Direct access restore [DAR] is the ability to restore specific files without having to go through the ENTIRE TAPE. DAR provides the EXACT offset at restore time to the backup application. With this information, the backup application is able to JUMP directly to the DESIRED OFFSET and recover the file without having to read the ENTIRE backup IMAGE sequentially.
DAR requires:
1. NDMP version 3 or later.
2. File History [HIST environment variable) to be enabled.
Note: "File History" is the term used for an INDEX of files that has been backed up. An NDMP client [Backup Application] may request FILE HISTORY from the NDMP Data Service at the time of initiating the backup. If the file history is requested, the backup engine sends information like -File name and path, file status information and file positioning information (i.e. the address of the file in the backup data stream).
It is important to note that the "File History" and "DAR" are not the same, and this is b'cos you can choose to restore a file without the 'File Positioning Information', and this will be called Non-DAR restore, in other words, you have an ability to browse the folders/files and then select the specific file(s) to restore, but a large portion of the data from the backup that included the file must be read, could well be the entire tape.
Whereas, in DAR : Only the portion of the TAPE which contains the data to be restored is read.
Hence, they both are two different things.
This further means that the offset map [DAR] file generation can result in a significant impact to performance if a large number of small files are backed up. In such a case, you may want to disable the offset map generation WITHOUT disabling the FILE HISTORY and there by increasing the performance and yet have the ability to browse folders/files.
So, Keep FILE HIST=T, and disable DAR [offset_map]
To do so, on the NetApp console, use the following options command.
7-mode: [Disable offset_map]
7-mode> options ndmpd.offset_map.enable off
ONTAP [cmode] - [Disable offset_map]
::>vserver services ndmp modify -vserver vservername -offset-map-enable [true|false]
Directory DAR:
Is the ability to provide a directory name to the backup application such that the backup software EXPANDS the directory contents and recovers each FILE using the DAR process.
To be able to implement Directory DAR, ONTAP needs to record the offset of each file on the backup image. Thus, on restore time the restore application can EXPAND the directory to be recovered, and then LOAD the information about the offsets, and do DAR for all files underneath the specified DIRECTORY.
Enhanced DAR: Is nothing but combination of FILE HISTORY & OFFSET map both enabled .Thus to benefit from the Enhanced DAR feature, HIST & OFFSET must be enabled.
DAR requires:
1. NDMP version 3 or later.
2. File History [HIST environment variable) to be enabled.
Note: "File History" is the term used for an INDEX of files that has been backed up. An NDMP client [Backup Application] may request FILE HISTORY from the NDMP Data Service at the time of initiating the backup. If the file history is requested, the backup engine sends information like -File name and path, file status information and file positioning information (i.e. the address of the file in the backup data stream).
It is important to note that the "File History" and "DAR" are not the same, and this is b'cos you can choose to restore a file without the 'File Positioning Information', and this will be called Non-DAR restore, in other words, you have an ability to browse the folders/files and then select the specific file(s) to restore, but a large portion of the data from the backup that included the file must be read, could well be the entire tape.
Whereas, in DAR : Only the portion of the TAPE which contains the data to be restored is read.
Hence, they both are two different things.
This further means that the offset map [DAR] file generation can result in a significant impact to performance if a large number of small files are backed up. In such a case, you may want to disable the offset map generation WITHOUT disabling the FILE HISTORY and there by increasing the performance and yet have the ability to browse folders/files.
So, Keep FILE HIST=T, and disable DAR [offset_map]
To do so, on the NetApp console, use the following options command.
7-mode: [Disable offset_map]
7-mode> options ndmpd.offset_map.enable off
ONTAP [cmode] - [Disable offset_map]
::>vserver services ndmp modify -vserver vservername -offset-map-enable [true|false]
Directory DAR:
Is the ability to provide a directory name to the backup application such that the backup software EXPANDS the directory contents and recovers each FILE using the DAR process.
To be able to implement Directory DAR, ONTAP needs to record the offset of each file on the backup image. Thus, on restore time the restore application can EXPAND the directory to be recovered, and then LOAD the information about the offsets, and do DAR for all files underneath the specified DIRECTORY.
Enhanced DAR: Is nothing but combination of FILE HISTORY & OFFSET map both enabled .Thus to benefit from the Enhanced DAR feature, HIST & OFFSET must be enabled.
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