From: Michael DiDomenico <mdidomenico4@gmail.com>
To: dm-devel@lists.linux.dev
Subject: delete on snapshot does not free space
Date: Tue, 30 Apr 2024 13:54:39 -0400 [thread overview]
Message-ID: <CABOsP2PL9AM5phv4ZhucGmpk6V1x+DozXLaPTBVaWqMdy-KdPA@mail.gmail.com> (raw)
this is probably expected behavior, but i'd like to confirm as i'm not
a device-mapper expert
we're using COW snapshots overtop of a readonly block device. this
seems to be working fine, however, when a person deletes files the
blocks are not freed. case in point a RO block device with a COW
filled up to 100% as shown in df. when i deleted some files to free
space the usage never decreased
to create the mount we do
echo "0 <realsize> linear /dev/loop1 0" | dmsetup create roimage-base-real
echo "0 <cowsize> linear /dev/loop2 0" | dmsetup create roimage-snap-cow
echo "0 <realsize> snapshot-origin /dev/mapper/roimage-base-real" |
dmsetup create roimage-base
echo "0 <realsize> snapshot /dev/mapper/roimage-base-real
/dev/mapper/roimage-snap-cow n 1" | dmsetup create roimage-cow
mount /dev/mapper/roimage-cow /roimagemntpnt
this is working as the origin device is Read-Only, but we can change files.
so two questions come up
1 when we delete files, how do we reclaim the space
2 should/can we roll the snapshots to reclaim space?
- if i wanted to create a new snapshot and discard the existing one,
do i basically just to the fourth step again to a different device? i
haven't found a "clear" how-to on this. most people just stop after
the fourth line and then tear it all down
ideally what i'm looking to do is create a filesystem where the base
is read-only, but we can make changes. i'm okay with all those
changes getting lost after a reboot/unmount/etc. the issue right now
is when df shows 100% usage it seems the only recourse is reboot(aka
clear it all out) and restart. i'm hoping there's something more
graceful
next reply other threads:[~2024-04-30 17:54 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-30 17:54 Michael DiDomenico [this message]
2024-04-30 20:35 ` delete on snapshot does not free space Zdenek Kabelac
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