NetApps SnapMirror document.

 NetApps SnapMirror document.       

SnapMirror

·        This enables you replicate the data from volume or qtree to the volume or qtree.

·        This requires a separate license.

·        Modes of Snapmirror

·        Asynchronous mode :-replicates the snapshots are specified intervals

·        Synchronous mode:- replicates the snapshots as soon as they are written to the source

·        Semi-synchronous mode:-The destination volume lags behind the source volume by 10 Sec

·        SnapMirror can be used either for Flex Volume and also Traditional Volume.

·              How it works

·        Files and options involved in snapmirror ( /etc/snapmirror.conf, snapmirror.access,/etc/snapmirror.allow)

·        It takes the snapshot copy of the source volume and copies to the destination in the read only mode.

·        Updates the changes in the destination in the incremental mode. As result you get an online read only volume or qtree.

Application of snapmirror

·        Disaster recovery = You can make the destination writable so clients can access the data.

·        Disaster recovery testing = FlexClone technology in the destination without affecting the replication process

·        Data restoration = you can reverse the destination and source volume or qtree and sync the data.

·        Application testing = you can copy the data using the snapmirror so that source data is not disturbed.

·        Load Balancing = you can copy the data using the snapmirror and distribute the load.

·        Off - Loading the tape backup= you can take backup from the destination volume and offloading the source utilization.

·       Access for remote users.

Synchronous SnapMirror

·        In this mode the data is written to the destination as soon as it is written to the sourcce filer

·        The data's can synced either between the systems or between the system in the active/active cluster either via ip or fcp

·        We require separate license apart from the snapmirror license for the synchronous transfer of data.

·        This can be used only in the case of the volume not with the qtree's and that two the volume should of same type.As there are two types of volume they are Traditional volume and also FlexVol Volume.\

There are two types of synchronous modes

·         sync

·         semi-sync ( This is used for balancing between the sync and also asynchronous mode of transfer)

·        you cannot set both sync and semi-sync relationship  between filers of active/active configuration, and the definition has to be done in the /etc/snapmirror.conf file.

·        Sync = The source acknowledges the client only after the write opertation is done at both the source and also destination and recovery point objective is almost 0 seconds.

·        Semi-sync = The source acknowledges the client write operation immediately after it is written in the source.The data is written to the destination with time lag of 10 sec. This implies the recovery point objective is 10 sec which means the data of only 10 sec would lost at the tiime recovery. Adv of it is that the performance compared to the sync option.

·        If neither of the two option are not specified then the it is set to asynchronous.

·        Syntax :- systemA:volA systemB:volB – semi-sync

·        How Sync Works:-Before the data is written to the disk the data's is written to the NVRAM and at the consistency point the data is transferred to the disk. In the case synchronous when the data's are written to the NVRAM the source sends the data to the destination NVRAM's and at the consistency point the source asks the destination to write the data to the disk and source also writes the data to the disk, After this source waits for the acknowledgement from the destination before it starts the next write operation

·        How it handles network issues :- If there is any network issues then the snapmirror goes into the asynchronous mode.Snapmirror follows the steps in the case network issues

·        snapmirror sets the mode in the asynchronous mode.

·        In asynchronous mode the source tries to communicate the destination with the time interval of one minute.

·        After the connection is re- established then the source replicates the destination asynchronously to the destination.

·        Snapmirror then gradually transitions replication mode from the asynchronous to synchronous mode.

·        The transition is possible only in the case of latest common snapshot is available otherwise the need to break the snap(snapmirror break) and then resync it with (snapmirror resync).

·        This can also be overcomed with the option “replication.volume.use_auto_resync on”.The default vaule of it is off.

·        Things to consider before growing aggregate with synchronous snapmirror destination volume.

·        Add minimum of four disk.

·        New RAID group is created with atleast four disks.

·        RAID group size is 16 or fewer disk.

·        Tuning snapmirror :- options snapmirror.enable on (This persist even after the reboot)

Consideration for use of snapmirror

Pre-requisites

·        Need to enable the license at the filers where we are going to use the snapmirror.

·        Snapmirror volume replication requires the destination volume to be in the restricted mode and needs to be created also.

·       The destination filer for the snapmirror volume replication needs to be same version or the later one comparing to the source.If you use it for DR then both filer should be off same version.

If you are upgrading the filer destination needs to be done before source.

·        For QSM , the destination qtree should not be created its automatically created at the time initialization (snapmirror initialize).

·        For QSM OnTap version should be more than 6.2.

·        IP needs to be resolvable in the both the ends (Source and destination).

Restrictions

·        Source volume needs to be online.

·        Destination needs to be more than the source or equal to that.

·        In QSM the destination needs to be more than 5% more than the source qtree consumes.

·        In VSM the destination volume cannot be the root volume but the source volue can root volume.

·       In QSM. Destination qtree can be on the root that is /etc but cannot be /etc qtree.

Point of Caution while snapmirror\

·        Do not delete the snapshot copies that the snapmirror creates in the source before copying to the destination.The newely created snapshot is called NCS(Newely created snapshot copy).Incremental copy depends upon the NCS.

·        Don't use “snapmirror release or snapmirror break” in the destination less you don't require the incremental changes from the source.

·        Don't take the destination volume offline.

Recommenations of SnapMirror

·        The schedule time of snapmirror and also snapshot should not happen at the same time.Then the snapshot would fail saying in the log file that already a snapshot copy is available.

·        If the source and destination are FlexVol Volume then there is no problem with the RAID configuation.

Deduplication with volume snapmirror

·        In the case deduplication, The deduplication metadata is kept in the aggregate level outside of the volume, so metadata is not replicated along with the volume so in the detination the deduplication needs to be started using the “sis start -s “( Without -s options only newly written data is scanned for the deduplication)

Destination accessability when using CIFS with snapmirror

·        The directories needs to be unicode format. This can be done with the following options “vol options vol_name convert_ucode on”

Deployment of snapMirror

·        It consists of source volume and qrees, and destination volume and qtree .

·        Source volume or qtree's :-These are data objects  that are need to synchronized or replicated.Normally users access these data's object and have read write access to the data's.

·        Destination volume or qtree's:- These are data object where the data is synchronized . Destination volume or qtree are only read only. The destination volume needs to be writable if we use QSM

·        The volumes can be used for both traditional and FlexVol Volumes.

·        VSM

·        This supported only same kind of volume either traditional volume or FlexVol Volume.

·        QSM

·        It supports different kind volumes. i.e traditional and also FlexVol Volume.

SnapMirror commands

·        Snapmirror on ( Use to enable the snapmirror, Between snapmirror on and snapmirror off command you need wait for 60 seconds for proper transfer of controls)

·        To enable you can also use options snapmirror.enable on

·        vol create and vol restrict (To create destination volume snapmirror)

·        snapmirror initialize (Used to start the initial transfer of snapshot(Baseline copy) from the source to destination .

·        snapmirror status (view the status of the snapmirror)

·        snapmirror update (Manually updating the snapmirror destination )

·        snapmirror quiesce (Stabilize the contents of the destination volume before snapshot is taken, and allowing the active snapmirror transfer to finish and temporarily preventing the new transfers).

·        Snapmirror resume (Resume the normal transfer after quiesce).

·        Snapmirror abort (Stopping the active snapmirror)

·        Snapmirror break (Used to break the relationship between the source and the destination volume and convert the volume in writable volume or qtree)

·        Snapmirror resync (Reestablish the relationship between the source and destination volume , This command is generally issued after the “snapmirror break” so that we avoid the initial transfer which is the baseline copy)

·        Snapmirror release (The snapshot copies are deleted)

·        Snapmirror off (Turn off the snapmirror functionality) alternatively options snapmirror.enable off

·        Snapmirror store and snapmirror use(Copy the volume to the local tape, and  continue the same on subsequent tapes).

·        Snapmirror retrive and snapmirror use (Initial or restore the volume from the local tape)

·        Snapmirror destination (Used in the case of cascading the snapmirror destination )

Snapmirror options

·        Snapmirro.enable on|off ( Default is off)

·        Snapmirror.access host=list(list is hostnames separated by comma)(Default = legacy this means the access depends upon the list of hostnames available in /etc/snapmirror.allow)(If both options and /etc/snapmirror.allow are used then options takes precedence)

·        Snapmirror.log.enable on|off(Whether the snapmirror activity is being logged /etc/log/snapmirror.x files (Default on)

·        Replication.volume.use_auto_resync on|off (specifies automatic re-synchronisation for synchronous snapmirror relationship) (Default off)

·        Replication.volume.reserved_transfers n (Specifies the number of reserved transfers for the snapmirror volumes and n is the number of times and it depends upon the system model default is 0)

·        Replication.logical.reserved_transfers n (specifies the number of reserved transfers for snapvault and snapmirror qtrees where n is the number of times and default is 0).

·        Replication.volume.transfer_limits {current|previous}(Specifies the maximum number of transfers the volume snapmirror can run concurrently)(Default is current)(Current is set as 7.3.x limit and previous is set as 7.2.x limit)

·        Replication.logical.transfer_limits {current|previous}(Specifies the maximum number of transfers the qtree snapmirror and snapvault  can run concurrently)(Default is current)(Current is set as 7.3.x limit and previous is set as 7.2.x limit)

Snapmirror files:-

·        /etc/snapmirror.conf ( They are used to specify the relationship between the source and the destination along with the following

·        Snapmirror update schedule for relationship

·        Type of relationship (single, multipath or failover)

·        Other options

·        /etc/snapmirror.allow ( Specifies the snapmirror destination that are allowed to copy from the system)

·        /etc/log/snapmirror ( Latest the snapmirror and the older ones are snapmirror.0 snapmirror.1)

·        /etc/hosts( Used for resolving the host names)

Setting up a basic snapmirror operation:-

·        License need to be enabled in the source and also destination filer.

·        Need to specify the destination systems name on the source filer in the /etc/snapmirror.allow

·       If the source volume contains the directories that are being accessed by clients then the volume needs to be changed to the Unicode format before the replication.

STEPS:-

·        license add snapmirror_license_code

·        s_systemA>options snapmirror.access host=d_systemA

·        In the destination /etc/snapmirror.allow “s_systemA:vol0 d_systemA:vol1 kbs=200,restart=always 15 * * 1,2,3,4,5” and “s_systemA:vol0 d_systemA:vol1 – sync”

·        In both source and destination the  “snapmirror on” needs to be given.

·        VSM :- “d_systemA>snapmirror initialize –S systemA:vol0 systemB:vol2”

·       QSM:- “d_systemA>snapmirror initialize –S systemA:/vol0/vol1/qtree4 systemB:/vol2/vol1/qtree4”

Firewall usage with snapmirror:-

·        They use socket/bind/listen/accept sequence on the TCP socket.

·        Snapmirror source is binded to the 10566 port number. Synchronous snapmirror requires extra one more port which is 10569. Destination listens on the 10565,10567,10578.

·        So ports between 10565 to 10569 needs to opened.

With Regards,

Santhosh Kumar R