Dell PowerVault 720N, Справочник по командам, Страница: 267 / 606

Snapshots 9-15
system before the filer took the
hourly.0
snapshot, the
snap list
command would
have displayed the following output:
%/used %/total date name
-------- --------- ------------ ---------
0% (0%) 0% (0%) May 05 16:00 hourly.0
20% (20%) 1% (1%) May 05 12:00 hourly.1
20% (20%) 1% (1%) May 05 08:00 hourly.2
In the %/used column, the cumulative values for
hourly.1
and
hourly.2
are both 20%,
but the cumulative value for
hourly.2
is not 40%. This is because both snapshots
point to the same 20 MB of data, the data that you just deleted.
The cumulative values for
hourly.1
and
hourly.2
are different if you delete and create
data between snapshots in the following way:
1. Delete 20 MB of data and create 20 MB of new data after
hourly.2
.
2. Delete the 20 MB of data created in Step 1 after
hourly.1
.
After the data deletions and additions, the
snap list
command displays the follow-
ing output:
%/used %/total date name
-------- --------- ------------ ---------
0% (0%) 0% (0%) May 05 16:00 hourly.0
20% (20%) 1% (1%) May 05 12:00 hourly.1
33% (20%) 2% (1%) May 05 08:00 hourly.2
In this scenario,
hourly.1
and
hourly.2
each consume 20% of the used disk space:
20 MB out of 100 MB. However, this time they reference different data blocks. Cumu-
latively, they consume 40 MB, which is about 33% of the disk space used: 120 MB,
which is 40 MB used by snapshots plus 80 MB in the file system.
Before trying to conserve space by deleting a large snapshot file, examine the cumu-
lative values in the
snap list
output. If two adjacent snapshot files show little
difference in the cumulative values, most of the data referenced by the snapshots is
the same. In this case, removing one of the snapshots doesn’t free much disk space.
If you find snapshots confusing and hard to manage, use the default snapshot sched-
ule and the default snapshot reserve because these settings are appropriate for most
environments. When you create a new volume on a filer, remember that the new vol-
ume inherits the snapshot schedule from the root volume. After you use the volume
for a while, check how much disk space the snapshots consume in the volume. If the
disk space seems high, decrease the amount of time that snapshots are kept or
increase the snapshot reserve.
As you use snapshots, continue to watch the statistics change over time. The statis-
tics help you gain a better understanding of how snapshots work.