XenServer Storage Management and Troubleshooting

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XenServer Storage Management and Troubleshooting

Daniel Lazar

Lead Escalation Engineer

May 11, 2010

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• XenServer Storage Overview

• Management and Monitoring

• Troubleshooting and Diagnosing Common Storage Issues

• Q & A

Citrix Confidential - Do Not Distribute

Agenda

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XenServer Storage Overview

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• XenServer Storage Objects

• SRs, VDIs, PBDs and VBDs

• Virtual Disk Data Formats

• File-based VHD, LVM and StorageLink

XenServer Storage Overview

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What is an SR (Shared Repository)?

XenServer Storage Objects

• 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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VDIs, PBDs, VBDs

XenServer Storage Objects

• 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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PBD PBD PBD

SR

VDI

VDI VBD

VBD XenServer Host VBD

XenServer Host

Virtual Machine

XenServer Storage Objects

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File-based VHD

Virtual Disk Data Formats

• 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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Logical Volume (LVM)-based VHDs

Virtual Disk Data Formats

• 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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StorageLink (LUN per VDI)

Virtual Disk Data Formats

• 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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StorageLink Architecture

Virtual Disk Data Formats

• 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

Storage Repository Storage Repository

LUN LUN

LVM Volume Group LVM Volume Group

LVM Logical Volume LVM Logical Volume

VHD header

VHD header VHD headerVHD header

XenServer 5.5 iSCSI / FC XenServer 5.5

iSCSI / FC

LUN LUN

Storage Repository Storage Repository XenServer 5.5

iSCSI / FC XenServer 5.5

iSCSI / FC + +

LUN LUN

VM Virtual Disk

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Storage Management and

Monitoring

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• Understanding how XenServer Perceives the Storage

• Monitoring Storage

• Protecting Your Data

Management and Monitoring Overview

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Understanding the physical disk layout

Management and Monitoring

# 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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Understanding the physical disk layout (continued)

Management and Monitoring

# 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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# 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)

Management and Monitoring

Host Number

Bus SCSI

ID

LUN SCSI

Type

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Understanding the physical disk layout (continued)

Management and Monitoring

# 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

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

This SCSI device is

mapped to /dev/sda

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Understanding the physical disk layout (continued)

Management and Monitoring

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

with the SR in

XenCenter

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LVM-related commands

Management and Monitoring

# 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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LVM (continued)

Management and Monitoring

# lvs # Lists the logical volumes

LV VG Attr LSize

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

Represents Logical Volume containers for individual VDIs.

Tip: Type ‘lvm help’ for a

complete list of LVM command options.

The ‘a’ and ‘o’

attributes indicate the LV is ‘active’

and ‘open’ implying

it is attached to a

running VM

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

Management and Monitoring

# xe sr-list type=lvmoiscsi params=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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Understanding how the physical storage is represented as virtual objects in XenServer using the XenAPI (continued)

Management and Monitoring

# xe pbd-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 Monitoring

# xe vdi-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 Monitoring

# xe vbd-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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Fibre Channel LUN Zoning

Management and Monitoring

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

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Fibre Channel LUN Zoning

Xen1 Xen1 Xen2 Xen2 Xen3 Xen3

Pool1 Pool2

FC Switch

Storage

FC Switch example

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iSCSI Isolation

Management and Monitoring

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)

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Storage

iSCSI Isolation

Xen1 Xen1 Xen2 Xen2 Xen3 Xen3

Pool1 Pool2

Network Switch

iSCSI Example

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Monitoring XenServer Storage - Alerts

Management and Monitoring

• 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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Monitoring XenServer Storage – CLI Commands

Management and Monitoring

# 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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Monitoring XenServer Storage – CLI Commands

Management and Monitoring

# 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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Monitoring XenServer Storage – CLI Commands

Management and Monitoring

# 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

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

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

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

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Monitoring XenServer Storage – Additional Tips

Management and Monitoring

• 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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• 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.

Management and Monitoring

Protecting Your Data – Backup VM Metadata

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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Protecting Your Data – Exporting VMs

Management and Monitoring

• 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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Protecting Your Data – Creating VM Snapshots

Management and Monitoring

• 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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Protecting Your Data – Creating VM Snapshots (continued)

Management and Monitoring

• 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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Protecting Your Data – Third-Party Solutions

Management and Monitoring

• 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 and Diagnosing

Common Storage Issues

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Native Troubleshooting Tools – XenServer Logs

Troubleshooting XenServer Storage

• 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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Native Troubleshooting Tools – XenAPI commands

Troubleshooting XenServer Storage

• 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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Native Troubleshooting Tools – VHD commands

Troubleshooting XenServer Storage

• 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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Storage Multipathing

Troubleshooting XenServer Storage

• 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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SAN Debugging

Troubleshooting XenServer Storage

• 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 Storage

• 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.

Additional Scenarios

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Questions? Comments?

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