Avamar: Backups and sparse files
Summary: This article discusses the behavior of Avamar where a client is backing up a file system which contains sparse files.
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Symptoms
Issue 1: A backup could run slowly if it includes a sparse file which has been modified:
UNIX and Linux file system support for sparse file detection and management is file-system-dependent.
When a sparse file is modified, Avamar's avtar process reads the file as presented by the file system.
Avtar performs sticky byte factoring on the entire file. This means, it breaks the file into chunks, compress those chunks, and create a hash of each chunk. For a large file, this may take a significant length of time.
This scenario can be detected by reviewing the avtar status messages in the backup log.
For further details, see: How to analyse Avamar client backup performance and identify performance bottlenecks.
Issue 2: Performing a restore of a sparse file may result in the restored file having a different size:
Although avtar does not detect 'sparse files', it detects regions of sparseness within a file and attempt to handle them efficiently.
The logic which handles the detection of sparseness is not 100% deterministic. It relies on a heuristic that looks for large regions of consecutive zeroes. This is described in Engineering Esczilla record 24164.
It is expected that the space or number of blocks consumed by the restored file, may not be identical as for the original file.
UNIX and Linux file system support for sparse file detection and management is file-system-dependent.
When a sparse file is modified, Avamar's avtar process reads the file as presented by the file system.
Avtar performs sticky byte factoring on the entire file. This means, it breaks the file into chunks, compress those chunks, and create a hash of each chunk. For a large file, this may take a significant length of time.
This scenario can be detected by reviewing the avtar status messages in the backup log.
For further details, see: How to analyse Avamar client backup performance and identify performance bottlenecks.
Issue 2: Performing a restore of a sparse file may result in the restored file having a different size:
Although avtar does not detect 'sparse files', it detects regions of sparseness within a file and attempt to handle them efficiently.
The logic which handles the detection of sparseness is not 100% deterministic. It relies on a heuristic that looks for large regions of consecutive zeroes. This is described in Engineering Esczilla record 24164.
It is expected that the space or number of blocks consumed by the restored file, may not be identical as for the original file.
Cause
One or more of the files backed up by an Avamar client are "sparse files".
Resolution
Current releases of Avamar do not detect for or handle file sparseness during backups.
Workaround for Issue 1:
Use a separate dataset to back up sparse files. This helps avoid issues with reliability and long run times for the regular files when sparse files are modified.
If possible, exclude sparse files from the backup.
An enhancement request has been made to include this functionality in a future version of Avamar.
Workaround for Issue 1:
Use a separate dataset to back up sparse files. This helps avoid issues with reliability and long run times for the regular files when sparse files are modified.
If possible, exclude sparse files from the backup.
Issue 2:
No resolution applicable. This is expected behaviour.
Additional Information
According to Wikipedia: In computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when blocks allocated to the file are mostly empty.
The default ls command will not help identify a sparse file.
The default ls command will not help identify a sparse file.
ls -lh lastlog
-rw-r--r-- 1 root tty 1.2T 2013-01-14 03:03 lastlog
Unless the -s flag is added.
admin@client:/var/log/>: ls -slh sparsefile
36K -rw-r--r-- 1 root tty 1.2T 2013-01-14 03:03 sparsefile
Επηρεαζόμενα προϊόντα
AvamarΠροϊόντα
AvamarΙδιότητες άρθρου
Article Number: 000038476
Article Type: Solution
Τελευταία τροποποίηση: 06 Φεβ 2025
Version: 5
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