Add lfs migrate examples to storage guide (#297)

This adds the `lfs migrate` recommendation/example that was removed
along with the Gordon-Bell guide (#240).

This also adds an example to recursively update all files in a directory
(finding files using `lfs find`).

@mpasserini can you or someone else from the storage team comment if you
think these are ok recommendations. For example:
- (this already exists) do you think the recommended block sizes and
progressive layouts are sane recommendations?
- Are there any concerns with using e.g. `xargs -P` to migrate files in
parallel (the example is currently sequential which can be slow, but I'm
not sure if doing a parallel `lfs migrate` adds unnecessarily high load
on lustre)?

Note that I changed the short options (`-c`/`-S`/`-E`) to long options
as I find that it's easier to follow for someone like me who doesn't
know the options by heart. However, the commands do end up being longer
(wider than the regular page, and require scrolling), and I'm happy to
change that back (it's not the important part of this PR).

I'm not attempting to move or restructure the storage guide otherwise.

Link to generated docs:
https://docs.tds.cscs.ch/297/guides/storage/#lustre-tuning

Author: msimberg

docs/guides/storage.md

@@ -136,19 +136,36 @@ The simplest way to have the correct layout is to copy to a directory with the c
     ... so it is a good choice when reading a file sequentially or in large chunks, but if one reads shorter chunks in random order it might be better to reduce the size, the performance will be smaller, but the performance of your application might actually increase.
     See the [Lustre documentation](https://doc.lustre.org/lustre_manual.xhtml#managingstripingfreespace) for more information.
 
-
 !!! example "Settings for large files"
-    *Remember:* Settings only apply to files added to the directory after this command.
-    ```console
-    lfs setstripe --stripe-count -1 --stripe-size 4M <big_files_dir>`
+    ```bash
+    lfs setstripe --stripe-count -1 --stripe-size 4M <big_files_dir>
     ```
+
+    *Remember:* Settings applied with `lfs setstripe` only apply to files added to the directory after this command.
+    [Use `lfs migrate` to update the settings for existing files][ref-guides-storage-examples-lfs-migrate].
+
 Lustre also supports composite layouts, switching from one layout to another at a given size `--component-end` (`-E`).
 With it it is possible to create a Progressive file layout switching `--stripe-count` (`-c`), `--stripe-size` (`-S`), so that fewer locks are required for smaller files, but load is distributed for larger files.
 
 !!! example "Good default settings"
-    ```console
-    lfs setstripe -E 4M -c 1 -E 64M -c 4 -E -1 -c -1 -S 4M <base_dir>
+    ```bash
+    lfs setstripe --component-end 4M --stripe-count 1 --component-end 64M --stripe-count 4 --component-end -1 --stripe-count -1 --stripe-size 4M <base_dir>
+    ```
+
+[](){#ref-guides-storage-examples-lfs-migrate}
+!!! example "Updating settings for existing files"
+    While `lfs setstripe` applies to newly created files, `lfs migrate` can be used to re-layout existing files.
+    For example, to set the recommended settings above on an existing file:
+    ```bash
+    lfs migrate --component-end 4M --stripe-count 1 --component-end 64M --stripe-count 4 --component-end -1 --stripe-count -1 --stripe-size 4M <file>
+    ```
+
+    Alternatively, to migrate all files recursively in a directory:
+    ```bash
+    lfs find --type file <base_dir> | xargs lfs migrate --verbose --component-end 4M --stripe-count 1 --component-end 64M --stripe-count 4 --component-end -1 --stripe-count -1 --stripe-size 4M
     ```
+    The `--verbose` flag makes `lfs migrate` print the path of each file after the file has been migrated.
+    Also note the use of `lfs find` instead of regular `find` as `lfs` can more efficiently retrieve the list of files recursively.
 
 ### Iopsstor vs Capstor