2009-07-29 10:40 PM
We are implementing SMVI left and right and would like to go from 7 daily snaps and add 3 weeklies. Our aggr. are mainly 70% to 90% full and
I d like to avoid a scenario where we add weekly snaps and we enter 90%+.
Given that I ve got 7 daily snaps and I can see how much space they use is there a good/solid way of calculating how much space
1 weekly will take?
7 per volume
Total Snap Consumed GB
In the numbers above you can see that .snapshot across X volumes in aggr03 consumes close to 2TB. Can I deduct from this that any of the 3 weeklies will
consume 2TB on top of what is used here? I dont think its that simple as there will be common blocks etc. but right now I cant get my head around it.
Thanks in advance for any help!
2009-07-30 03:31 AM
It is not about how many snapshots you keep, but for how long you keep them & what's your data change rate (only unique blocks are preserved across a number of snapshots of the same volume).
So, say your oldest snapshot now is 1 week old & all snapshots consume 2TB of space. If you change retention period to 3 weeks, then you will need (roughly) 6TB space for snapshots (or 4TB on top of what already has been consumed).
This assumes your data change rate is fairly steady & does not change over time.
2009-07-30 04:45 PM
Thanks for your answer. I have heard something similar:
After day 7 when I take my first weekly.1 I will only add to my weekly snapshot new blocks occuring between daily.0 and weekly.0
snapshots listed chronologically where snapshot names are incremental, 0 (zero) is the most recent one:
So in week 2 when I take my weekly.2 the delta in blocks will be between the daily.0 from week 2 and weekly.1
Given that all blocks from daily.0-daily.6 will be kept in weekly.1 you are right. That is a lot of space used to have 1 months work
for the record: what we are snapping here has NOT been designed for snapshots. there is a lot of transient data, sql dumps etc etc. that should be segrated
at volume level.
2009-07-31 04:34 AM
"for the record: what we are snapping here has NOT been designed for snapshots. there is a lot of transient data, sql dumps etc etc. "
That suggests to me that you might well have some data perhaps created on Tuesday then deleted on Friday. That kind of data will (hopefully obviously!) not be picked up by a weekly snapshot taken on the Saturday, so although your latest week keeps almost 2TB in snapshot space, subsequent ones *could* be quite a bit less.
As a broad rule of thumb, I would typically suggest a weekly snapshot might take 4-5x the daily snapshot amount....but if data is very transient then I might expect it to be closer to 1-2x a daily.