RackCorp Cloud Blog

JUNE 17TH, 2015
By ADMIN

Virtual Server Image Backups – Performance Improvement

Tonight we’ve deployed the latest version of our Virtual Server backup manager. This software is responsible for:
1) Creating LVM snapshots of Virtual Servers
2) Reading entire snapshots, 1MB at a time and generating data hashes
3) Comparing those data hashes to our backup repositories (usually in off-site datacenters)
4) Copying any changed blocks to our backup repositories

Traditionally, most of our customers run on small VMs (20GB-30GB), so the above process has worked quite well albeit not optimal. This year we’ve seen a significant uptake of large 200GB+ VM’s which has made us re-evaluate our backup manager. Tonight’s update is a significant one in terms of performance:

Change 20150516-001: Backup Repository no longer re-calculates hashes during a backup. They’re calculated once when the backup is first written, used for comparison purposes, and updated as new blocks come in.
Change 20150601-001: snapshot blkio limiting. Previously, backup processes were allowed to utilise full sequential read speeds due to oversight in the blkio settings. This has now been remedied, with sequential reads now limited to 60% of the virtual server’s allocated device speed.
Change 20150601-002: posix_fadvise(POSIX_FADV_SEQUENTIAL) utilisation. Added libc binding to make a call to posix_fadvise to set the POSIX_FADV_SEQUENTIAL flag. This should mean the Linux Disk Cache doesn’t get blown away during backups.

Coming up in the future will be the much requested feature addition to allow file-level mounts of backups!