Add a document about existing assets

A short overview of the assets currently stored on the NFS mount and
the distribution of their file sizes.

Fixes: #103

docs/existing_assets.md

@@ -0,0 +1,61 @@
+# Existing assets
+
+This document is a short overview of the assets currently stored in the NFS mount. On integration the sizes of the directories under `/mnt/uploads` are:
+
+``` bash
+@integration-asset-master-1:~$ cd /mnt/uploads/
+@integration-asset-master-1:/mnt/uploads$ du -h --max-depth=1
+35G ./asset-manager
+629G    ./whitehall
+7.6M    ./publisher
+14G ./support-api
+16K ./lost+found
+677G    .
+```
+
+Comparing this to the [production Grafana dashboard](https://grafana.publishing.service.gov.uk/dashboard/db/assets) (674.99G today) leads me to believe that integration has all of the assets that are on production.
+
+The asset manager application stores a record in MongoDB for each asset. On integration the number of records is
+
+``` bash
+@integration-backend-1:/var/apps/asset-manager$ sudo su - deploy
+deploy@integration-backend-1:~$ cd /var/apps/asset-manager
+deploy@integration-backend-1:/var/apps/asset-manager$ govuk_setenv asset-manager bundle exec rails c
+Loading production environment (Rails 4.2.7.1)
+irb(main):002:0> Asset.count
+=> 57232
+```
+
+We generate a list of all the files stored in NFS in the asset manager directory
+
+``` bash
+@integration-asset-master-1:/mnt/uploads/asset-manager$ find . -type f | xargs ls -s > ~/file_sizes.txt
+```
+
+This indicates that there are 58,613 files in the NFS mount (which is slightly more than the number of records in MongoDB).
+
+``` bash
+12:18 $ wc -l file_sizes.txt
+   58613 file_sizes.txt
+```
+
+I haven't investigated yet why this difference exists. However we can take a look at the file sizes of the files on the mount
+
+``` bash
+cat file_sizes.txt | tr -d ' ' | awk -F"[.]/" '{print $1","$2}
+```
+
+Loading this file into R allows us to calculate the distribution of file sizes
+
+``` r
+library(readr)
+d <- read_csv('file_sizes.csv', col_names=c('size', 'filename'))
+quantile(d$size, c(.5, .8, .95, .99, 1))
+```
+
+```
+   50%       80%       95%       99%      100%
+204.00    732.00   2376.00   6031.52 174844.00
+```
+
+The median file size is 204k, 95% of all assets are under 2.3Mb and the largest asset is just over 174Mb.