title: GCE SSD persistent disk slower than Standard persistent disk
body: <p>We are using GCE for a MongoDB replica set with three members. As our data is quite large the initial sync for a new member is taking quite a lot. In our case the initial sync takes 7 hours for copying records and then 30 hours to create indexes.</p>
  
  <p>The database is stored on a separate disk with these properties (copy-paste from the GCE console):</p>
  
  <pre><code>Type: Standard persistent disk
  Size: 2000 GB
  Zone: us-central1-c
  Sustained random IOPS limit - estimated (R/W): 1,500 / 3,000
  Sustained throughput limit (MB/s) - estimated (R/W): 180 / 120
  </code></pre>
  
  <p>To speed up we tried to add an SSD disk:</p>
  
  <pre><code>Type: SSD persistent disk
  Size: 1000 GB
  Zone: us-central1-c
  Sustained random IOPS limit - estimated (R/W): 15,000 / 15,000
  Sustained throughput limit (MB/s) - estimated: 240 / 240
  </code></pre>
  
  <p>One would expect that SSD disk should be quite faster than a Standard disk. But our results are different. During the initial MongoDB sync Standard disk was several time faster than the SSD. While it took 7 hours for the Standard disk to copy all data, the SSD disk after 12 hours had copied just half of data. We used Linux tool iostat and measured, Standard disk is achieving around 80,000 kB_wrtn/s while the SSD disk is around 8,000 kB_wrtn/s. How is possible that SSD disks is 10 times slower than the Standard disk?</p>