[Gluster-devel] few queries regarding file snapshot feature in gluster

Prasanna Kumar Kalever pkalever at redhat.com
Wed Dec 23 06:17:48 UTC 2015

On Wednesday, December 23, 2015 5:20:09 AM, Vijay Bellur wrote:
> On 12/21/2015 06:47 AM, Prasanna Kumar Kalever wrote:
> > Hi Team,
> >
> > I have few queries regarding file snapshot feature in gluster
> >
> > what are the limitations of Block device translator and qemu-block
> > translator ?
> > Are we using them some where ?
> > Do we have plans to improve them ?
> > Or someone have a design to implement file snapshot feature ?
> >
> I have not heard much feedback about these two translators. The
> qemu-block functionality can be handy for exposing virtual block devices
> and due to features like native snapshotting, it could be quite
> interesting in use cases that require virtual block devices.
> The qemu-block translator has not been tested of late. A few months back
> I tried mkfs.xfs after attaching the block device and it failed. I did
> not find more time to look into that further.

XFS does not provide direct support for snapshots, as it expects the
snapshot process to be implemented by the volume manager. Taking a
snapshot of an XFS filesystem involves temporarily halting I/O ...
More @ https://en.wikipedia.org/wiki/XFS#Snapshots

In my opinion BTRFS and ZFS are the file systems that provide direct
snapshots but may not be production ready at this state. And we should
not wait for them.

> file snapshotting could be a possibility with dht2 but I don't think
> much work has happened in that direction.
> The other possibilities for file snapshotting include reflinks and
> overlay. Given that reflinks support is most probably going to land
> sometime in xfs, we might be able to leverage this feature and explore
> the possibility of providing snapshots with backend filesystems that are
> reflink capable (btrfs, xfs etc.).
> >
> > I am really thankful if some one gives more details about qemu-block
> > xlator,
> > I don't see enough documentation regarding this :(
> >
> qemu-block xlator provides the ability to snapshot files by relying on
> qcow2/qed image formats. what specific details are you looking for?

I am just investigating, are this features enough from the customer
perspective. One of the limitation could be file should be in QCOW2 or QED

My thought is using rsnapshot mechanism for file snapshots in gluster,
rsnapshot is a utility (stable) that uses rsync in the backend

Similar utilities: http://www.nongnu.org/rdiff-backup/


> Avati & Brian probably have the maximum context about this translator.
> Regards,
> Vijay

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