Understanding ‘Total inlined data savings’ When Using ’isi_cstats’
Thu, 12 May 2022 14:22:45 -0000
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Recently a customer contacted us to tell us that he thought that there was an error in the output of the OneFS CLI command ‘isi_cstats’. Starting with OneFS 9.3, the ‘isi_cstats’ command includes the accounted number of inlined files within /ifs. It also contains a statistic called “Total inlined data savings”.
This customer expected that the ‘Total inlined data savings’ number was simply ‘Total inlined files’ multiplied by 8KB. The reason he thought this number was wrong was that this number does not consider the protection level.
In OneFS, for the 2d:1n protection level, each file smaller than 128KB is stored as 3X mirrors. Take the screenshot below as an example.
If we do some calculation here,
379,948,336 * 8KB = 3,039,586,688KiB = 2898.78GiB
we can see that the 2,899GiB from the command output is calculated as one block per inlined file. So, in our example, the customer would think that ‘Total inlined data savings’ should report 2898.78 GiB * 3, because of the 2d:1n protection level.
Well, this statistic is not the actual savings, it is really the logical on-disk cost for all inlined files. We can't accurately report the physical savings because it depends on the non-inlined protection overhead, which can vary. For example:
- If the protection level is 2d:1n, without the data inlining in 8KB inode feature, each of the inlined files would cost 8KB * 3.
- If the protection level is 3d:1n1d, it will become 8KB * 4.
One more thing to consider, if a file is smaller than 8KB after compression, it will be inlined into an inode as well. Therefore, this statistic doesn't represent logical savings either, because it doesn't take compression into account. To report the logical savings, total logical size for all inlined files should be tracked.
To avoid any confusion, we plan to rename this statistic to “Total inline data” in the next version of OneFS. We also plan to show more useful information about total logical data of inlined files, in addition to “Total inline data”.
For more information about the reporting of data reduction features, see the white paper PowerScale OneFS: Data Reduction and Storage Efficiency on the Info Hub.
Author: Yunlong Zhang, Principal Engineering Technologist