Merge branch 'sh-nfs-read-perf-docs' into 'master'

Improve NFS benchmarking doc to include read performance test

See merge request gitlab-org/gitlab-ce!24874

doc/administration/operations/filesystem_benchmarking.md

@@ -7,13 +7,72 @@ systems.
 Normally when talking about filesystem performance the biggest concern is
 with Network Filesystems (NFS). However, even some local disks can have slow
-IO. The information on this page can be used for either scenario.
-## Write Performance
-The following one-line command is a quick benchmark for filesystem write
+I/O. The information on this page can be used for either scenario.
+## Executing benchmarks
+### Benchmarking with `fio`
+
+We recommend using
+[fio](https://fio.readthedocs.io/en/latest/fio_doc.html) to test I/O
+performance. This test should be run both on the NFS server and on the
+application nodes that talk to the NFS server.
+
+To install:
+
+- On Ubuntu: `apt install fio`.
+- On `yum`-managed environments: `yum install fio`.
+
+Then run the following:
+
+```sh
+fio --randrepeat=1 --ioengine=libaio --direct=1 --gtod_reduce=1 --name=test --filename=/path/to/git-data/testfile --bs=4k --iodepth=64 --size=4G --readwrite=randrw --rwmixread=75
+```
+
+This will create a 4GB file in `/path/to/git-data/testfile`. It performs
+4KB reads and writes using a 75%/25% split within the file, with 64
+operations running at a time. Be sure to delete the file after the test
+completes.
+
+The output will vary depending on what version of `fio` installed. The following
+is an example output from `fio` v2.2.10 on a networked solid-state drive (SSD):
+
+```
+test: (g=0): rw=randrw, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, iodepth=64
+    fio-2.2.10
+    Starting 1 process
+    test: Laying out IO file(s) (1 file(s) / 1024MB)
+    Jobs: 1 (f=1): [m(1)] [100.0% done] [131.4MB/44868KB/0KB /s] [33.7K/11.3K/0 iops] [eta 00m:00s]
+    test: (groupid=0, jobs=1): err= 0: pid=10287: Sat Feb  2 17:40:10 2019
+      read : io=784996KB, bw=133662KB/s, iops=33415, runt=  5873msec
+      write: io=263580KB, bw=44880KB/s, iops=11219, runt=  5873msec
+      cpu          : usr=6.56%, sys=23.11%, ctx=266267, majf=0, minf=8
+      IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, >=64=100.0%
+         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=0.1%, >=64=0.0%
+         issued    : total=r=196249/w=65895/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
+         latency   : target=0, window=0, percentile=100.00%, depth=64
+
+    Run status group 0 (all jobs):
+       READ: io=784996KB, aggrb=133661KB/s, minb=133661KB/s, maxb=133661KB/s, mint=5873msec, maxt=5873msec
+      WRITE: io=263580KB, aggrb=44879KB/s, minb=44879KB/s, maxb=44879KB/s, mint=5873msec, maxt=5873msec
+```
+
+Notice the `iops` values in this output. In this example, the SSD
+performed 33,415 read operations per second and 11,219 write operations
+per second. A spinning disk might yield 2,000 and 700 read and write
+operations per second.
+
+### Simple benchmarking
+
+NOTE: **Note:** This test is naive but may be useful if `fio` is not
+available on the system. It's possible to receive good results on this
+test but still have poor performance due to read speed and various other
+factors.
+
+The following one-line commands provide a quick benchmark for filesystem write and read
 performance. This will write 1,000 small files to the directory in which it is
-executed.
+executed, and then read the same 1,000 files.
 1. Change into the root of the appropriate
    [repository storage path](../repository_storage_paths.md).
@@ -27,13 +86,18 @@ executed.
     ```sh
     time for i in {0..1000}; do echo 'test' > "test${i}.txt"; done
     ```
+1. To benchmark read performance, run the command:
+
+    ```sh
+    time for i in {0..1000}; do cat "test${i}.txt" > /dev/null; done
+    ```
 1. Remove the test files:
    ```sh
    cd ../; rm -rf test
    ```
-The output of the `time for ...` command will look similar to the following. The
+The output of the `time for ...` commands will look similar to the following. The
 important metric is the `real` time.
 ```sh
@@ -42,12 +106,13 @@ $ time for i in {0..1000}; do echo 'test' > "test${i}.txt"; done
 real	0m0.116s
 user	0m0.025s
 sys	0m0.091s
+
+$ time for i in {0..1000}; do cat "test${i}.txt" > /dev/null; done
+
+real    0m3.118s
+user    0m1.267s
+sys 0m1.663s
 ```
 From experience with multiple customers, this task should take under 10
-seconds to indicate good filesystem performance. 
-
-NOTE: **Note:**
-This test is naive and only evaluates write performance. It's possible to 
-receive good results on this test but still have poor performance due to read 
-speed and various other factors. 
\ No newline at end of file
+seconds to indicate good filesystem performance.