doc: add some thoughts about large chunk sizes

[proxmox-backup.git] / README.rst

``rustup`` Toolchain
====================

We normally want to build with the ``rustc`` Debian package. To do that
you can set the following ``rustup`` configuration:

    # rustup toolchain link system /usr
    # rustup default system


Versioning of proxmox helper crates
===================================

To use current git master code of the proxmox* helper crates, add::

   git = "git://git.proxmox.com/git/proxmox"

or::

   path = "../proxmox/proxmox"

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
==================

This repository ships with a ``.cargo/config`` that replaces the crates.io
registry with packaged crates located in ``/usr/share/cargo/registry``.

A similar config is also applied building with dh_cargo. Cargo.lock needs to be
deleted when switching between packaged crates and crates.io, since the
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;