Filesystem freeze vs blockdevice snapshot
Hello all,
please excuse the silly question...
What is the benefit of performing an ext4 fs freeze before taking a snapshot of the underlying block device with LVM2 ?
Is the freeze step useful or I can go with the snapshot directly?
And/or is there another use case for the freeze?
Thank you
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Thanks but it's still not clear to me.
What do you mean with "clean fs"? is that in any way different from an image after a sudden power loss?
Secondarily, how can LVM know what filesystem is underneath and call the proper freeze? That's a bare block device, there might not even be a filesystem under LVM; or it might be exported via iSCSI to an XYZ operating system using the Xyzfs filesystem which Linux knows nothing about... Thank you
Quoted 25 lines · Amir Goldstein
LVM2 already does the fs freeze for you before taking the snapshot. it's the only way to get a block device snapshot of a "clean" fs. hope that answers your question.
On Fri, Jan 13, 2012 at 5:30 PM, Asdo <[email protected] <mailto:[email protected]>> wrote:
Hello all,
please excuse the silly question...What is the benefit of performing an ext4 fs freeze before taking a snapshot of the underlying block device with LVM2 ?
Is the freeze step useful or I can go with the snapshot directly?
And/or is there another use case for the freeze?
Thank you
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Quoted 3 lines · Asdo
Thanks but it's still not clear to me.
What do you mean with "clean fs"? is that in any way different from an image after a sudden power loss?
Yes. It is different:
a) the filesystem will be marked as clean so journal will not be replayed on mount of the snapshot
b) dirty data will be flushed to disk during freezing so you will get most recent file content. On sudden power failure you might miss quite a lot of data.
Quoted 5 lines
Secondarily, how can LVM know what filesystem is underneath and call the proper freeze? That's a bare block device, there might not even be a filesystem under LVM; or it might be exported via iSCSI to an XYZ operating system using the Xyzfs filesystem which Linux knows nothing about...
Well, if the kernel has the device mounted, it knows it and it can call
proper fs method to freeze the filesystem. If it is some more complex scenario (like export to other host), then kernel doesn't know enough and doesn't attempt to freeze the filesystem.
Honza
Quoted 33 lines · Amir Goldstein
Quoted 17 lines
LVM2 already does the fs freeze for you before taking the snapshot. it's the only way to get a block device snapshot of a "clean" fs. hope that answers your question.
On Fri, Jan 13, 2012 at 5:30 PM, Asdo <[email protected] <mailto:[email protected]>> wrote:
Hello all,
please excuse the silly question...What is the benefit of performing an ext4 fs freeze before taking a snapshot of the underlying block device with LVM2 ?
Is the freeze step useful or I can go with the snapshot directly?
And/or is there another use case for the freeze?
Quoted 9 lines
Thank you
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Jan Kara <[email protected]> SUSE Labs, CR