doc: add some thoughts about large chunk sizes

[proxmox-backup.git] / README.rst

diff --git a/README.rst b/README.rst
index 6bc1d0165c9fbd6a75a2df43fa5fb664f7d6599d..e880a1d7aea6fc26f6e78894a618bb9a24d62f7c 100644 (file)
--- a/README.rst
+++ b/README.rst
@@ -13,7 +13,7 @@ Versioning of proxmox helper crates
 
 To use current git master code of the proxmox* helper crates, add::
 
-   git = "ssh://gitolite3@proxdev.maurer-it.com/rust/proxmox"
+   git = "git://git.proxmox.com/git/proxmox"
 
 or::
 
@@ -22,6 +22,7 @@ or::
 to the proxmox dependency, and update the version to reflect the current,
 pre-release version number (e.g., "0.1.1-dev.1" instead of "0.1.0").
 
+
 Local cargo config
 ==================
 
@@ -35,3 +36,79 @@ checksums are not compatible.
 To reference new dependencies (or updated versions) that are not yet packaged,
 the dependency needs to point directly to a path or git source (e.g., see
 example for proxmox crate above).
+
+
+Build
+=====
+on Debian Buster
+
+Setup:
+  1. # echo 'deb http://download.proxmox.com/debian/devel/ buster main' >> /etc/apt/sources.list.d/proxmox-devel.list
+  2. # sudo wget http://download.proxmox.com/debian/proxmox-ve-release-6.x.gpg -O /etc/apt/trusted.gpg.d/proxmox-ve-release-6.x.gpg
+  3. # sudo apt update
+  4. # sudo apt install devscripts debcargo clang
+  5. # git clone git://git.proxmox.com/git/proxmox-backup.git
+  6. # sudo mk-build-deps -ir
+
+Note: 2. may be skipped if you already added the PVE or PBS package repository
+
+You are now able to build using the Makefile or cargo itself.
+
+
+Design Notes
+============
+
+Here are some random thought about the software design (unless I find a better place).
+
+
+Large chunk sizes
+-----------------
+
+It is important to notice that large chunk sizes are crucial for
+performance. We have a multi-user system, where different people can do
+different operations on a datastore at the same time, and most operation
+involves reading a series of chunks.
+
+So what is the maximal theoretical speed we can get when reading a
+series of chunks? Reading a chunk sequence need the following steps:
+
+- seek to the first chunk start location
+- read the chunk data
+- seek to the first chunk start location
+- read the chunk data
+- ...
+
+Lets use the following disk performance metrics:
+
+:AST: Average Seek Time (second)
+:MRS: Maximum sequential Read Speed (bytes/second)
+:ACS: Average Chunk Size (bytes)
+
+The maximum performance you can get is::
+
+  MAX(ACS) = ACS /(AST + ACS/MRS)
+
+Please note that chunk data is likely to be sequential arranged on disk, but
+this it is sort of a best case assumption.
+
+For a typical rotational disk, we assume the following values::
+
+  AST: 10ms
+  MRS: 170MB/s
+
+  MAX(4MB)  = 115.37 MB/s
+  MAX(1MB)  =  61.85 MB/s;
+  MAX(64KB) =   6.02 MB/s;
+  MAX(4KB)  =   0.39 MB/s;
+  MAX(1KB)  =   0.10 MB/s;
+
+Modern SSD are much faster, lets assume the following::
+
+  max IOPS: 20000 => AST = 0.00005
+  MRS: 500Mb/s
+
+  MAX(4MB)  = 474 MB/s
+  MAX(1MB)  = 465 MB/s;
+  MAX(64KB) = 354 MB/s;
+  MAX(4KB)  =  67 MB/s;
+  MAX(1KB)  =  18 MB/s;