Introduction to the principles of Vnx and Vnxe snapsure

The article is reproduced from the EMC Chinese Support forum.

EMC Snapsure is a software feature for creating and managing file snapshots where snapshots are not a copy or mirror image of the original file system, and the snapshot "filesystem" is just a logical image of the point-in-time of the production file system at a specific time, the file system is a read-only view of the production file system before a change occurs at a specific time.

Solution:

Brief introduction

Both VNX and Vnxe have snapsure snapshot capabilities, which provide users with multiple point-in-time views of their data. For example, as shown in the following illustration, the user's real-time production data is my_file, and if the user needs to access the version of the file a few days before, you can take a snapshot of the production data through the Snapsure feature, making it easy to access the read-only version of the file (shown as a different time version

Introduction to Terminology

Pfs:nas file system, PFS must be loaded, and PFS is typically exported for read and write access by the client.

Snapshot: The logical point-in-time view of PFS. VNX SnapSure uses a combination of real-time PFS data and stored data to display the file system at a specific point in time.

SavVol: Each PFS with snapshots has a related storage volume (that is, SavVol). The initial change to each PFS block after the snapshot triggers the VNX SnapSure to copy the block of data to SavVol. It also saves changes made to writable snapshots.

Bitmap: VNX SnapSure creates and maintains a bitmap that retains 1-bit space for each block of data in PFS. The bit value is initialized to zero, indicating that the value of the corresponding PFS block has not changed since the checkpoint was established. After the first write to a block of data, the bit value is changed to 1.

Block diagram: A block diagram that maintains SavVol to record the address of each saved block of data in SavVol.

Base snapshot: Read a snapshot from which you can create a writable snapshot.

Case Analysis

This case will analyze the Snapsure snapshot creation principle and how to capture data from file system modifications and how to provide data to users and applications.

Create a snapshot: The case shows PFS with a block of data (including letters A through F). After the first system snapshot was created, a savvol is also created on the disk to store the bitmap, the raw data from PFS, and the block diagram for that particular snapshot. Each single-digit value of a bitmap refers to a block of data on the PFS.

Write PFS with snapshots: The user or application modifies the PFS. In this case, write "H" in the position of "B" and write "K" in the position "E". Before these writes are made, SnapSure will retain the I/O and copy "B" and "E" to SavVol. The block diagram is then updated based on the position of the data in the Savvol. In this example, the first column in the Block diagram is the data block address in PFS, and the left column is the address of the block in SavVol. After that, the bitmap is updated with "1" regardless of where the data block changes in PFS. "0" indicates that the data block has not been updated.

After all this process occurs, SnapSure will release the reservation and the write can be established. If the same two blocks of data have been modified again, no content will be saved in the SavVol after the write is completed. The first write-time copy principle results in this, and the original data for that point in time has been stored, and any subsequent changes are independent of snapshot 1.