Re: Writing to RBD image while it's snapshot is being created causes I/O errors

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On Fri, 14 Jun 2013, Karol Jurak wrote:
> Hi,
> 
> I noticed that writing to RBD image using kernel driver while it's snapshot 
> is being created causes I/O errors and the filesystem (reiserfs) eventually 
> aborts and remounts itself in read-only mode:
> 
> [192507.327359] end_request: I/O error, dev rbd7, sector 818528
> [192507.331510] end_request: I/O error, dev rbd7, sector 819200
> [192507.348478] end_request: I/O error, dev rbd7, sector 408
> [192507.352647] REISERFS abort (device rbd7): Journal write error in 
> flush_commit_list
> 
> Is this happening by design or is it a bug? I know that I should freeze the 
> filesystem before attempting to create a snapshot but these I/O errors seem 
> unnecessary to me. If it's really impossible to write to an image while it's 
> snapshot is being created couldn't write requests be blocked until 
> snapshotting completes?
> 
> I'm using ceph 0.56.6 and kernel 3.4.48.

This is definitely a bug; you should be able to create a snapshot at any 
time.  After a rollback, it should look (to the fs) like a crash or power 
cycle.

How easy is this to reproduce?  Does it happen every time?

sage



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> Karol Jurak
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