Red Hat ENTERPRISE LINUX - ONLINE STORAGE RECONFIGURATION GUIDE... User manual

Red Hat Enterprise Linux
Online Storage Reconfiguration Guide (BETA)
Red Hat Enterprise Linux Documentation
Don Domingo
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Abstract
This guide outlines the different procedures involved in reconfiguring iSCSI,
Fibre Channel and SAS storage devices.
Beta Document.
This document is currently in Beta. Red Hat welcomes any suggestions and
corrections; for more information about submitting feedback, refer to
Section 1.2, “We Need Feedback!”.
1. Introduction ........................................................................................................... 2
1.1. Document Conventions ............................................................................... 3
1.2. We Need Feedback! .................................................................................... 4
2. Fibre Channel ........................................................................................................ 5
2.1. Fibre Channel API ....................................................................................... 5
2.2. Native Fibre Channel Drivers and Capabilities ............................................... 6
3. iSCSI .................................................................................................................... 6
3.1. iSCSI API ................................................................................................... 6
4. Persistent Naming .................................................................................................. 7
5. Scanning for New Devices ...................................................................................... 8
6. Removing Devices ................................................................................................. 8
7. Modifying Link Loss Behavior .................................................................................. 9
7.1. Fibre Channel ............................................................................................. 9
7.2. iSCSI Settings With dm-multipath .............................................................10
7.3. iSCSI Root .................................................................................................12
8. Controlling the SCSI Command Timer and Device Status ........................................12
9. Troubleshooting ....................................................................................................13
Index .......................................................................................................................14
1. Introduction
This manual outlines the different procedures involved in reconfiguring online storage devices
on Red Hat Enterprise Linux 5 host systems. Online storage devices typically use any of three
protocols: fibre channel, iSCSI, and Serial Attached SCSI (SAS).
The scope of this manual is limited to adding, removing, monitoring and management of online
storage devices. This manual does not discuss the fibre channel, iSCSI, or SAS protocols in
detail. Refer to other documentation for more information about these protocols.
This manual assumes that you have advanced working knowledge of Red Hat Enterprise Linux
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5, along with first-hand experience in managing storage devices in Linux.
Before consulting this book, verify if your host bus adapter vendor or hardware vendor have
their own documentation. It is recommended that you consult such documents before reading
this manual.
1.1. Document Conventions
Certain words in this manual are represented in different fonts, styles, and weights. This
highlighting indicates that the word is part of a specific category. The categories include the
following:
Courier font
Courier font represents commands,file names and paths, and prompts .
When shown as below, it indicates computer output:
Desktop about.html logs paulwesterberg.png
Mail backupfiles mail reports
bold Courier font
Bold Courier font represents text that you are to type, such as: service jonas start
If you have to run a command as root, the root prompt (#) precedes the command:
#gconftool-2
italic Courier font
Italic Courier font represents a variable, such as an installation directory:
install_dir/bin/
bold font
Bold font represents application programs and text found on a graphical interface.
When shown like this: OK , it indicates a button on a graphical application interface.
Additionally, the manual uses different strategies to draw your attention to pieces of information.
In order of how critical the information is to you, these items are marked as follows:
Note
Document Conventions
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A note is typically information that you need to understand the behavior of the
system.
Tip
A tip is typically an alternative way of performing a task.
Important
Important information is necessary, but possibly unexpected, such as a
configuration change that will not persist after a reboot.
Caution
A caution indicates an act that would violate your support agreement, such as
recompiling the kernel.
Warning
A warning indicates potential data loss, as may happen when tuning hardware
for maximum performance.
1.2. We Need Feedback!
If you find a typographical error in this manual, or if you have thought of a way to make this
manual better, we would love to hear from you! Please submit a report in Bugzilla:
http://bugzilla.redhat.com/bugzilla/ against the product Red_Hat_Enterprise_Linux.
When submitting a bug report, be sure to mention the manual's identifier:
Online_Storage_Reconfiguration_Guide
If you have a suggestion for improving the documentation, try to be as specific as possible when
describing it. If you have found an error, please include the section number and some of the
surrounding text so we can find it easily.
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2. Fibre Channel
This section discusses the fibre channel API, native Red Hat Enterprise Linux 5 fibre channel
drivers, and the fibre channel capabilities of these drivers.
2.1. Fibre Channel API
Below is a list of /sys/class/ directories that contain files used to provide the userspace API.
In each item, host numbers are designated by <H>, bus numbers are <B>, targets are <T>, LUNs
are <L>, and remote port numbers are <R>.
Transport: /sys/class/fc_transport/target<H>:<B>:<T>/
•port_id — 24-bit port ID/address
•node_name — 64-bit node name
•port_name — 64-bit port name
Remote Port: /sys/class/fc_remote_ports/rport-<H>:<B>-<R>/
•port_id
•node_name
•port_name
•dev_loss_tmo — number of seconds to wait before marking a link as "bad". Once a link
is marked bad, IO running on its corresponding path (along with any new IO on that path)
will be failed.
The default dev_loss_tmo value varies, depending on which driver/device is used. If a
Qlogic adapter is used, the default is 35 seconds, while if an Emulex adapter is used, it is
30 seconds. The dev_loss_tmo value can be changed via the scsi_transport_fc
module parameter dev_loss_tmo, although the driver can override this timeout value.
The maximum dev_loss_tmo value is 600 seconds. If dev_loss_tmo is set to zero or any
value greater than 600, the driver's internal timeouts will be used instead.
•fast_io_fail_tmo — length of time to wait before failing IO executed when a link
problem is detected. IO that reaches the driver will fail. If IO is in a blocked queue, it will
not be failed until dev_loss_tmo expires and the queue is unblocked.
Host: /sys/class/fc_host/host<H>/
•port_id
•issue_lip — instructs the driver to rediscover remote ports.
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2.2. Native Fibre Channel Drivers and Capabilities
Red Hat Enterprise Linux 5 ships with the following native fibre channel drivers:
•lpfc
•qla2xxx
•zfcp
•mptfc
Table 1, “Fibre-Channel API Capabilities” describes the different fibre-channel API capabilities
of each native Red Hat Enterprise Linux 5 driver. X denotes support for the capability.
lpfc qla2xxx zfcp mptfc
Transport
port_id
XXXX
Transport
node_name
XXXX
Transport
port_name
XXXX
Remote Port
dev_loss_tmo
XXXX
Remote Port
fast_io_fail_tmo
X
Host port_id XXXX
Host issue_lip X X
Table 1. Fibre-Channel API Capabilities
3. iSCSI
This section describes the iSCSI API and the iscsiadm utility.
3.1. iSCSI API
To get information about running sessions, run:
iscsiadm -m session -P 2
This command displays the session/device state, session ID (sid) and some negotiated
parameters.
For shorter output (for example, to display only the sid-to-node mapping), run:
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iscsiadm -m session -P 0
or
iscsiadm -m session
These commands print the list of running sessions with the format:
driver [sid] ip:port,target_portal_group_tag targetname
For example:
iscsiadm -m session
tcp [2] 10.15.84.19:3260,2 iqn.1992-08.com.netapp:sn.33615311
tcp [3] 10.15.85.19:3260,3 iqn.1992-08.com.netapp:sn.33615311
For more information about the iSCSI API, refer to
/usr/share/doc/iscsi-initiator-utils-version/README.
4. Persistent Naming
The /dev/disk/ directory contains symlinks to different symbolic names that point back to any
attached raw device. These symbolic names are useful in determining what names for each
device are persistent regardless of which ports or protocols are used.
The symlinks used by these persistent names are divided by type in the following /dev/disk/
subdirectories:
by-id/
Names devices by SCSI VPD page 0x80 or 0x83 data
by-uuid/
Names devices based on file system Universally Unique Identifier (UUID)
by-label/
Names devices based on file system label
by-path/
Names devices by sysfs path. For fibre channel this may name the device using the PCI
info and Host:BusTarget:LUN info.
For iSCSI devices, by-path/ names use the target name and portal information. Note that
by-path names are not reliable if you have two multiple paths to the same portal.
Persistent Naming
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Note
In addition to this, you can also use udev rules to implement persistent naming.
For more information about this, refer to man udev.
5. Scanning for New Devices
If you load a driver before adding the corresponding storage device, you will likely need to
manually add the new storage to the operating system. As such, you will need the
corresponding logical unit number (LUN) of the added storage device.
To scan all buses and targets for new LUNs, use:
echo - - - > /sys/class/scsi_host/<host>/scan
Here, <host> refers to the host number. This can be host0,host1,host2, and so on.
For iSCSI, if the targets sends an iSCSI async event indicating new storage is added, then the
scan is done automatically. Cisco MDS™and EMC Cellera™support this feature.
For fibre channel, you may need to rediscover the target by executing the command issue_lip.
Note that the proc interface is deprecated; as such, do not use it.
6. Removing Devices
If remove a device on a target, the driver will normally not remove it automatically. To properly
remove a device, perform the following steps:
Procedure 1. Ensuring a Clean Device Removal
1. Close all users of the device.
2. Unmount any file systems that mounted the device.
3. File systems that mounted the device should be unmounted using umount. In addition, you
should also remove the device from any dm,md, LVM, multipath or RAID devices using it.
4. Remove the device from the SCSI layer using:
echo 1 > /sys/block/<device name>/device/delete
As stated in the previous subsection, the proc interface is deprecated; as such, do not use it.
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7. Modifying Link Loss Behavior
This section describes how to modify the link loss behavior of devices that use either fibre
channel or iSCSI protocols.
7.1. Fibre Channel
If a driver implements the Transport dev_loss_tmo callback, access attempts to a device
through a link will be blocked when a transport problem is detected. To verify if a device is
blocked, run the following command:
cat /sys/block/<device>/device/state
This command will return blocked if the device is blocked. If the device is operating normally,
this command will return running.
Procedure 2. Determining The State of a Remote Port
1. To determine the state of a remote port, run the following command:
cat /sys/class/rport/rport-H:B:R/port_state
2. This command will return Blocked when the remote port (along with devices accessed
through it) are blocked. If the remote port is operating normally, the command will return
Online.
3. If the problem is not resolved within dev_loss_tmo seconds, the rport and devices will be
unblocked and all IO running on that device (along with any new IO sent to that device) will
be failed.
Procedure 3. Changing dev_loss_tmo
• To change the dev_loss_tmo value, echo in the desired value to the file. For example, to
set dev_loss_tmo to 30 seconds, run:
echo 30 > /sys/class/rport/rport-H:B:R/dev_loss_tmo
For more information about dev_loss_tmo, refer to Section 2.1, “Fibre Channel API”.
When a device is blocked, the fibre channel class will leave the device as is; i.e. /dev/sdxwill
remain /dev/sdx. This is because the dev_loss_tmo expired. If the link problem is fixed at a
later time, the SCSI device will be used again.
Fibre Channel
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Fibre Channel: remove_on_dev_loss.
If you prefer that devices are removed at the SCSI layer when links are marked bad (i.e. expired
after dev_loss_tmo seconds), you can use the scsi_transport_fc module parameter
remove_on_dev_loss. When a device is removed at the SCSI layer while remove_on_dev_loss
is in effect, the device will be added back once all transport problems are corrected.
Warning
The use of remove_on_dev_loss is not recommended, as removing a device at
the SCSI layer does not automatically unmount any file systems from that device.
When file systems from a removed device are left mounted, the device may not
be properly removed from multipath or RAID devices.
Further problems may arise from this if the upper layers are not hotplug-aware.
This is because the upper layers may still be holding references to the state of
the device before it was originally removed. This can cause unexpected behavior
when the device is added again.
7.2. iSCSI Settings With dm-multipath
If dm-multipath is implemented, it is advisable to set iSCSI timers to immediately defer
commands to the multipath layer. To configure this, nest the following line under device { in
/etc/multipath.conf:
features "1 queue_if_no_path"
This ensures that I/O errors are retried and queued if all paths are failed in the dm-multipath
layer.
You may need to adjust iSCSI timers further to better monitor your SAN for problems. Available
iSCSI timers you can configure are NOP-Out Interval/Timeouts and replacement_timeout,
which are discussed in the following sections.
7.2.1. NOP-Out Interval/Timeout
To help monitor problems the SAN, the iSCSI layer sends a NOP-Out request to each target. If
a NOP-Out request times out, the iSCSI layer responds by failing any running commands and
instructing the SCSI layer to requeue those commands when possible.
When dm-multipath is being used, the SCSI layer will fail those running commands and defer
them to the multipath layer. The multipath layer then retries those commands on another path. If
dm-multipath is not being used, those commands are retried five times before failing
altogether.
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