24.44%, avg=1914.96, stdev=1130.29
  write: io=372416KB, bw=635586B/s, iops=38, runt=600003msec
    clat (usec): min=6, max=2574, avg=57.38, stdev=79.65
    bw (KB/s) : min=    0, max=11119, per=26.07%, avg=679.63, stdev=517.84
  cpu          : usr=0.08%, sys=0.63%, ctx=64513, majf=0, minf=109
  IO depths    : 1=107.4%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=69474/23276, short=0/0
     lat (usec): 10=10.23%, 20=8.89%, 50=4.15%, 100=11.66%, 250=0.83%
     lat (usec): 500=1.48%, 750=1.41%, 1000=0.82%
     lat (msec): 2=0.83%, 4=1.56%, 10=47.07%, 20=5.91%, 50=4.24%
     lat (msec): 100=0.55%, 250=0.29%, 500=0.06%, 750=0.01%, 1000=0.01%
     lat (msec): 2000=0.01%
sla-1: (groupid=0, jobs=1): err= 0: pid=16702
  read : io=963360KB, bw=1605KB/s, iops=100, runt=600180msec
    clat (usec): min=4, max=2396K, avg=9934.23, stdev=30986.37
    bw (KB/s) : min=    0, max= 4657, per=21.64%, avg=1695.89, stdev=1273.00
  write: io=326000KB, bw=556206B/s, iops=33, runt=600180msec
    clat (usec): min=6, max=3882, avg=55.07, stdev=77.92
    bw (KB/s) : min=    0, max=10708, per=23.74%, avg=618.92, stdev=559.01
  cpu          : usr=0.08%, sys=0.53%, ctx=55500, majf=0, minf=129
  IO depths    : 1=108.5%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=60210/20375, short=0/0
     lat (usec): 10=11.36%, 20=9.63%, 50=3.56%, 100=11.97%, 250=0.81%
     lat (usec): 500=0.66%, 750=0.50%, 1000=0.37%
     lat (msec): 2=0.33%, 4=0.74%, 10=49.56%, 20=3.78%, 50=5.48%
     lat (msec): 100=0.60%, 250=0.43%, 500=0.16%, 750=0.04%, 1000=0.01%
     lat (msec): 2000=0.01%, >=2000=0.01%
sla-2: (groupid=0, jobs=1): err= 0: pid=16703
  read : io=827584KB, bw=1379KB/s, iops=86, runt=600012msec
    clat (usec): min=397, max=2396K, avg=11569.59, stdev=31237.03
    bw (KB/s) : min=    0, max= 4237, per=18.60%, avg=1457.59, stdev=1113.89
  write: io=276192KB, bw=471358B/s, iops=28, runt=600012msec
    clat (usec): min=8, max=8339, avg=63.95, stdev=121.52
    bw (KB/s) : min=    0, max= 8531, per=20.52%, avg=534.85, stdev=478.91
  cpu          : usr=0.07%, sys=0.54%, ctx=57019, majf=0, minf=89
  IO depths    : 1=109.9%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=51724/17262, short=0/0
     lat (usec): 10=0.98%, 20=5.38%, 50=3.53%, 100=13.68%, 250=0.92%
     lat (usec): 500=0.60%, 750=0.39%, 1000=0.22%
     lat (msec): 2=0.24%, 4=2.26%, 10=59.15%, 20=4.90%, 50=6.28%
     lat (msec): 100=0.78%, 250=0.48%, 500=0.18%, 750=0.03%, 1000=0.01%
     lat (msec): 2000=0.01%, >=2000=0.01%
sla-3: (groupid=0, jobs=1): err= 0: pid=16704
  read : io=865920KB, bw=1443KB/s, iops=90, runt=600005msec
    clat (usec): min=369, max=2396K, avg=11052.97, stdev=32396.85
    bw (KB/s) : min=    0, max= 5984, per=19.47%, avg=1525.97, stdev=1164.42
  write: io=285568KB, bw=487365B/s, iops=29, runt=600005msec
    clat (usec): min=7, max=11910, avg=65.72, stdev=154.09
    bw (KB/s) : min=    0, max=11064, per=21.38%, avg=557.30, stdev=534.59
  cpu          : usr=0.07%, sys=0.57%, ctx=59458, majf=0, minf=109
  IO depths    : 1=109.5%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=54120/17848, short=0/0
     lat (usec): 10=0.99%, 20=5.11%, 50=3.58%, 100=13.64%, 250=0.89%
     lat (usec): 500=0.71%, 750=0.48%, 1000=0.30%
     lat (msec): 2=0.70%, 4=4.00%, 10=57.63%, 20=5.21%, 50=5.40%
     lat (msec): 100=0.70%, 250=0.43%, 500=0.16%, 750=0.03%, 1000=0.01%
     lat (msec): 2000=0.01%, >=2000=0.01%
sla-4: (groupid=0, jobs=1): err= 0: pid=16705
  read : io=934752KB, bw=1558KB/s, iops=97, runt=600007msec
    clat (usec): min=187, max=2396K, avg=10236.87, stdev=26080.98
    bw (KB/s) : min=    0, max=11419, per=20.74%, avg=1625.28, stdev=1338.26
  write: io=304528KB, bw=519721B/s, iops=31, runt=600007msec
    clat (usec): min=7, max=7572, avg=67.29, stdev=117.27
    bw (KB/s) : min=    0, max=10772, per=22.06%, avg=575.17, stdev=560.68
  cpu          : usr=0.08%, sys=0.60%, ctx=63685, majf=0, minf=129
  IO depths    : 1=108.7%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=58422/19033, short=0/0
     lat (usec): 10=0.81%, 20=4.77%, 50=3.62%, 100=13.77%, 250=0.97%
     lat (usec): 500=1.45%, 750=0.64%, 1000=0.53%
     lat (msec): 2=1.75%, 4=4.71%, 10=53.48%, 20=6.92%, 50=5.53%
     lat (msec): 100=0.56%, 250=0.37%, 500=0.08%, 750=0.02%, 1000=0.01%
     lat (msec): 2000=0.01%, >=2000=0.01%

Run status group 0 (all jobs):
   READ: io=4593MB, aggrb=7836KB/s, minb=1412KB/s, maxb=1897KB/s, mint=600003msec, maxt=600180msec
  WRITE: io=1528MB, aggrb=2607KB/s, minb=471KB/s, maxb=635KB/s, mint=600003msec, maxt=600180msec

Disk stats (read/write):
  dm-0: ios=298995/596154, merge=0/0, ticks=3107720/433061790, in_queue=436170340, util=99.68%, aggrios=0/0, aggrmerge=0/0, aggrticks=0/0, aggrin_queue=0, aggrutil=0.00%
    sdb: ios=0/0, merge=0/0, ticks=0/0, in_queue=0, util=-nan%

Summing up read and write IOPS for each job (why doesn't fio include this in its summary?) I get 647, which seems like it's exceeding their specified service levels. Anything obvious I'm missing, or are their metrics skewed massively for some workloads (specifically I'm interested in PostgreSQL with data-warehouse workloads).

share|improve this question
    
I would hope they'd let you use more resources than their SLA if they aren't currently limited. When doing networking SLAs I've usually configured the QoS buckets such that users can use the entire pipe if they are the only users, but it starts limiting if there is contention. Is there some reason to think they haven't done something similar? –  polynomial Oct 5 '11 at 1:08
    
IO performance hasn't been stellar. As part of figuring out what exactly the problems are, I'm trying to replicate their own measurements. –  rcoup Oct 5 '11 at 2:05
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1 Answer

SQL and data warehouses are more like 8:1 reads to writes, all small block, all random. In any case, anything but random reads is easy to cache, and is not likely causing you disk performance issues. Without knowing how they do their disks, it's hard to really help much, but consider asking them what they mean when they specify "RAID5-SAN environment".

Since they specify an SLA as IOPS per TB, I'd hazard a guess that each volume they provide to you is supposed to be on a separate RAID-5, allowing for more IOPS as they add volumes. Bad performance could easily be caused by bad raid neighbours: volumes on the same raid as you that take more than their fair share of storage resources. The problem with this is that sometimes your SLA will be exceeded, but sometimes you'll have to deal with high latency.

Start off by warning them you're unhappy with performance, and they might simply move you to a lower utilized raid which might solve all your problems. Also ask them if they have some raid-10 storage available, and maybe ask for a volume there instead of the raid 5. If the problem comes back, then consider getting your own storage or finding some other host that can provide you better performance.

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