Date: Sun, 7 Sep 2008 13:30:45 +0200 From: Stefan `Sec` Zehl <sec+freebsdstable@42.org> To: freebsd-stable@freebsd.org Subject: snapshots and disk usage Message-ID: <20080907113045.GA11293@ice.42.org>
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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
--
Consider the need for having to type "www.domain.name" a little IQ test
that you have to take before you can access my web site.' -- Wietse
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