snapshots and disk usage

[Stefan `Sec` Zehl]

Hi,

I am using ufs snapshots on RELENG_7 for some time now, and am generally
happy with it. I have noticed a strange behaviour when removing large
amount of files, and wanted to ask if this is expected.

Before starting, we check the free space on /usr:

| ice:/usr>df -h .
| Filesystem         Size    Used   Avail Capacity  Mounted on
| /dev/ad4s2.elid    9.7G    7.6G    1.3G    64%    /usr

Then delete /usr/obj and run df again:

| ice:/usr>sudo rm -rf obj 2>/dev/null
| ice:/usr>df -h .
| Filesystem         Size    Used   Avail Capacity  Mounted on
| /dev/ad4s2.elid    9.7G    5.7G    3.2G    64%    /usr

This is unexpected. With snapshots, removing something should not
release space.

Sure enough, in the course of the next minute, the fake free space
vanishes....

| ice:/usr>df -h .
| Filesystem         Size    Used   Avail Capacity  Mounted on
| /dev/ad4s2.elid    9.7G    5.9G    3.0G    66%    /usr
| ice:/usr>df -h .
| Filesystem         Size    Used   Avail Capacity  Mounted on
| /dev/ad4s2.elid    9.7G    6.6G    2.3G    74%    /usr
| ice:/usr>df -h .
| Filesystem         Size    Used   Avail Capacity  Mounted on
| /dev/ad4s2.elid    9.7G    8.6G    269M    97%    /usr

and all the free space is allocated in the snapshot:

| ice:~>sudo snapshot list
| Filesystem          User   User%     Snap   Snap%  Snapshot
| /usr                 8GB   89.3%      2GB   21.5%  daily.1
| /usr                 8GB   89.3%    344MB    3.5%  daily.0
| /usr                 8GB   89.3%    344MB    3.5%  weekly.0
| /usr                 8GB   89.3%    344MB    3.5%  hourly.1
| /usr                 8GB   89.3%      7MB    0.1%  hourly.0

My understanding so far was that df may underreport free space, but i
find overreporting it a bit troublesome. -- What would happen if I tried
to use that space before it was allocated to the snapshot?

Case in point: I created a few unkillable hung process on /usr a few
weeks ago, by running make world which was running out of diskspace, and
deleting files from another windows. -- At leat thats what I think has
happened. 

CU,
    Sec
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