add technical FAQ (FATQ)

Signed-off-by: simonsan <[email protected]>

src/FATQ.md

@@ -0,0 +1,22 @@
+# Frequently asked technical questions
+
+> **Question**: *You mention a "huge decrease in memory requirement"; are you
+> streaming blobs to the data store without keeping them in memory and just
+> keeping the hashes in memory? How does the memory footprint look like?*
+
+The largest memory consuming part is the index which is kept in-memory by restic
+and rustic. For restic, however, the index took about 3-4 times the theoretical
+size - maybe due to Golangs garbage collector additions or other Golang-specific
+stuff.
+
+In rustic, there are two basic improvements:
+
+- the memory consumption is exactly as expected as Rust allows to directly
+  define the in-memory structures
+
+- For some operations, not all index information is needed. Rustic allows
+  several kinds of index types to be loaded into memory (only trees, only ids,
+  full index) and uses only the type needed for a given command. For instance
+  with backup you need the list of all present blob IDs, but not the position
+  within the repository - rustic thus only saves the IDs in memory for the
+  backup command and therefore uses less memory.

src/SUMMARY.md

@@ -4,6 +4,7 @@
 - [Contributing to rustic](./contributing-to-rustic.md)
 - [Comparison `rustic` and `restic`](./comparison-restic.md)
 - [FAQ](./FAQ.md)
+- [Technical FAQ](./FATQ.md)
 - [Installation](./installation.md)
 - [Getting Started](./getting_started.md)
 - [Preparing a new repository](./commands/init/intro.md)