Re: Reconstructing files from shards

[Krutika Dhananjay <kdhananj@xxxxxxxxxx>]

The short answer is - no there exists no script currently that can piece the shards together into a single file.

Long answer:

IMO the safest way to convert from sharded to a single file _is_ by copying the data out into a new volume at the moment.

Picking up the files from the individual bricks directly and joining them, although fast, is a strict no-no for many reasons - for example, when you have a replicated volume
and the good copy needs to be carefully selected and must remain a good copy through the course of the copying process. There could be other consistency issues with

file attributes changing while they are being copied. All of this is not possible, unless you're open to taking the volume down.

Then the other option is to have gluster client (perhaps in the shard translator itself)) do the conversion in the background within the gluster translator stack, which is safer

but would require that shard lock it until the copying is complete. And until then no IO can happen into this file.
(I haven't found the time to work on this, as there exists a workaround and I've been busy with other tasks. If anyone wants to volunteer to get this done, I'll be happy to help).

But anway, why is copying data into new unsharded volume disruptive for you?

 

-Krutika

On Sat, Apr 21, 2018 at 1:14 AM, Jamie Lawrence <jlawrence@xxxxxxxxxxxxxxx> wrote:

Hello,

So I have a volume on a gluster install (3.12.5) on which sharding was enabled at some point recently. (Don't know how it happened, it may have been an accidental run of an old script.) So it has been happily sharding behind our backs and it shouldn't have.

I'd like to turn sharding off and reverse the files back to normal.  Some of these are sparse files, so I need to account for holes. There are more than enough that I need to write a tool to do it.

I saw notes ca. 3.7 saying the only way to do it was to read-off on the client-side, blow away the volume and start over. This would be extremely disruptive for us, and language I've seen reading tickets and old messages to this list make me think that isn't needed anymore, but confirmation of that would be good.

The only discussion I can find are these videos[1]: http://opensource-storage.blogspot.com/2016/07/de-mystifying-gluster-shards.html , and some hints[2] that are old enough that I don't trust them without confirmation that nothing's changed. The video things don't acknowledge the existence of file holes. Also, the hint in [2] mentions using trusted.glusterfs.shard.file-size to get the size of a partly filled hole; that value looks like base64, but when I attempt to decode it, base64 complains about invalid input.

In short, I can't find sufficient information to reconstruct these. Has anyone written a current, step-by-step guide on reconstructing sharded files? Or has someone has written a tool so I don't have to?

Thanks,

-j


[1] Why one would choose to annoy the crap out of their fellow gluster users by using video to convey about 80 bytes of ASCII-encoded information, I have no idea.
[2] http://lists.gluster.org/pipermail/gluster-devel/2017-March/052212.html


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