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XenServer Storage Management and Troubleshooting. Daniel Lazar Lead Escalation Engineer May 11, 2010. Agenda. XenServer Storage Overview Management and Monitoring Troubleshooting and Diagnosing Common Storage Issues Q & A. XenServer Storage Overview. XenServer Storage Overview.

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Xenserver storage management and troubleshooting

XenServer Storage Management and Troubleshooting

Daniel Lazar

Lead Escalation Engineer

May 11, 2010


Agenda
Agenda

  • XenServer Storage Overview

  • Management and Monitoring

  • Troubleshooting and Diagnosing Common Storage Issues

  • Q & A

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Xenserver storage overview1
XenServer Storage Overview

  • XenServer Storage Objects

    • SRs, VDIs, PBDs and VBDs

  • Virtual Disk Data Formats

    • File-based VHD, LVM and StorageLink

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Xenserver storage objects
XenServer Storage Objects

What is an SR (Shared Repository)?

Describes a particular storage target in which Virtual Disk Images (VDIs) are stored.

Flexible—supports a wide variety of storage types.

Centralized—easier to manage, more reliable with a XenServer pool.

Must be accessible to each XenServer host.

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Xenserver storage objects1
XenServer Storage Objects

VDIs, PBDs, VBDs

Virtual Disk Images are a storage abstraction that is presented to a VM.

Physical Block Devices represent the interface between a physical server and an attached SR.

Virtual Block Devices are connector objects that allow mappings between VDIs and VMs.

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XenServer Storage Objects

SR

PBD

VDI

VBD

XenServer Host

Virtual Machine

VDI

VBD

PBD

XenServer Host

Virtual Machine

VDI

VBD

PBD

XenServer Host


Virtual disk data formats
Virtual Disk Data Formats

File-based VHD

VM images are stored as thin-provisioned VHD format files on either a local non-shared file system (EXT type SR) or a shared NFS target (NFS type SR).

What is VHD?

  • A Virtual Hard Disk (VHD) is a file formatted to be structurally identical to a physical Hard Disk Drive.

  • Image Format Specification was created by Microsoft in June, 2005.

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Virtual disk data formats1
Virtual Disk Data Formats

Logical Volume (LVM)-based VHDs

The default XenServer block device-based storage inserts a Logical Volume manager on a disk. VDIs are represented as volumes within the Volume manager.

Introduced LVHD in XenServer 5.5

  • Enhances LVM for SRs

  • Hosts VHD files directly on LVM volumes

  • Adds Advanced Storage features like Fast Cloning and Snapshots

  • Fast and simple upgrade

  • Backwards compatible

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Virtual disk data formats2
Virtual Disk Data Formats

StorageLink (LUN per VDI)

LUNs are directly mapped to VMs as VDIs by SR types that provide an array-specific plug-in (NetApp, Equallogic or StorageLink type SRs). The array storage abstraction therefore matches the VDI storage abstraction for environments that manage storage provisioning at an array level.

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Virtual disk data formats3
Virtual Disk Data Formats

StorageLink Architecture

  • XenServer calls direct to Array API‘s to provision and adjust storage on demand.

  • Fully leverages array hardware capabilities.

  • Virtual disk drives are individual LUNs.

  • High performance storage model.

  • Only the server running a VM connects to the individual LUN(s) for that VM.

  • A special master server coordinates which servers connect to which LUNs

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LVM vs. StorageLink

XenServer 5.5

iSCSI / FC

XenServer 5.5

iSCSI / FC

+

Storage Repository

Storage Repository

LUN

LUN

LUN

VHD header

VHD header

LVM

Logical

Volume

LVM

Logical

Volume

LVM Volume Group

VM Virtual Disk



Management and monitoring overview
Management and Monitoring Overview

  • Understanding how XenServer Perceives the Storage

  • Monitoring Storage

  • Protecting Your Data

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Management and monitoring
Management and Monitoring

Understanding the physical disk layout

# fdisk –l # Lists the physical block devices on the host

Disk /dev/cciss/c0d0: 146.7 GB, 146778685440 bytes

255 heads, 32 sectors/track, 35132 cylinders

Units = cylinders of 8160 * 512 = 4177920 bytes

Device Boot Start End Blocks Id System

/dev/cciss/c0d0p1 * 1 981 4002464 83 Linux

/dev/cciss/c0d0p2 982 1962 4002480 83 Linux

/dev/cciss/c0d0p3 1963 35132 135333600 83 Linux

Denotes a SCSI block device locally attached to the system (HP RAID array in this case)

The first partition on the disk contains the boot information for the OS.

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Management and monitoring1
Management and Monitoring

Understanding the physical disk layout (continued)

# fdisk –l # Continued output

Disk /dev/sda: 107.3 GB, 107374182400 bytes

255 heads, 63 sectors/track, 13054 cylinders

Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/sda doesn't contain a valid partition table

Implies a block device using the SCSI Generic (sg) driver. It is likely attached via a separate interface such as iSCSI or FC HBA

This disk is part of a Storage Repository using an LVM file system and therefore does not require a local partition table.

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Management and monitoring2
Management and Monitoring

# sg_map –x # Displays the mapping between Linux sg and regular SCSI devices

/dev/sg0 0 0 0 0 13

/dev/sg1 0 0 0 1 0 /dev/sda

/dev/sg2 0 0 0 2 0 /dev/sdb

/dev/sg3 1 0 0 0 13

/dev/sg4 1 0 0 1 0 /dev/sdc

/dev/sg5 1 0 0 2 0 /dev/sdd

Understanding the physical disk layout (continued)

Host Number

Bus

SCSI ID

LUN

SCSI Type

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Management and monitoring3
Management and Monitoring

Understanding the physical disk layout (continued)

# ll /dev/disk/by-id # List the attached block devices by SCSI ID.

cciss-3600508b1001035373120202020200003 -> ../../cciss/c0d0

cciss-3600508b1001035373120202020200003-part1 -> ../../cciss/c0d0p1

cciss-3600508b1001035373120202020200003-part2 -> ../../cciss/c0d0p2

cciss-3600508b1001035373120202020200003-part3 -> ../../cciss/c0d0p3

scsi-360a98000503350642f4a553833616b57 -> ../../sda

This SCSI device is mapped to /dev/sda

Unique ID assigned by udev. It corresponds to individual block devices.

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Management and monitoring4
Management and Monitoring

Understanding the physical disk layout (continued)

To identify a specific SR based on the SCSI ID, compare /dev/disk/by-id with the SR in XenCenter

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Management and monitoring5
Management and Monitoring

LVM-related commands

# pvs # Lists physical volumes

PV VG Fmt Attr PSize PFree

/dev/sda VG_XenStorage-40bbf542-b9d9-ffa1-6efe-aa9c56aadd95 lvm2 a- 99.99G 59.88G

# vgs # Lists volume groups

VG #PV #LV #SN Attr VSize VFree

VG_XenStorage-40bbf542-b9d9-ffa1-6efe-aa9c56aadd95 1 4 0 wz--n- 99.99G 59.88G

Linux sg device

LVM Volume Group stored on the physical volume.

SR UUID

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Management and monitoring6
Management and Monitoring

LVM (continued)

# lvs # Lists the logical volumes

LV VG AttrLSize

VHD-c67a887f-3a1a-41f4-8d40-1b21f6307c4a VG_XenStor... -wi--- 24.00G

VHD-c9b919a7-b93b-49ea-abe5-00acb8240cf5 VG_XenStor... -wi-ao 8.00G

VHD-f3d26dde-254f-4d80-a3bb-d993e904bd63 VG_XenStor... -wi--- 24.00G

LV-e056f479-b0f3-49f3-bc5d-6c226657ae6c VG_XenStor... -wi-ao 10.00G

LV-ebdcad46-66d9-4020-baa1-0d5b6ac439c7 VG_XenStor... -wi-ao 24.00G

The ‘a’ and ‘o’ attributes indicate the LV is ‘active’ and ‘open’ implying it is attached to a running VM

Tip: Type ‘lvm help’ for a complete list of LVM command options.

Represents Logical Volume containers for individual VDIs.

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Management and monitoring7
Management and Monitoring

Understanding how the physical storage is represented as virtual objects in XenServer using the XenAPI

# xesr-list type=lvmoiscsiparams=name-label,uuid,VDIs,PBDs

# Lists the SRs configured for the pool

name-label ( RW) : NetApp - iSCSI

uuid ( RO) : 40bbf542-b9d9-ffa1-6efe-aa9c56aadd95

VDIs (SRO) : f3d26dde-254f-4d80-a3bb-d993e904bd63; c67a887f-3a1a-41f4...

PBDs (SRO) : 27d05ffc-07d3-4f02-d265-3594a2179f8f

Note that the VDI UUID is the same as the logical volume ID. We will make a note of this UUID to refer back to.

Using the PBD UUID from this command output we will query for its characteristics in the next slide…

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Management and monitoring8
Management and Monitoring

Understanding how the physical storage is represented as virtual objects in XenServer using the XenAPI (continued)

# xepbd-list uuid=27d0… params=uuid,sr-uuid,device-config,currently-attached

# List PBD params

uuid ( RO) : 27d05ffc-07d3-4f02-d265-3594a2179f8f

sr-uuid ( RO): 40bbf542-b9d9-ffa1-6efe-aa9c56aadd95

device-config (MRO): port: 3260; SCSIid: 360a98000503350642f4a553833616b57; target: 10.12.45.10; targetIQN: iqn.1992-08.com.netapp:sn.135027806

currently-attached ( RO): true

‘device-config’ describes all the physical characteristics of the block device attached to this PBD. Note the SCSIid as referenced earlier from /dev/disk/by-id

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Management and monitoring9
Management and Monitoring

Understanding how the physical storage is represented as virtual objects in XenServer using the XenAPI (continued)

# xevdi-list uuid=f3d26dde-254f-4d80-a3bb-… params=uuid,sr-uuid,vbd-uuids

# List VDI params

uuid ( RO) : f3d26dde-254f-4d80-a3bb-d993e904bd63

sr-uuid ( RO): 40bbf542-b9d9-ffa1-6efe-aa9c56aadd95

vbd-uuids (SRO): 69afb055-3b52-57e3-63fa-d26b82a9b01d

This tells us what VBDs are attached to this VDI. We will use this UUID in the next slide to query for the VBD characteristics and determine which VM this disk is attached to.

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Management and monitoring10
Management and Monitoring

Understanding how the physical storage is represented as virtual objects in XenServer using the XenAPI (continued)

# xevbd-list uuid=69afb055-3b52-… params=uuid,vm-uuid,vm-name-label,vdi-uuid,mode

# List VBD params

uuid ( RO) : 69afb055-3b52-57e3-63fa-d26b82a9b01d

vm-uuid ( RO): 2c3a0e82-3f96-eab8-4982-db33fdb3bd88

vm-name-label ( RO): Windows 7 Test

vdi-uuid ( RO): f3d26dde-254f-4d80-a3bb-d993e904bd63

mode ( RW): RW

This tells us which VM (name and UUID) this VBD is attached to, and which VDI it is providing to the VM.

Tip: You can issue ‘xe help <command>’ to get syntax help for any ‘xe’ commands.

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Management and monitoring11
Management and Monitoring

FibreChannel LUN Zoning

Since Enterprise SANs consolidate data from multiple servers and operating systems, many types of traffic and data are sent through the interface, whether it is fabric or the network.

With Fibre Channel, to ensure security and dedicated resources, an administrator creates zones and zone sets to restrict access to specified areas. A zone divides the fabric into groups of devices.

Zone sets are groups of zones. Each zone set represents different configurations that optimize the fabric for certain functions.

WWN - Each HBA has a unique World Wide Name (similar to an Ethernet MAC)

node WWN (WWNN) - can be shared by some or all ports of a device

port WWN (WWPN) - necessarily unique to each port


Fibre channel lun zoning
Fibre Channel LUN Zoning

Pool1

Pool2

Xen1

Xen3

Xen2

Zone1

Zone2

Xen2 WWN

Xen1 WWN

Xen3 WWN

FC Switch

Storage WWN

Storage WWN

Initiator Group

Xen3

Initiator Group

Xen1, Xen2

Storage

LUN0

LUN1

LUN2

FC Switch example


Management and monitoring12
Management and Monitoring

iSCSI Isolation

With iSCSI type storage a similar concept of isolation as fibre-channel zoning can be achieved by using IP subnets and, if required, VLANs.

IQN – Each storage interface (NIC or iSCSI HBA) has configured a unique iSCSI Qualified Name

Target IQN – Typically associated with the storage provider interface

Initiator IQN – Configured on the client side, i.e. the device requesting access to the storage.

IQN format is standardized:

iqn.yyyy-mm.{reversed domain name} (e.g. iqn.2001-04.com.acme:storage.tape.sys1.xyz)


Iscsi isolation
iSCSI Isolation

Pool1

Pool2

Xen1

Xen3

Xen2

VLAN2 / Subnet2

VLAN1 / Subnet1

Xen1 Initiator IQN

Xen2 Initiator IQN

Xen3 Initiator IQN

Network Switch

Controller 1 Target IQN

Controller 2 Target IQN

Controller Interface 2

Controller Interface 1

Storage

LUN0

LUN1

LUN2

iSCSI Example


Management and monitoring13
Management and Monitoring

Monitoring XenServer Storage - Alerts

XenServer will generate alerts for certain storage events:

  • Missing or duplicate IQNs configured

  • HA state file lost or inaccessible

  • PBD plug failure on server startup

    XenServer can be configured to send alert notifications via email too.

    See the XenServer Administrator’s Guide for more information about configuring alerts.

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Management and monitoring14
Management and Monitoring

Monitoring XenServer Storage – CLI Commands

# iostat –k # Reports basic I/O stats for devices and partitions

avg-cpu: %user %nice %system %iowait %steal %idle

0.12 0.00 0.05 0.09 0.02 99.72

Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn

cciss/c0d0 4.05 0.52 32.11 164361 10156264

sda 0.11 1.38 1.79 437259 566151

Note: iostat is not a great performance indicator for shared storage devices because it is unaware of external bottlenecks, for example the network in the case of iSCSI.

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Management and monitoring15
Management and Monitoring

Monitoring XenServer Storage – CLI Commands

# hdparm –t /dev/<device> # Performs timed sequential reads

/dev/cciss/c0d0:

Timing buffered disk reads: 286 MB in 3.00 seconds = 95.19 MB/sec

  • Has some limitations:

  • Does not measure non-sequential disk reads.

  • Does not measure disk write speed

  • May not be accurate with non-local storage devices since it is unaware of underlying bus architecture (iSCSI, FC, etc.)

  • Must be sampled repeatedly over time to get an accurate picture of I/O read performance.

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Management and monitoring16
Management and Monitoring

Monitoring XenServer Storage – CLI Commands

# dd if=<infile> of=<outfile> # Simple, common block device copy utility

# dd if=/dev/<device> of=/dev/null

1998929+0 records in

1998929+0 records out

1023451648 bytes (1.0 GB) copied, 13.8456 seconds, 73.9 MB/s

if = ‘infile’, the source dd reads from.

of = ‘outfile’, the target dd writes to.

WARNING: NEVER run dd specifying an active, running VHD as the outfile—it WILL destroy the VM container making it unreadable!!

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Management and monitoring17
Management and Monitoring

Monitoring XenServer Storage – Additional Tips

iSCSI storage throughput can usually be tied directly to network performance. If there is slow throughput for an iSCSI storage array, perform network diagnostics first!!

Many SAN arrays have native logging and monitoring tools that can identify bottlenecks affecting storage performance.

Refer to the Citrix Knowledge Base for best practices and known issues relating to storage performance.

  • http://support.citrix.com/article/CTX121634

  • http://support.citrix.com/article/CTX122806

  • http://support.citrix.com/article/CTX120737

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Management and monitoring18
Management and Monitoring

Protecting Your Data – Backup VM Metadata

  • Can use xsconsole or the CLI.

  • Makes the SR “portable”.

  • Can be used as part of a Disaster Recovery solution, or, as part of regular maintenance of the environment.

  • Can be scheduled within xsconsole.

For more information relating to using XenServer as a Disaster Recovery solution, refer to the Citrix Knowledge Center:

http://support.citrix.com/article/CTX117258

http://support.citrix.com/article/CTX121099

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Management and monitoring19
Management and Monitoring

Protecting Your Data – Exporting VMs

Virtual machines can be exported directly out of XenServer into XVA files that contain a complete clone of the VM and all of its attached VDIs.

Can be initiated via XenCenter or from the XenServer CLI.

VM must be offline (shutdown) during export process.

Since it backs up all the VM data it can take a very long time depending on the size of the VM!

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Management and monitoring20
Management and Monitoring

Protecting Your Data – Creating VM Snapshots

Snapshots create VDI clones of a VM that can be used for backup or quickly provisioned into new VMs or templates.

XenServer supports two types in version 5.5

  • Regular – Supports all guest environments, including Linux

  • Quiesced – Takes advantage of Windows Volume Shadow Copy Service (VSS). It requires the manual installation of in-guest components to enable.

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Management and monitoring21
Management and Monitoring

Protecting Your Data – Creating VM Snapshots (continued)

New in XenServer 5.6!

  • Introduces snapshot “Revert”, a.k.a. “Checkpoint”.

  • Introduces a new snapshot mode: “Snapshot with disk and memory”

  • XenCenter GUI enhanced for easier management of VM snapshots and to support Checkpoint feature.

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Management and monitoring22
Management and Monitoring

Protecting Your Data – Third-Party Solutions

There are also Third-Party backup options:

  • In-guest backups can be performed using any guest-supported solution (backup agents running in Windows or Linux, for example).

  • Volume snapshots performed directly on the storage via StorageLink plugins (for Dell and NetApp).

  • Backup solutions that plug into the XenAPI to capture VM data, or clone the LVM data directly.

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Troubleshooting xenserver storage
Troubleshooting XenServer Storage

Native Troubleshooting Tools – XenServer Logs

Always check the logs first! XenServer creates several logs that are useful for diagnosing storage problems

  • /var/log/messages # General messages and system related stuff

  • /var/log/xensource.log # Logging specific to XenAPI

  • /var/log/SMlog # Logging specific to XenServer storage manager

    Often errors logged in any of these files can be searched for in the Citrix Knowledge Center for a solution. See http://support.citrix.com.

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Troubleshooting xenserver storage1
Troubleshooting XenServer Storage

Native Troubleshooting Tools – XenAPI commands

The XenAPI (xe) can be used to troubleshoot storage issues too

  • # xe sr-scan # Force XAPI to sync the database with local VDIs present in the underlying substrate.

  • # xe sr-probe # Using device-config parameters you can probe a block device for its characteristics, such as existing VM metadata and SR uuid.

  • # xe pbd-plug/unplug # Manually plug or unplug a PBD for an SR. This can be useful when repairing an SR in XenCenter fails.

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Troubleshooting xenserver storage2
Troubleshooting XenServer Storage

Native Troubleshooting Tools – VHD commands

See and verify mount point of VHD SR

  • # /var/run/sr-mount/<SR UUID>

    “full provision” VHD SR

  • vhd-util

  • See http://support.citrix.com/article/CTX118842

    Check VHD architecture

  • # hexdump -vC <VDI-UUID>.vhd | less

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Troubleshooting xenserver storage3
Troubleshooting XenServer Storage

Storage Multipathing

Ensure that multipathing is enabled if you have multiple paths zoned to the XenServer

  • Use ‘sg_map –x’ and check the host and bus IDs

    Problems if you do not enable multipath

  • I/O Errors

  • Decrease in performance

  • Introduce errors with SR.create

    What is multipath.conf vs multipath-enabled.conf

  • multipath.conf is symlink to multipath-enable.conf or multipath-disabled.conf

    DMP vs. MPP multipathing

  • http://support.citrix.com/article/ctx121364

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Troubleshooting xenserver storage4
Troubleshooting XenServer Storage

SAN Debugging

Always start at the hardware adapter, use the Qlogic or Emulex CLI tools to verify the LUNs known to the adapter

  • For QLogic, run ‘scli’

  • For Emulex, run ‘hbanywhere’

    Use ‘xe sr-probe type=lvmohba’ to trigger a bus refresh

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Troubleshooting xenserver storage5
Troubleshooting XenServer Storage

Additional Scenarios

  • Unable to create SRs

    • Verify that XenServer can see the storage/LUN

      • Use fdisk and /dev/disk/xxx

    • Verify that HBA can see the LUN

      • Use the HBA CLI tools

    • Verify that iSCSI can login:

      • # iscsiadm –m node –L all # Will force iscsid service to log into the storage array.

  • Clearing the device mappings via CMD line

    • # echo 1 > /sys/class/scsi_devices/x:x:x:x/device/delete

    • Be extremely careful what device is being deleted!

  • Clean up of orphaned VDIs, XC not displaying the right amount of free storage

    • If a logical volume has no corresponding VDI it can be deleted. Be extremely careful with this because if you delete a parent disk, then you lost all differentiated disks.



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