Direct APIs for Changed Block Tracking Between Two Backups

Using SCSI GET LBA STATUS

You can send SCSI GET LBA STATUS commands to both boot volumes and block volumes to find and get the changed blocks on the volume for the specified starting logical block address (LBA) and the length to scan to identify the changed blocks (scan length). The time it takes to return provisioning status of the blocks on a device depends on the scan length provided in the command, and the performance level configured for the volume.

You can build and install sg3_utils from sg3_utils-1.48. See also Manpage.

Limitations and Considerations for Using LBA Status

• The 32-byte command for Linux is the only supported version. The 16-byte command for Linux isn't supported.
• No commands are supported for Windows.
• Only iSCSI-attached volumes are supported.
• Recommended scan length is up to 1 GB worth of logical blocks. See the example in the next section for scan length (-s) usage. Scan length greater than 1 GB isn't supported and the output is undefined. If the scan length is specified as 0 or omitted, the behavior is undefined.
• The default allocation length is 24 bytes, for 1 descriptor only.
• If report type is 0 (all), provisioning statuses for uninitialized blocks are also included in the result as deallocated blocks leading to is no distinction between uninitialized and explicitly deallocated blocks.

Example

The details in this section provide an example of running the commands on a Linux instance to get the changed blocks.

To only get provisioning status about mapped and deallocated blocks, and not uninitialized blocks, use report type 1 to specify a non-zero provisioning status.

sudo sg_get_lba_status -b -T -l 96 -m 32768 -s 2097152 -t 1 /dev/sdc

In the preceding example, the logical block size is 512 bytes and the block device is /dev/sdc. This example uses the 32-byte command (-T) and scans 1 GB (or 2097152 contiguous logical blocks), starting from lba 96 with an allocation length of 32768 bytes. The specified report type is 1. Specifying the -b option prints the output in brief.

Understanding Output

The following is example output from the running the previous command example:

Completion condition=3
RTP=1
0x0000000000000060  0x10  3  0
0x0000000000000070  0x8  1  0
0x0000000000000078  0x10  3  0
0x0000000000000088  0x8  1  0
0x0000000000000090  0x10  3  0
0x00000000000000a0  0xfff60  1  0

The preceding output contains six descriptors, each with a starting lba, count, and provisioning status of that many contiguous blocks.

For example, the first descriptor's starting lba is 96, count is 16, and provisioning status is 3, which means blocks from 96 to 112 are mapped.

RTP=1 means the report type sent in the command was considered in processing of the Get Lba Status command.

The four completion condition types in the example output are:

• Completion condition = 0: the device server didn't return any output.
• Completion condition = 1: the device server stopped processing as the allocation length was met, and the next command should use starting lba after the last lba in the output.
• Completion condition = 2: the device server scanned the entire scan length and returned the output. Therefore, the next command can use starting lba after the scan length sent before.
• Completion condition = 3: the device server reached the end of the volume, and no more commands should be sent after the last lba in the output.