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From lvcreate(1) on an Ubuntu 22.04 LTS system I have CLI on: -s|--snapshot Create a snapshot. Snapshots provide a "frozen image" of an or

From lvcreate(1) on an Ubuntu 22.04 LTS system I have CLI on: -s|--snapshot Create a snapshot. Snapshots provide a "frozen image" of an ori‐ gin LV. The snapshot LV can be used, e.g. for backups, while the origin LV continues to be used. This option can create a […] * Also: https://manpages.ubuntu.com/manpages/lunar/en/man8/lvcreate.8.html https://manpages.ubuntu.com/manpages/lunar/en/man8/lvcreate.... The word 'frozen' to me means unmoving / fixed. As mentioned in the Wikipedia article, the analogy comes from photography where a picture / snap(shot) is a moment frozen in time. I've admined NetApps in the past, used Veritas VxFS back in the day, and currently run a lot of ZFS (first using it on Solaris 10), and "snapshot" has meant read-only for the past few decades whenever I've run across it.

pm215 3y ago
The LVM howto https://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html https://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html says "In LVM2, snapshots are read/write by default". And RedHat's docs https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_logical_volumes/snapshot-of-logical-volumes_configuring-and-managing-logical-volumes https://access.redhat.com/documentation/en-us/red_hat_enterp... include text "Since the snapshot is read/write". So I think LVM2 snapshots are indeed read/write. Perhaps that manpage sentence was not updated since LVM1 read-only snapshots ? (I agree with you that 'snapshot' to me strongly suggests read/write; I'm just saying that you can't actually rely on that assumption because it's not just bcachefs that doesn't use that meaning.)
- yrro 3y ago
  The man page continues: > This option can create a COW (copy on write) snapshot, or a thin snapshot (in a thin pool.) [...] COW snapshots are created when a size is specified. The size is allocated from space in the VG, and is the amount of space that can be used for saving COW blocks as writes occur to the origin or snapshot. Likely snapshots _were_ originally read-only, and the description of creating thin and COW snapshots was added later, but the man page text was not re-written completely; rather the description of thin and COW snapshots were added to the end of the existing text.
- pm215 3y ago
  I've just noticed that I wrote "I agree with you that 'snapshot' to me strongly suggests read/write" and of course I meant "read only"! Hope that wasn't too confusing...

pm215 3y ago

The LVM howto https://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html https://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html says "In LVM2, snapshots are read/write by default". And RedHat's docs https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_logical_volumes/snapshot-of-logical-volumes_configuring-and-managing-logical-volumes https://access.redhat.com/documentation/en-us/red_hat_enterp... include text "Since the snapshot is read/write". So I think LVM2 snapshots are indeed read/write. Perhaps that manpage sentence was not updated since LVM1 read-only snapshots ? (I agree with you that 'snapshot' to me strongly suggests read/write; I'm just saying that you can't actually rely on that assumption because it's not just bcachefs that doesn't use that meaning.)

yrro 3y ago
The man page continues: > This option can create a COW (copy on write) snapshot, or a thin snapshot (in a thin pool.) [...] COW snapshots are created when a size is specified. The size is allocated from space in the VG, and is the amount of space that can be used for saving COW blocks as writes occur to the origin or snapshot. Likely snapshots _were_ originally read-only, and the description of creating thin and COW snapshots was added later, but the man page text was not re-written completely; rather the description of thin and COW snapshots were added to the end of the existing text.
pm215 3y ago
I've just noticed that I wrote "I agree with you that 'snapshot' to me strongly suggests read/write" and of course I meant "read only"! Hope that wasn't too confusing...