Document iscsi and multipath implementations.

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+==================
+iSCSI and Anaconda
+==================
+
+
+Introduction
+------------
+
+iSCSI device is a SCSI device connected to your computer via a TCP/IP
+network. The communication can be handled either in hardware or in software, or
+as a hybrid --- part software, part hardware.
+
+The terminology:
+
+- 'initiator', the client in the iscsi connection. The computer we are running
+  Anaconda on is typically an initiator.
+- 'target', the storage device behind the Network. This is where the data is
+  physically stored and read from. You can turn any Fedora/RHEL machine to a
+  target (or several) via scsi-target-utils.
+- 'HBA' or Host Bus Adapter. A device (PCI card typically) you connect to a
+  computer. It acts as a NIC and if you configure it properly it transparently
+  connects to the target when started and all you can see is a block device on
+  your system.
+- 'software initiator' is what you end up with if you emulate most of what HBA is
+  doing and just use a regular NIC for the iscsi communication. The modern Linux
+  kernel has a software initiator. To use it, you need the Open-ISCSI software
+  stack [1, 2] installed. It is known as iscsi-initiator-utils in Fedora/RHEL.
+- 'partial offload card'. Similar to HBA but needs some support from kernel and
+  iscsi-initiator-utils. The least pleasant to work with, particularly because
+  there is no standardized amount of the manual setting that needs to be done
+  (some connect to the target just like HBAs, some need you to bring their NIC
+  part up manually etc.). Partial offload cards exist to get better performing
+  I/O with less processor load than with software initiator.
+- 'iBFT' as in 'Iscsi Boot Firmware Table'. A table in the card's bios that
+  contains its network and target settings. This allows the card to configure
+  itself, connect to a target and boot from it before any operating system or a
+  bootloader has the chance. We can also read this information from
+  /sys/firmware/ibft after the system starts and then use it to bring the card
+  up (again) in Linux.
+- 'CHAP' is the authentication used for iSCSI connections. The authentication
+  can happen during target discovery or target login or both. It can happen in
+  both directions too: the initiator authenticates itself to the target and the
+  target is sometimes required to authenticate itself to the initiator.
+
+
+What is expected from Anaconda
+------------------------------
+
+We are expected to:
+
+- use an HBA like an ordinary disk. It is usually smart enough to bring itself
+  up during boot, connect to the target and just act as an ordinary disk.
+- allow creating new software initiator connections in the UI, both IPv4 and IPv6.
+- facilitate bringing up iBFT connections for partial offload cards.
+- install the root and/or /boot filesystems on any iSCSI initiator known to us
+- remember to install dracut-network if we are booting from an iSCSI initiator that
+  requires iscsi-initiator-utils in the ramdisk (most of them do)
+- boot from an iSCSI initiator using dracut, this requires generating an
+  appropriate set of kernel boot arguments for it [3].
+
+
+How Anaconda handles iscsi
+--------------------------
+
+iSCSI comes into play several times while Anaconda does its thing:
+
+In loader, when deciding what NIC we should setup, we check if we have iBFT
+information from one of the cards. If we do we set that card up with what we
+found in the table, it usually boils down to an IPv4 static or IPv4
+DHCP-obtained address. [4][5]
+
+Next, after the main UI startup during filtering (or storage scan, whatever
+comes first) we startup the iscsi support code in Anaconda [6]. This currently
+involves:
+- manually modprobing related kernel modules
+- starting the iscsiuio daemon (required by some partial offload cards)
+- most importantly, starting the iscsid daemon
+
+All iBFT connections are brought up next by looking at the cards' iBFT data, if
+any. The filtering screen has a feature to add advanced storage devices,
+including iSCSI. Both connection types are handled by libiscsi (see below). The
+brought up iSCSI devices appear as /dev/sdX and are treated as ordinary block
+devices.
+
+When DeviceTree scans all the block devices it uses the udev data (particularly
+the ID_BUS and ID_PATH keys) to decide if the device is an iscsi disk. If it is,
+it is represented with an iScsiDiskDevice class instance. This helps Anaconda
+remember that:
+
+- we need to install dracut-network so the generated dracut image is able to
+  bring up the underlying NIC and establish the iscsi connection.
+- if we are booting from the device we need to pass dracut a proper set of
+  arguments that will allow it to do so.
+
+
+Libiscsi
+--------
+
+How are iSCSI targets found and logged into? Originally Anaconda was just
+running iscsiadm as an external program through execWithRedirect(). This
+ultimately proved awkward especially due to the difficulties of handling the
+CHAP passphrases this way. That is why Hans de Goede <hdegoede@redhat.com>, the
+previous maintainer of the Anaconda iscsi subsystem decided to write a better
+interface and created libiscsi (do not confuse this with the libiscsi.c in
+kernel). Currently libiscsi lives as a couple of patches in the RHEL6
+iscsi-initiator-utils CVS (and in Fedora package git, in somewhat outdated
+version). Since Anaconda is libiscsi's only client at the moment it is
+maintained by the Anaconda team.
+
+The promise of libiscsi is to provide a simple C/Python API to handle iSCSI
+connections while being somewhat stable and independent of the changes in the
+underlying initiator-utils (while otherwise being tied to it on the
+implementation level).
+
+And at the moment libiscsi does just that. It has a set of functions to discover
+and login to targets software targets. It supports making connections through
+partial offload interfaces, but the only discovery method supported at this
+moment is through firmware (iBFT). Its public data structures are independent of
+iscsi-initiator-utils. And there is some python boilerplate that wraps the core
+functions so we can easily call those from Anaconda.
+
+To start nontrivial hacking on libiscsi prepare to spend some time familiarizing
+yourself with the iscsi-initiator-utils internals (it is complex but quite
+nice).
+
+
+Debugging iSCSI bugs
+--------------------
+
+There is some information in anaconda.log and storage.log but libiscsi itself is
+quite bad at logging. Most times useful information can be found by sshing onto
+the machine and inspecting the output of different iscsiadm commands [2][7],
+especially querying the existing sessions and known interfaces.
+
+If for some reason the DeviceTree fails at recognizing iscsi devices as such,
+'udevadm info --exportdb' is of interest.
+
+The booting problems are either due to incorrectly generated dracut boot
+arguments or they are simply dracut bugs.
+
+Note that many of the iscsi adapters are installed in different Red Hat machines
+and so the issues can often be reproduced and debugged.
+
+
+Future of iSCSI in Anaconda
+---------------------------
+
+- extend libiscsi to allow initializing arbitrary connections from a partial
+  offload card. Implement the Anaconda UI to utilize this. Difficulty hard.
+- extend libiscsi with device binding support. Difficulty hard.
+- work with iscsi-initiator-utils maintainer to get libiscsi.c upstream and then
+  to rawhide Fedora. Then the partial offload patches in the RHEL6 Anaconda can
+  be migrated there too and partial offload can be tested. This is something
+  that needs to be done before RHEL7. Difficulty medium.
+- improve libiscsi's logging capabilities. Difficulty easy.
+
+
+
+[1] http://www.open-iscsi.org/
+[2] /usr/share/doc/iscsi-initiator-utils-6.*/README
+[3] man 7 dracut.kernel
+[4] Anaconda git repository, anaconda/loader/ibft.c
+[5] Anaconda git repository, anaconda/loader/net.c, chooseNetworkInterface()
+[6] Anaconda git repository, anaconda/storage/iscsi.py
+[7] 'man 8 iscsiadm'
+
+
+---
+Red Hat Author(s): Ales Kozumplik <akozumpl@redhat.com>