performance.rst: small updates, mention (lack of) MDMF

refs #1398

diff --git a/docs/performance.rst b/docs/performance.rst
index 4657c1516e5d06a29a1c14de7e13a90899fd0a11..1766d381ff02c38f736ac287266d160b19e5a28b 100644 (file)
--- a/docs/performance.rst
+++ b/docs/performance.rst
@@ -28,6 +28,11 @@ written
 
 ``G`` indicates the number of storage servers on your grid
 
+Most of these cost estimates may have a further constant multiplier: when a
+formula says ``N/K*S``, the cost may actually be ``2*N/K*S`` or ``3*N/K*S``.
+Also note that all references to mutable files are for SDMF-formatted files;
+this document has not yet been updated to describe the MDMF format.
+
 Publishing an ``A``-byte immutable file
 =======================================
 
@@ -210,10 +215,15 @@ cpu: variable, between ~A and ~N/K*A
 
 network: variable; between A and N/K*A
 
-memory footprint: (1+N/K)*S
+memory footprint (immutable): (1+N/K)*S
+              (SDMF mutable): (1+N/K)*A
 
 notes: To repair a file, Tahoe-LAFS downloads the file, and
 generates/uploads missing shares in the same way as when it initially
 uploads the file.  So, depending on how many shares are missing, this
 can cost as little as a download or as much as a download followed by
 a full upload.
+
+Since SDMF files have only one segment, which must be processed in its
+entirety, repair requires a full-file download followed by a full-file
+upload.