Slack space required for fulls, active or synthetic

[mrt]

regarding slack space required for full backups, active or synthetic, when doing forward incrementals

Am I right in assuming that at least 3x the initial vbk size needs to be available on the volume in order for either type of full backup to occur?
I.e. you’d need space for the oldest vbk, some vibs, the newer vbk, some more vibs, and then space to build the newest vbk, then when that’s done, the old vbk’s/vib’s get purged. So for a moment, space for 3x vbk is needed. Is this right?

[foggy]

Space required depends on retention settings for the given job. Please review this topic for explanation on how retention works in case of forward incremental mode. Feel free to ask if you need any additional clarification. Thanks.

[mrt]

i understand the presence of the vibs and vbk's, i just need a little clarity on how the newest full is created. if it is a new file altogether, then i think the 3x logic is sound, but if you guys somehow just inject changes into the previous vbk, then 3x the size of vbk might not be needed. thanks

[foggy]

With forward incremental backup mode, the new full backup is always an entirely new VBK file. I'm not quite understand the logic under 3x amount of space as the space required for the new full to be created at any moment in time is typically similar to the size of the previous existing full file. A previous full backup files remain on disk until automatically deleted according to the retention policy (basically, depending on the retention, you might need to store from 2 to whatever number of fulls on the backup storage).

[mrt]

right, space for two fulls on disk (7 point retention, daily inc backups, once weekly synthetic full), and then space for the new full = 3x the vbk storage for the moments before the oldest vbk gets purged....right?

[foggy]

If you have daily backups (including weekend) with 7 points retention, the first VBK will be removed along with its subsequent increments right after the 7th restore point in the next week's backup chain is created. So at the time of creation the next VBK, you will have only one VBK file on the repository. If weekend is excluded, then you will need space for a third one, indeed.