How to estimate time and disk space requirements for collecting a metasave

Problem

How to estimate time and disk space requirements for collecting a metasave

Solution

Table of Contents

Introduction
Size estimate
Time estimate

Introduction

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The metasave process reads through the metadata of a VxFS file system, processes through each individual inode entry. The amount of time that is required to create a metasave depends on how quickly the system can perform I/O reads, along with the overall level of system resources available (e.g. physical memory, cpu speed, etc.)

It is possible to estimate the time and size requrements for a metasave by using the "fsadm -DE" command. This utility reads the same metadata as those read by the metasave process. As a result, the time required to run both utilities should be similar.

Note: Significant levels of file system corruption may affect time and size estimates. For example, if the metadata contains inode that incorrectly references an extent that is impossibly large (as a result of corruption) the metasave could be significantly larger than the estimate. Either monitor the metasave output, as it takes place, or direct the output file to a non-critical volume to avoid the possibility of filling up a critical volume.

Size estimate

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Basically the size of the metasave output will the sum of following.

1. The sum of the sizes of all structural inodes in fileset 1
2. Directory blocks in all filesets (file system checkpoint)
3. Indirect blocks used in the extent maps of all the files and directories in all filesets

Item 1 can be obtained by running the shell script "metasize" in the attachment section of this acticle. Basically, the "metasize" reads all the inodes in the fileset 1, eliminates the redundant copies (matchino) and sums up the blocks used by each inode.  

Figure 1

Items 2 and 3 can be obtained from the 'fsadm -DE" output.
 

Figure 2 - Using /opt/VRTS/bin/fsadm with the native time command

• The sum of the sizes of all structural inodes in fileset 1 = 434.0 MB
• Directory blocks in all filesets (Directory Total Blocks) = 24.6 MB
• Indirect blocks used in the extent maps = 43.0 MB
• The estimated size of the metasave output will be 434.0 + 24.6 + 43.0 = 501.6 MB

Time estimate

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Use the native command "time" with "fsadm -dE" to emulate the process of a metasave.

For the above example in Figure 2, one can estimate that the metasave process will take approximately 51 seconds.

Figure 3

For real time monitoring of the metasave process, we can divide the estimated size of the metasave output by the I/O rate of the volume. For example, the following vxstat command will display the number of volume blocks (512 bytes) read per 5 seconds. By dividing the number of blocks read by 2 (blocks per KB), it gives the number of KB read per 5 seconds.  Further dividing the previous result by 5 seconds will give us the read rate per second. For example, 63616 means 6361 KB/s or 6.3 MB/s.

Figure 4

Now, divide the estimated size of the metasave output by 6 MB per second. It gives us an estimated run time of the metasave process. Continuing with the above example:

Figure 5