02 Snapshots and Replication

8/7/2019 02 Snapshots and Replication

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2-2Snapshots, Replication, and Clones






A clone is a bit-for-bit copy of an existing volume.

Cloning a volume creates a new volume with a new name, a new iSCSI Qualified Name(IQN), a new access control record, snapshot space, etc.

Once the clone is created, there is no continuing relationship between the original volumeand the clone; they clone and the original volume are from that point forward two separateentities

You can create a clone from a volume, snapshot, or replica




Creating a new volume, by copying an existing object

• From base volume, snapshot or replica

• Non-shared storage (unlike snapshot) once copy completes

• Full volume operations available once copy completes

Cloning process

• Coordinated (like snapshot) creation

• Instant creation and accessibility (while copy is occurring)

• Background copy

• Space balancing enforcement







Snapshot overview

• Snapshots enable you to capture the contents of a volume at a specific point in timeand can be used for backups or testing.

• Snapshot creation does not disrupt access to the volume.

• Like volumes, snapshots appear on the network as iSCSI targets and can be setonline and accessed by hosts with iSCSI initiators.

• You can create a snapshot of a volume at the current time, or you can set upschedules to automatically create a snapshot at a specific time in the future or on aregular basis.

Fast creation

• Synchronized across PS Series Group

• Distributed snapshot

• Follows the same distribution of the base volume

Copy Original data before write (COW)

• Tracked at page level; facilitates data movement of pages

• Tracked at 64kb increment within a page on initial allocation

• Unmodified pages are shared between volume and snapshots




The physical storage is divided into logical segments called pages, consisting of multiplelogical blocks (LBs). This is similar to the way most file systems aggregate physical disksectors into "clusters" or "chunks."

When you create a volume (called the base volume), an internal table (called the basevolume table) is created,

• Base volume table containing pointers to the pages that make up the base volume.

When you create a snapshot of the base volume

• A new volume (the snapshot) is created, whose page table is a copy of the one forthe base volume.

• No data is moved or copied, nor are any new storage pages allocated

• The page tables for the base volume and snapshot are at this point identical




When WRITES occur to the base volume the software then performs the following sequence:

• Determines which page in the base volume will be changed by the upcoming WRITE.

• Allocates a new page out of the snapshot reserve space and copies the original data from the

base volume page into the new page.

• Changes the pointer in the original snapshot page table (but not in the base volume table) to

point to that new page, which contains the original data.

• Commits the WRITE to the base volume, this sequence is called a copy-on-write operation.

Only the first WRITE operation to a given page on the base volume will cause the page allocation

and copy-on-write operations to occur.




Modifying the snapshot reserve.

• You can change the amount of space, as a percentage of the volume size, that isreserved for snapshots.

Space allocated is group space consumed by the volume.

Snapshot reserve is group space reserved for snapshots of the volume.

Free group space is unused group space.

• The table shows how space is utilized with the current snapshot reserve and with thenew reserve value. If increasing the snapshot reserve will exceed the capacity of thegroup, the free group space table cell will turn red.

Snapshot space recovery policy.

• Choose the action to take when the reserved snapshot space has been exceeded,either delete oldest snapshot or put the volume and its snapshots offline.

• If a snapshot has active iSCSI connections, the connections will be terminatedbefore the snapshot is deleted or before the volume and its snapshots are set offline.

• You can prevent the loss of snapshot data due to automatic snapshot deletion bycloning the snapshot or restoring the base volume from the snapshot.

Modifying the warning level.

• You can change the threshold at which a warning message is generated becausethe amount of free reserved snapshot space has fallen below this limit.




Before creating a snapshot of a volume, check that snapshot space is reserved for thevolume.

When a snapshot is created, a name is automatically generated using the volume name plusa timestamp plus an identification number (for example, staff1-2004-01-08-11:29:29.1).

An iSCSI target name for a snapshot is also automatically generated. The iSCSI target namewill consist of the iSCSI prefix (iqn.2001-05.com.Dell EqualLogic, by default), followed by agenerated string, followed by the snapshot name.

As with volumes, host access to snapshots is controlled through access control records.

To create a snapshot at the current time, click:

Volumes > volume_name > Create snapshot




When a snapshot is created, a name is automatically generated using the volume name plusa timestamp plus an identification number

• For example, staff1-2004-01-08-11:29:29.1

An iSCSI target name for a snapshot is also automatically generated. The iSCSI target namewill consist of the iSCSI prefix (iqn.2001-05.com.Dell EqualLogic, by default), followed by agenerated string, followed by the snapshot name.




Select the add button and then specify the following information:

• Schedule name, a name that you are familiar with

• Identify whether the schedule will be used for snapshots or replication.

Type of schedule, either once, on an hourly basis, or on a daily basis.

• You can also choose to use an existing schedule and modify it to meet your needsby selecting Reuse existing schedule.

Based on the schedule type, set the following parameters:

• Dates that the schedule will start and, optionally, stop running.

• How often to run the schedule.

• Time to create the snapshots or replicas.

• Frequency of snapshots or replicas.

• Maximum number of snapshots created by the schedule that the group will retain.

• If you reach the maximum snapshots, the oldest snapshots created by the

schedule will be automatically deleted before a new snapshot is created.

• If a snapshot to be deleted has active iSCSI connections, the connectionswill be terminated before the snapshot is deleted.

• Whether the snapshots will have read-write permission.




There are three different ways to recover data from a snapshot of a volume:

• Restore the volume from the snapshot: this option returns the original volume to thestate that it was in when the snapshot was taken.

• Clone the snapshot: this option creates a new volume that is a copy of the original

volume as it existed when the snapshot was taken.• Attach to the volume with an iSCSI initiator: the user logs into the snapshot using an

iSCSI initiator. If changes are made, then the snapshot will no longer be an image ofthe original volume as it existed when the snapshot was taken.




You can restore a volume from a snapshot. The restored volume will contain the data thatexisted in the volume at the time the snapshot was created and will have the original volumename and iSCSI target name.

The volume and the snapshot must be offline during the restore operation. All members that

contain data from the base volume or the snapshot must be online to restore the volumefrom the snapshot.

To set a volume and snapshot offline and restore the volume from the snapshot:

• Set the volume offline - Volumes > volume_name > Set volume offline

• Set the snapshot offline - Volumes > volume_name > snapshot_timestamp




Cloning a snapshot creates a new volume that is a copy of the original volume as it was atthe time that the snapshot was created

• The new volume will consume as much space as the original volume

• The new volume will have its own volume characteristics such as volume name,

iSCSI Qualified name, access control, snapshot reserve, etc. After you have created the clone, you can log in to the clone with an iSCSI initiator, and

recover files or directories from the clone

Cloning the snapshot does not cause any changes to occur to either the snapshot or theoriginal volume

After you have recovered the missing files, you can log off of the clone and delete the clone




Connecting the iSCSI initiator to a snapshot is OS dependent

• Windows volume can be mounted on the same Windows server or a different server(just drive x).

• You must leave the snapshot read/write.

• Run chkdsk on the volume before use

• NetWare volume - can NOT be mounted on same server

• You would need to mount that volume on another host.

• The original and the clone/snapshot can't be mounted on the same server atthe same time.

• Linux - RH or SLES

The snapshot will still be subject to automatic deletion in accordance with either the spacerecovery policy or snapshot schedule













Typical replication solutions are often time-consuming and require you to back up data andmanually transport the backups to a different location.

Through Dell EqualLogic’s Snapshots and Auto-Replication capability, end-to-end dataprotection capability is now possible for customers.

• At the primary site, snapshots are use to provide quick recovery, based on• Volume changes

• Snapshot schedule

• Risk

• Need to recover

• At the Remote site

• Critical volumes or possibly all volumes are replicated

• Backup to tape occurs at the replication secondary site, allowing for allbackups to happen at a central site

• Secondary site is available for operation if the primary sites should fail Replication with Dell EqualLogic PS Series Storage arrays is performed between groups

• Primary site is one group

• Secondary site is a second group

Other solutions require expensive hardware and the ability to synchronously copy dataacross long distances, which can adversely affect application performance.

With Dell EqualLogic’s Auto-Replication, you can protect data from serious failures, rangingfrom the destruction of a volume to a complete site disaster.




Easy configuration and monitoring

• By configuring two separate PS Series groups as replication partners, volume datacan be automatically replicated (copied) across storage management groups.

• Replicas are similar to snapshots, in that they represents the contents of the

volume at a specific point in time.• The replica set includes all the completed replicas for a volume.

• The replica volume (not visible to the user, but equivalent to the most recentcomplete replica) ensures that the secondary group always preserves acomplete, stable copy of the volume data the key goal of Dell EqualLogicreplication technology.

• Replication process

• The first replication is a complete volume data transfer

• After the second replication finishes (only the changed volume datais transferred), the second replica becomes the most recent

complete replica.• Now, the replica volume is formed from the first replica and the

changed data from the second replica. As additional replicas arecreated, this process is repeated.

Fast Replica Failback

Replication Slow Link Support

• In the event that the replication partnership is over a slow data link, this optionprovides a mechanism to save the initial replica image to external media, physicallyship it to the remote site, and restore the image from the external media

• The Manual Transfer Wizard is used to copy only non-zero data to external media




Ability to optimize recovery back to primary site after an outage

Optimized for slow links, reduces mean time to recover, both in terms of time theadministrator must spend to launch / complete the recovery and the overall recovery time

Works with collections so you can fail back the data for an application quickly




Ability to securely transfer initial replications to the remote site using transportable media ofyour choice

Optimized to minimize space required for the data being transferred

Thin provisioning aware




Collections allow you to now group related volumes together to provide for point-in-timecopies of all volumes within the same collection at the same time

• This allows you to create a snapshot of 2 or more related volumes at the same timeby placing them in the same collection

• This allows you to create a replica of 2 or more related volumes at the same time byplacing them in the same collection

• A volume can exist in multiple collections at the same time

• Under normal circumstances it is recommended that a volume only exist in asingle collection

• You should be cautious with this because the results could be volumes thatare out of sync resulting, example

• “Collection_one” contains two volumes – DBvol-1 and Log-vol1

• “Collection_two” contains three volumes –mail_store-1, mail_store-1 and Log-vol1

• Snapshot of “Collection_two” is performed at 1:00

• Snapshot of “Collection_one” is performed at 1:10

• You do a recover of you Database and use the snapshot of Log-vol1to rebuild your Database, if you use the snapshot taken with the1:00 snapshot, could result in rebuild issues







Cloning a replica creates a new volume that is a copy of the original volume as it was at thetime that the replica was created

• The new volume will consume as much space as the original volume

• The new volume will have its own volume characteristics such as volume name,

iSCSI Qualified name, access control, snapshot reserve, etc. After you have created the clone, you can log in to the clone with an iSCSI initiator, and

recover files or directories from the clone

Cloning the replica does not cause any changes to occur to either the replica or the originalvolume

After you have recovered the missing files, you can log off of the clone and delete the clone

Note that when you create a clone, you do so from a specific replica, rather than the replicaset




Temporarily promoting a replica set is useful when the primary volume is unavailable. Thetemporary volume can be used while the primary volume is offline.

Temporary promotions are space efficient. When you promote the volume, delegated spaceis reduced by the amount of space required for the promotion. The net available group

space stays constant. Changes made to the temporary volume can be failed back to the primary when it becomes

available

Before promoting the volume, ensure that there are no iSCSI connections to the originalvolume, and set the volume offline

When the primary volume becomes available, disconnect from the temporary volume andfollow failback procedures to send changes back to the primary

If the primary volume turns out to be permanently unavailable, it is possible to convert thetemporary promotion to a permanent promotion




A volume should be permanently promoted when there is no plan to return operations to theoriginal volume. It is assumed that the volume is destroyed.

Promotion transforms the replica set into a volume and set of snapshots that correspond toeach replica in the set

Replication for that volume will be disabled. Delegated space used by the replica set will bereleased







Local reserve is on the Primary group side

• Settable value between 5% to 100% of the volume size

• Used to preserve the state of the volume while a replication data transfer is in progress.

• If volume writes occur during a replication data transfer, the original (unmodified) volume datais stored in the local reserve, progressively consuming space.

• Once the data transfer completes, any volume data in the local reserve is released and is notused again until volume writes occur during the next replication data transfer.

• Can vary for each replication and is based on the workload and the time it takes for the datatransfer to complete.

• If the local reserve is not big enough to store volume data during a replication data transferthe in-progress replication will fail

Replica reserve is on the secondary side group

• Is a portion of the space that the secondary group must allocate to the primary group. Thisspace is know on the secondary side as Delegated Space. This delegated space is used tostore the volume replica sets from the primary group, can be thought of as having two parts.

• First part is the initial replication of a volume and is a complete transfer of the basevolume data and consumes 100% of the base volume size.

• The Second part is each subsequent replica data transfer consisting only of thedeltas between the volume contents at start of the previously completed replication

and the volume contents at the start of the current replication.• Used to store the deltas while the data transfer is on-going and also to

store complete replicas other than the most recent.

• When the second part of the replica reserve is consumed, the oldestreplicas are automatically deleted to make room for new replicas.

• If all the replicas except the most recent are deleted, but the size of thesecond part of the replica reserve is still not big enough to store the deltasfor a replication operation, the data transfer will pause until theadministrator increases the replica reserve.

• The second part of the replica reserve is the most difficult to size becauseof workload variations and replication scheduling. The minimum space is5% of the volume size.




Local reserve considerations:

• Set the local reserve to the default (100%) to ensure that the size of the local reservewill be sufficient and a replication will never fail because of lack of local reserve.

• Set the local reserve to less than 100% (minimum 5%) if you are certain that thevolume changes during replication will be consistently less than 100% of the volumesize. However, if the local reserve is insufficient, the in-progress replication will fail.

• Set the local reserve to less than 100% and enable the feature that lets youtemporarily borrow free group space when the local reserve is too small. However, ifthe local reserve is insufficient and there is insufficient free group space, the in-progress replication will fail.

Replica reserve considerations

• You may want to be conservative when sizing the replica reserve. Insufficient replicareserve will result in the replication operation being paused

• If you cannot increase the replica reserve, the paused replication operation will notbe able to complete, and you may have to manually cancel the paused replicationoperation and re-plan your overall replication design.

Replication times

• It is important to design a replication environment that enables you to reliablyperform replication operations.

• A key performance factors to take into account

• volume sizes

• change rates

• replication frequency

• network bandwidth













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02 Snapshots and Replication