N series Snapshot: A Technical Discussion
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1. Snapshot version of X remains composed of blocks A B amp C1 Consumes space incrementally Figure 5 Step 4 Active pointers Understanding Snapshots in detail A small percentage of the drive s available space is used to store file system related data and can be considered as overhead A file system splits the remaining space into small consistently sized segments In the UNIX world these segments are known as inodes Understanding that the WAFL file system is a tree of blocks rooted by the root inode is the key to understanding Snapshots To create a virtual copy of this tree of blocks WAFL simply duplicates the root inode Figure 6 on page 8 illustrates how this works WAFL creates a Snapshot by duplicating the root inode that describes the inode file and avoids changing blocks in the Snapshot by writing new data to new locations on disk N series Snapshot A Technical Discussion 7 8 A B C Before After After Block Snapshot Snapshot Update New Root New Root Root Inode Snapshot Inode Snapshot Inode Figure 6 WAFL duplicates root inode avoids changing blocks by writing new data to new locations on disk Column B in Figure 6 shows WAFL creating a new Snapshot by making a duplicate copy of the root inode This duplicate inode becomes the root of a tree of blocks representing the Snapshot just as the root inode represents the active file system When the Snapshot inode is crea2. File LUN X Snapshot File LUN X These particular blocks are frozen on disk e Consistent point in time copy e Ready to use read only Consumes minimal amount of space 4 KB and creates pointers to original data Figure 3 Step 2 Pointers are created 3 When a request to Block C occurs the original Block C1 is frozen in order to maintain a point in time copy The modified block C2 is written to another location on disk and becomes the active block as shown in Figure 4 on page 6 N series Snapshot A Technical Discussion 5 6 How Snapshot works 3 Snapshot File LUN X Active Data File LUN X oe ser eee yer Lod Lae oy Todd nic Client sends Modified data block is simply written to an optimal location on the disk in one I O operation new data block gt Some vendors require 3 operations Figure 4 Step 3 Modified data block written to disk in one operation 4 The final result is that the Snapshot now consumes 4 K C1 of space Active pointers for the point in time snapshot are unmodified blocks A B and point in time copy C1 as shown in Figure 5 on page 7 N series Snapshot A Technical Discussion How Snapshot works 4 Active Data File LUN X Snapshot File LUN X Po a aoe oot Active version of X is now composed of blocks A B amp C2 oooee e t PECE dd
3. Creating a Snapshot from SnapDrive for UNIX and Linux One of the better data protection solutions provided by IBM System Storage N series is the combination of Snapshot and SnapDrive This section explains how these two features work together to create a high availability solution allowing you to create modify delete or connect a Snapshot with just one command SnapDrive for UNIX includes AIX support You use the snapdrive snap create command to create Snapshot copies As previously mentioned they are point in time read only images of data on storage system volumes The snap create operation ensures that you have backed up your LUNs files and directory trees You can use the Snapshot copy you create to restore your data if you encounter corruption or other problems Note To ensure that a Snapshot copy is application consistent you usually need to stop or perform whatever steps are required to quiesce the application before taking the Snapshot copy Follow these guidelines when you enter commands that create Snapshot copies gt You can keep a maximum of 255 Snapshot copies per storage system volume This limit is set by the storage system The total number can vary depending on whether other tools use these Snapshot copies When the number of Snapshot copies has reached the maximum limit the snapshot create operation fails You must delete some of the old Snapshot copies before you can use SnapDrive for Linux and UNIX to t
4. Redbooks paper Alex Osuna Patrick Marcius M dice Bisi Sven Schaffranneck N series Snapshot A Technical Discussion Introduction and overview This IBM Redpaper discusses the N series Snapshot feature Snapshot is a standard feature of the Data ONTAP operating system It enables online backups to be maintained thus providing near instantaneous access to previous versions of data without requiring complete separate copies or resorting to offline backups Snapshots can be scheduled by an administrator and you can keep up to 255 Snapshots online at any one time The paper provides UNIX and Windows examples of creating Snapshots Snapshot technology makes extremely efficient use of storage by storing only block level changes between each successive Snapshot In this way it can be simply thought of as being similar to any modern source code control system that maintains only the changes made to the original source code in order to minimize space and maximize recoverability This analogy actually extends further and we explore it later in this document Copyright IBM Corp 2007 All rights reserved ibm com redbooks 1 2 Because the Snapshot process is automatic and virtually instantaneous backups are significantly faster and simpler Snapshots can also be coordinated with outside applications to ensure highly consistent data states as viewed from the application prior to performing Snapshot and other backup procedures
5. For example flushing data from a production database prior to Snapshot creation is a generally recognized best practice The primary focus of this paper is on the algorithms and data structures that Write Anywhere File Layout WAFL uses to implement Snapshots which are read only clones of the active file system WAFL uses a technique to minimize the disk space that Snapshots consume This paper also describes how WAFL uses Snapshots to eliminate the need for file system consistency checking after an unclean shutdown and gives examples of how to set up a Snapshot using the FilerView Data ONTAP CLI SnapDrive for Microsoft Windows and SnapDrive for Linux and UNIX The WAFL primary distinguishing characteristic is Snapshots which are read only copies of the entire file system WAFL creates and deletes Snapshots automatically at prescheduled times and it keeps up to 255 Snapshots online at once to provide easy access to old versions of files Figure 1 on page 3 gives an overview of Snapshot features N series Snapshot A Technical Discussion Snapshot A snapshot is a read only freeze framed version of a filer s file system frozen at some past point in time Block 1 A volume can maintain up to 255 snapshots concurrently Snapshots are readily accessible via special subdirectories that appear in the current or active file system Block 2 Snapshots consume space when the file s
6. and so on up to the root of the tree A B Before Block Update After Block Update Snapshot Root Snapshot Root Inode Inode Inode Inode inode File Indirect Black Inode File Black Regular File Indirect Block Regular File Data Block Figure 7 Updating the pointers in the block s ancestors requiring them to be written to new locations WAFL would be very inefficient if it wrote this many blocks for each NFS write request Instead WAFL gathers up many hundreds of NFS requests before scheduling a write episode During a write episode WAFL allocates disk space for all the corrupted data in the cache and schedules the required disk I O As a result commonly modified blocks such as indirect blocks and blocks in the inode file are written only once per write episode instead of once per NFS request N series Snapshot A Technical Discussion 9 Snapshot data structures and algorithms The Snapshot data structures and algorithms are unique to the N series storage systems and they are built upon WAFL and its design Snapshot utilizes the WAFL design characteristics so that operational overhead is kept to a minimum The block map file 10 Most file systems keep track of free blocks by using a bitmap with one bit per disk block If the bit is set then the block is in use However this technique does not work for WAFL because many Snapshots can reference a block at the same time Instead the WAFL block
7. map file contains a 32 bit entry for each 4 KB disk block Bit 0 is set if the active file system references the block bit 1 is set if the first Snapshot references the block and so on A block is in use if any of the bits in its block map entry are set Figure 8 on page 11 shows the life cycle of a typical block map entry At time t1 the block map entry is completely clear indicating that the block is available At time t2 WAFL allocates the block and stores file data in it When Snapshots are created at times t3 and t4 WAFL copies the active file system bit into the bit indicating membership in the Snapshot The block is deleted from the active file system at time t5 This can occur either because the file containing the block is removed or because the contents of the block are updated and the new contents are written to a new location on disk The block cannot be reused however until no Snapshot references it In Figure 8 on page 11 this occurs at time t8 after both Snapshots that reference the block have been removed N series Snapshot A Technical Discussion Block Map Description Entry 00000000 Block is unused 00000001 Block is allocated for active FS 00000011 Snapshot 1 is created 00000111 Snapshot 2 is created 00000110 Block is deleted from active FS 00000110 Snapshot 3 is created 00000100 Snapshot 1 is deleted 00000QQ0 Snapshot 2 is deleted block is Nunused Bit 0 set for active fil
8. H WrtualbisK2 0 1 1 EP sapshot_vol0_05_25_2007 J Vituabsk3 0 1 0 4 Removable Storage Disk Defragmenter Z Disk Management i Ra Services and Applications gt ja Snapshots 1 Figure 14 Snapshot created Creating a Snapshot from Data ONTAP CLI Another way to create Snapshots from a volume is from the Data ONTAP command line interface using the snap create command Note To view brief help information about the command type snap help create at the command line interface itsotuc3 gt snap help create snap create A V lt vol name gt lt snapshot name gt itsotuc3 gt 1 Logon to the N series Data ONTAP command line interface using Telnet SSH or the serial console 2 Use the snap create command to create a Snapshot of your target volume see Example 4 on page 20 N series Snapshot A Technical Discussion 19 20 Example 4 Snapshot creation on the Data ONTAP CLI itsotuc3 gt snap create vol_DominoDB snapshot_vol0_06 25 2007 itsotuc3 gt Where vol_DominoDB This is our volume name snapshot_vol0_06 25 2007 This is the name of the Snapshot 3 Verify the created Snapshot using the snap 1ist command see Example 5 This step is not mandatory but it is useful Note If you enter snap list without any options all existing Snapshots are returned Example 5 List the newly created Snapshot itsotuc3 gt snap list vol_DominoDB Volume vol_DominoDB working used tota
9. ake any more gt SnapDrive for UNIX does not support Snapshot copies that it does not create For example it does not support Snapshot copies that are created from the storage system console because such a practice can lead to inconsistencies within the file system gt You cannot use SnapDrive for UNIX to create Snapshot copies of the following Root disk groups The snap create operation fails when you try to take a Snapshot copy of a root disk group for a logical volume manager Boot device or swap device SnapDrive for UNIX does not take a Snapshot copy of a system boot device or a system swap device Note For more information about requirements hints and commands we highly recommend referring to IBM System Storage N series SnapDrive for UNIX Installation and Administration Guide which is available at the following site http www 1 ibm com support docview wss uid ssg1S7001600 amp aid 1 N series Snapshot A Technical Discussion 15 16 To create a Snapshot copy from the Linux or UNIX server using SnapDrive follow these steps 1 Create the Snapshot with the snapdrive snap create command see Example 1 Example 1 Creating the Snapshot of the Lotus Domino database and transactional log root dominol snapdrive snap create lun db SdDg log SdDg snapname snapl Successfully created snapshot snap1 on 2 filer volumes jtsotuc3 vol vol_DominoDB jtsotuc3 vol vol_DominoLog Snapshot snapl contains di
10. e United States other countries or both UNIX is a registered trademark of The Open Group in the United States and other countries Linux is a trademark of Linus Torvalds in the United States other countries or both Other company product or service names may be trademarks or service marks of others 24 N series Snapshot A Technical Discussion
11. esystem Bit 1 set for Snapshot 1 Bit 2 set for Snapshot 2 Bit 3 set for Snapshot 3 Figure 8 The life cycle of a block map file entry Creating a Snapshot The challenge in writing a Snapshot to disk is to avoid locking out incoming NFS requests The problem is that new NFS requests may need to change cached data that is part of the Snapshot that must remain unchanged until it reaches disk An easy way to create a Snapshot would be to suspend NFS processing write the Snapshot and then resume NFS processing However writing a Snapshot can take more than a second which is too long for an NFS server to stop responding Remember that WAFL creates a consistency point Snapshot at least every 10 seconds so performance is critical The WAFL technique for keeping Snapshot data self consistent is to mark all the corrupted data in the cache as IN_SNAPSHOT The rule during Snapshot creation is that data marked IN_SNAPSHOT must not be modified and data not marked IN_SNAPSHOT must not be flushed to disk NFS requests can read all file system data and they can modify data that is not IN_SNAPSHOT but processing for requests that need to modify IN SNAPSHOT data must be deferred N series Snapshot A Technical Discussion 11 12 To avoid locking out NFS requests WAFL must flush IN_SNAPSHOT data as quickly as possible To do this WAFL performs the following steps 1 It allocates disk space for all files with IN_SNAPSHOT b
12. l date name 0 0 0 0 Jun 26 15 44 snapshot_vol0_06 25 2007 itsotuc3 gt Where vol_DominoDB This is the name of the volume on which we created the Snapshot N series Snapshot A Technical Discussion The team that wrote this Redpaper This Redpaper was produced by specialists working at the International Technical Support Organization San Jose Center Alex Osuna is a Project Leader at the International Technical Support Organization San Jose Center He has more than 28 years of experience in the IT industry and has spent 19 years specializing in the storage area Alex holds 10 certifications from IBM Microsoft and Red Hat Before joining the ITSO he worked as a System Engineer for Tivoli Patrick M rcius M dice Bisi is an IT Specialist at IBM Global Services in IBM Brazil He has more than 10 years of experience in the IT industry He holds a degree in Business Administration from SEDES UVV and holds several certifications from Microsoft including MCSE Messaging and MCSA Messaging His areas of expertise include Microsoft Windows operating system Microsoft Exchange servers and Active Directory directory services Sven Schaffranneck is an IT Project Leader at Netzlink Informationstechnik GmbH an IBM Business Partner from Germany He has more than 10 years of experience in several areas of the IT industry He is responsible for a high availability hosting environment including customer IT outsourcing Microsoft E
13. locks WAFL caches inode data in two places in a special cache of in core inodes and in disk buffers belonging to the inode file When it finishes write allocating a file WAFL copies the newly updated inode information from the inode cache into the appropriate inode file disk buffer and clears the IN_SNAPSHOT bit on the in core inode When this step is complete no inodes for regular files are marked IN_SNAPSHOT and most NFS operations can continue without blocking Fortunately this step can be done very quickly because it requires no disk 1 0 2 It updates the block map file For each block map entry WAFL copies the bit for the active file system to the bit for the new Snapshot 3 It writes all IN _SNAPSHOT disk buffers in cache to their newly allocated locations on disk As soon as a particular buffer is flushed WAFL restarts any NFS requests waiting to modify it 4 It duplicates the root inode to create an inode that represents the new Snapshot and turn the root inode s IN_SNAPSHOT bit off The new Snapshot inode must not reach disk until after all other blocks in the Snapshot have been written If this rule were not followed an unexpected system shutdown could leave the Snapshot in an inconsistent state After the new Snapshot inode has been written no more IN_SNAPSHOT data exists in cache and any NFS requests that are still suspended can be processed Under normal loads WAFL performs these four steps in less than a seco
14. nagement fet xi E File Action view Window Help le 8 x e m B2 m There are no items to show in this view Computer Management Local on System Tools Event Viewer Shared Folders Z Local Users and Groups Performance Logs and Alert 34 Device Manager Sa Storage Data ONTAP R DSM Manag SnapDrive Disks 9 VirtualDisk 2 0 1 1 Create Snapshot 9 VirtualDisk 3 Hp Removable Storage yieyy gt Disk Defragmenter New Window from Here Disk Management fe Services and Application Export List Help gt Creates a snapshot of a virtual disk Figure 12 Computer Management with SnapDrive MMC 4 Type in a name for the Snapshot then click OK see Figure 13 Enter a snapshot name snapthot_vol0_06_25_2007 Carcel Help Figure 13 Snapshot name 5 The Snapshot will be created and the information will be shown on the SnapDrive MMC detail window see Figure 14 on page 19 N series Snapshot A Technical Discussion Computer Management Ele Action yew Window Help 1a1 xl e AmE 2 mi Computer Management Local By System Tools 6 Event viewer C3 Shared Folders ro Local Users and Groups g Performance Logs and Alert Device Manager Storage Data ONTAPYR DSM Manag f SnapOrive B Disks
15. nd Step 1 can generally be done in just a few hundredths of a second and after WAFL completes it very few NFS operations need to be delayed Deleting a Snapshot Deleting a Snapshot is a trivial task WAFL simply zeros the root inode representing the Snapshot and clears the bit representing the Snapshot in each block map entry When creating Snapshots from LUNs the task can be accomplished by using SnapDrive software from the host and running the command from the Data ONTAP Command Line Interface CLI or running the command from the FilerView SnapDrive can be installed on Microsoft Windows servers and on UNIX servers such as IBM AIX HP UX Red Hat Linux and Solaris N series Snapshot A Technical Discussion Creating a Snapshot from the FilerView management interface FilerView is the Web management interface for the N series storage system To access FilerView using a Web browser go the URL http Hostname na_admin Follow these steps to create a Snapshot using the FilerView 1 On the FilerView window expand Volumes then expand Snapshots Click Manage to view the list of Snapshots that exist on the Filer see Figure 9 IBM System Storage N series FilerView About s E itsotuc4 Manage Snapshots e Filer 0O Volumes Snapshots Manage Volumes Add Manage Add Snapshot Restore E FlexClones View Volume All Volumes Qtrees 2 View Snapshots All Sna
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17. pshots 1 e Quotas Snapshots Volume Name Date Used Total Status Add Cl vao snapt Jun 20 14 53 46 95MB 81 15MB normal Configur O volo nightly 4 Jun 2400 00 8 895MB 34 2MB_ normal ge Ovo hourly S Jun 2408 00 692KB 25 3MB normal 3 Aggregates S OF von hourly 4 Jun 24 12 00 1 586MB 24 63MB normal 4 Figure 9 List of Snapshots 2 At the FilerView window click Add to add a new Snapshot as illustrated in Figure 10 on page 14 Select the Volume that you need to take the Snapshot provide a name for the Snapshot file and click Add N series Snapshot A Technical Discussion 13 a me IBM System Storage N series FilerView na_adminjimages dfm head jpg E itsotuc4 Filer gt Add Snapshot Volumes Snapshots gt Add e Volumes Add Manage Volume vlo Teri Select the volume for which the snapshot will be 2 added Only online volumes are displayed e FlexClones Oiroa 0 Snapshot Name SnapShot_Vol0_06 25 2 lt Enter the name of the new snapshot to be added e Quotas e Snapshots Add Configure Manage e Aggregates z Figure 10 Snapshot creation 3 A Success message is shown indicating that the Snapshot was successfully created see Figure 11 1 Success Created snapshot SnapShot _Vol0 906 25 2007 on volume volo Figure 11 Success creation message 14 N series Snapshot A Technical Discussion
18. s and cannot confirm the accuracy of performance compatibility or any other claims related to non IBM products Questions on the capabilities of non IBM products should be addressed to the suppliers of those products This information contains examples of data and reports used in daily business operations To illustrate them as completely as possible the examples include the names of individuals companies brands and products All of these names are fictitious and any similarity to the names and addresses used by an actual business enterprise is entirely coincidental COPYRIGHT LICENSE This information contains sample application programs in source language which illustrates programming techniques on various operating platforms You may copy modify and distribute these sample programs in any form without payment to IBM for the purposes of developing using marketing or distributing application programs conforming to the application programming interface for the operating platform for which the sample programs are written These examples have not been thoroughly tested under all conditions IBM therefore cannot guarantee or imply reliability serviceability or function of these programs You may copy modify and distribute these sample programs in any form without payment to IBM for the purposes of developing using marketing or distributing application programs conforming to IBM s application programming interfaces Copyright Interna
19. sk group db SdDg containing host volumes db SdHv filesystem notesdata db disk group log SdDg containing host volumes log_SdHv filesystem notesdata log root dominol Where snapname This is the short name of the Snapshot in our case it is snapl Note Unless you specify otherwise SnapDrive assumes that all entities that you specify on a given snap create command line are related that is the validity of updates to one entity may depend on updates to the other entities specified When storage entities have dependent writes in this way SnapDrive takes steps to create a Snapshot copy that is crash consistent for all storage entities as a group Example 2 illustrates the creation of a Snapshot from a normal LUN without using the LVM feature of Linux or UNIX Example 2 Creating a Snapshot from a single LUN root dominol snapdrive snap create lun itsotuc3 vol vol_DominoDB testlun Snapname snapshot_vol0_06_25 2007 Successfully created snapshot snapshot_vol0_06_25 2007 on itsotuc3 vol vol_DominoDB snapshot snapshot_vol0_06_25 2007 contains raw LUN itsotuc3 vol vol_DominoDB test1 un N series Snapshot A Technical Discussion root dominol Where lun This switch follows the long name of a LUN from which the Snapshot should be created snapname This is the short name of the Snapshot in our case it is snapshot_vol0 06 25 2007 2 Validate the created Snapshot Use the snapdrive snap lis
20. t command fora list of all Snapshots on the specified file system or volume See Example 3 Example 3 List all Snapshots root dominol snapdrive snap list vg db SdDg log _SdDg snap name host date snapped jtsotuc3 vol vol_DominoDB snapl1 dominol Jun 8 16 24 db SdDg log _SdDg jtsotuc3 vol vol_DominoLog snapl dominol Jun 8 16 24 db SdDg log SdDg root dominol Where vg This option means list all Snapshots of the specified volume groups Note As shown in Example 3 the Snapshot group is displayed at the end of every row At the time when the Snapshot is created SnapDrive stops all I O operations on db SdDg and log SdDg Creating a Snapshot from SnapDrive for Microsoft Windows SnapDrive for Microsoft Windows is an optional feature provided by N series for Microsoft Windows servers that can improve the management of storage resources from the host operating system After it is installed SnapDrive eases the creation and management of Snapshots from the host To create a Snapshot from SnapDrive for Microsoft Windows follow these steps 1 Open Computer Management SnapDrive will be available from the MMC 2 Expand SnapDrive expand Disks and expand the LUN that you wish to create a Snapshot of Note that despite the fact that you are selecting a LUN the actual Snapshot is volume based 3 Right click Snapshot and select Create Snapshot see Figure 12 N series Snapshot A Technical Discussion 17 18 im Computer Ma
21. ted it points to exactly the same disk blocks as the root inode so a brand new Snapshot consumes no disk space except for the Snapshot inode itself Column C in Figure 6 shows what happens when a user modifies data block D WAFL writes the new data to block D on disk and changes the active file system to point to the new block The Snapshot still references the original block D which is unmodified on disk Over time as files in the active file system are modified or deleted the Snapshot references more and more blocks that are no longer used in the active file system The rate at which files change determines how long Snapshots can be kept online before they consume an unacceptable amount of disk space WAFL s Snapshots duplicate the root inode instead of copying the entire inode file This reduces considerable disk I O and saves significant disk space By duplicating just the root inode WAFL creates Snapshots very quickly and with very little disk I O Snapshot performance is important because WAFL creates a Snapshot every few seconds to allow quick recovery after unclean system shutdowns N series Snapshot A Technical Discussion Figure 7 shows the transition from column A to column B in Figure 6 on page 8 in more detail When a disk block is modified and its contents are written to a new location the block s parent must be modified to reflect the new location The parent s parent in turn must also be written to a new location
22. tional Business Machines Corporation 2007 All rights reserved Note to U S Government Users Restricted Rights Use duplication or disclosure restricted by GSA ADP Schedule Contract with IBM Corp 23 TL Send us your comments in one of the following ways gt Use the online Contact us review redbook form found at ibm com redbooks gt Send your comments in an email to redbook us ibm com gt Mail your comments to IBM Corporation International Technical Support Organization Dept HYTD Mail Station P099 2455 South Road Poughkeepsie NY 12601 5400 U S A Trademarks The following terms are trademarks of the International Business Machines Corporation in the United States other countries or both AIX Lotus System Storage Domino Redbooks Tivoli IBM Redbooks logo The following terms are trademarks of other companies Snapshot WAFL SnapDrive FilerView Data ONTAP and the Network Appliance logo are trademarks or registered trademarks of Network Appliance Inc in the U S and other countries Snapshot Data ONTAP WAFL and NetApp logo are trademarks or registered trademarks of NetApp Corporation or its subsidiaries in the United States other countries or both Solaris and all Java based trademarks are trademarks of Sun Microsystems Inc in the United States other countries or both Active Directory Microsoft Windows and the Windows logo are trademarks of Microsoft Corporation in th
23. xchange and RIM BES infrastructure Sven s current focus is on presales and postsales work on Microsoft Windows Active Directory Exchange and IBM system storage installations Sven holds a Dipl Ing degree in Computer Sciences from the University of Applied Science in Braunschweig Wolfenbittel Germany N series Snapshot A Technical Discussion 21 22 N series Snapshot A Technical Discussion Notices This information was developed for products and services offered in the U S A IBM may not offer the products services or features discussed in this document in other countries Consult your local IBM representative for information on the products and services currently available in your area Any reference to an IBM product program or service is not intended to state or imply that only that IBM product program or service may be used Any functionally equivalent product program or service that does not infringe any IBM intellectual property right may be used instead However it is the user s responsibility to evaluate and verify the operation of any non IBM product program or service IBM may have patents or pending patent applications covering subject matter described in this document The furnishing of this document does not give you any license to these patents You can send license inquiries in writing to IBM Director of Licensing IBM Corporation North Castle Drive Armonk NY 10504 1785 U S A The following paragraph does
24. ystem changes Snapshots use no additional disk space when first taken Snapshots can be taken manually or automatically on a schedule Block 3 Figure 1 Snapshot features Snapshots use a technique to avoid duplicating disk blocks that are the same in a Snapshot as in the active file system Only when blocks in the active file system are modified or removed do Snapshots containing those blocks begin to consume disk space Users can access Snapshots through NFS to recover files that they have accidentally changed or removed and system administrators can use Snapshots to create backups safely from a running system In addition WAFL uses Snapshots internally so that it can restart quickly even after an unclean system shutdown High level Snapshot process In this section we provide an overview of the Snapshot process 1 Snapshots are taken from active data on the file system as shown in Figure 2 on page 4 N series Snapshot A Technical Discussion 3 How Snapshot works 1 SE e SN Active Data File LUN X fa le Le Figure 2 Step 1 Initial Snapshot is taken on active data 2 When an initial Snapshot is taken no initial data is copied Instead pointers are created to the original blocks for recording the point in time state of these blocks as shown in Figure 3 on page 5 4 N series Snapshot A Technical Discussion How Snapshot works 2 Active Data
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