We have started seeing misaligned partitions on Linux guests runnning certain HDFS distributions. How these partitions became mis-aligned is a bit of a mystery, because the only way I know how to do this on Linux is to create a partition using old DOS format like this (using -c=dos and -u=cylinders) Continue reading
High Response time and low throughput in vCenter performance charts.
Often we are presented with a vCenter screenshot, and an observation that there are “high latency spikes”. In the example, the response time is indeed quite high – around 80ms. Continue reading
Hyperconverged File Systems PT1 Taxonomy
One way of categorizing Hyperconverged filesystems (or any filesystem really) is by how data is distributed across the nodes, and the method used to track/retrieve that data. The following is based on knowledge of the internals of Nutanix and publicly available information for the other systems.
Metadata
Characteristics
Implemented by
| Distributed | Distributed data & metadata | Nutanix | Hash | Random data distribution, hash-lookup (object store) | VSAN | Dedupe | Data stored in HA-Pairs, Lookup by fingerprint | Simplivity | Dedupe | Random data distribution, Lookup by fingerprint | Springpath/Hyperflex | Psuedo Distributed | Data stored in HA pairs, Unified namespace via redirection | NetApp C-Mode |
- Nutanix uses a fully distributed metadata layer that allows the cluster to decide where to place data depending on the location of the VM accessing it. The data can move around to follow the VM. The Nutanix FS uses a lot of ideas from distributed systems research and implementation, rather than taking a classic filesystems approach and applying it to HCI.
Creating compressible data with fio.
Today I used fio to create some compressible data to test on my Nutanix nodes. I ended up using the following fio params to get what I wanted.
buffer_compress_percentage=50 refill_buffers buffer_pattern=0xdeadbeef
- buffer_compress_percentage does what you’d expect and specifies how compressible the data is
- refill_buffers Is required to make the above compress percentage do what you’d expect in the large. IOW, I want the entire file to be compressible by the buffer_compress_percentage amount
- buffer_pattern This is a big one. Without setting this pattern, fio will use Null bytes to achieve compressibility, and Nutanix like many other storage vendors will suppress runs of Zero’s and so the data reduction will mostly be from zero suppression rather than from compression.
Much of this is well explained in the README for latest version of fio.
Also NOTE Older versions of fio do not support many of the fancy data creation flags, but will not alert you to the fact that fio is ignoring them. I spent quite a bit of time wondering why my data was not compressed, until I downloaded and compiled the latest fio.
Code Red – The healthcare.gov story.
A downtime classic, for several months in 2013 the troubles of a very particular website were front page news across the US. Full Story from Time Magazine (PDF)
