I was looking at my 630 trying to figure out if it was safe to add another nightly snapshot. According to the Sys-Admin manual on page 10-13 it shows an example where a snap reserve is set to 20% and a 'snap list' shows only 10% utilization. The suggestion is to drop the reserve from 20% down to 12 or 15% assuming usage patterns remain constant.
Great, this make sense to me. However, when I look at my system I have:
bigdisk> snap reserve Current snapshot reserve is 10% or 18976480 k-bytes.
bigdisk> snap list working...................................................... %/used %/total date name ---------- ---------- ------------ -------- 0% ( 0%) 0% ( 0%) Feb 18 12:01 hourly.0 1% ( 0%) 0% ( 0%) Feb 18 08:01 hourly.1 2% ( 1%) 1% ( 0%) Feb 18 00:01 nightly.0 2% ( 1%) 1% ( 0%) Feb 17 20:01 hourly.2 3% ( 1%) 1% ( 0%) Feb 17 16:01 hourly.3 3% ( 1%) 1% ( 0%) Feb 17 12:01 hourly.4 5% ( 3%) 2% ( 1%) Feb 17 08:01 hourly.5 7% ( 4%) 3% ( 1%) Feb 17 00:01 nightly.1
So, so far it would indicate that my 10% reserve is probably a pretty good number for the current setup and if I want to add another nightly snapshot I should probably bump my reserve up a couple of percent to be safe.
However, then I do a df:
bigdisk> df Filesystem kbytes used avail capacity Mounted on / 170788336 71454936 99333400 42% / /.snapshot 18976480 5180276 13796204 27% /.snapshot
I was expecting to see 70% utilization on my .snapshot file system here. Why is it only 27%? From this output I can only conclude that I have a lot of room and I don't need to bump up my reserve.
I must have missed something along the way.
-Rasmus
bigdisk> snap reserve Current snapshot reserve is 10% or 18976480 k-bytes.
...
bigdisk> snap list working...................................................... %/used %/total date name
...
7% ( 4%) 3% ( 1%) Feb 17 00:01 nightly.1
...
bigdisk> df Filesystem kbytes used avail capacity Mounted on / 170788336 71454936 99333400 42% / /.snapshot 18976480 5180276 13796204 27% /.snapshot
The question is:
Snap list seems to be saying that snapshots consume 7% of a total 10% in my snapshot reserve, for a total of 70% of reserve consumed.
Yet "df" says that I'm only consuming 27% of the snapshot reserve.
What gives?
What gives is that the first column (%/used) measures the space consumed by snapshots as a percentage of space USED, rather than as a percentage of the TOTAL available.
Thus, the first column is a good estimate of what snapshots would be consuming if the filesystem were full.
The second column (%/total) measures the space consumed by snapshots as a percentage of the TOTAL space in the filesystem. Since your filesystem is about half full, you would expect this to be about half of the first column.
In summary, your first reaction was correct. To safely handle another nightly snapshot, you may need to increase the snapshot reserve just a bit. Since the FS is only half full, you have plenty of spare reserve at the moment, but if the FS were full, you might not.
It's possible, depending on the nature of the data you are storing, that the rate of change won't increase as you store more data. In this case, the %/total column would be a better reflection of utilization patterns even when the FS is full. On the other hand, if you are adding additional users over time, and you expect them to behave about like the existing ones, then %/used is probably a safer measure.
Dave
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hitz＠netapp.com -
rasmus＠bellglobal.com -
rasmus＠lerdorf.on.ca