File Maintenance on ReFS

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Re: File Maintenance on ReFS

Veeam Logoby ChrisGundryCEGA » Wed Mar 15, 2017 9:11 am

I thought that with the way ReFS works and the fact that it allows the re-mapping of blocks, it would be the ideal candidate for Reverse Incremental? When the merge parts of the job are happening instead of re-writing all the merged blocks it just re-maps them?

My understanding being that Forward Incremental will use 1 write IO during the backup then 1 read and 1 write after the backup to do the merge. Where RI will use 3 IO during the whole backup process, 1 write of the new block, 1 read of an existing block and another write to put that existing block into the new file. My understanding was that with ReFS the 'existing block' IO is just re-mapped instead of actually read/written. So this improves the speed of the process and takes the actual IO load away from the disk array and does it at a software level instead. I understand that would cause fragmentation of the files, but that would happen anyway with ReFS no?

I thought that Reverse Incremental would give the benefit of the latest backup always being a full, but the IO load issue normally associated with it would be removed due to benefits of ReFS functionality.
Our problem with using Forward Incremental is that we rely on the full chain to go back to last night, if there is an issue with any of the chain the backup is no good. 99% of the time we only want to restore from last night. With Reverse Incremental we always have a full backup being last night.

Any comments on why Reverse Incremental is the worst for IO with ReFS? To me it seems like the ideal candidate to use with ReFS? Why is Forward Incremental so much better with ReFS?

Thanks!
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Re: File Maintenance on ReFS

Veeam Logoby foggy » Wed Mar 15, 2017 5:41 pm

You're correct, ReFS allows to save 2 of those 3 I/O's thanks to block cloning. Here's a good thread regarding this, btw.
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Re: File Maintenance on ReFS

Veeam Logoby ChrisGundryCEGA » Wed Mar 15, 2017 5:47 pm

OK thanks Foggy. I thought that was the case...

I guess my question was why Gostev was saying that he wouldn't use it on ReFS, wondered what the reason for it was? :)
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Re: File Maintenance on ReFS

Veeam Logoby foggy » Wed Mar 15, 2017 5:57 pm

Another disadvantage of this method is that it heavily defragments full backup file.
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Re: File Maintenance on ReFS

Veeam Logoby ChrisGundryCEGA » Wed Mar 15, 2017 5:59 pm

Where as Forward Incremental would be more sequential and less fragmented? I figured that as that was also using block cloning it would be similarly fragmented to reverse incremental?
Thanks
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Re: File Maintenance on ReFS

Veeam Logoby DaveWatkins » Wed Mar 15, 2017 7:06 pm

I'm also confused as to why Forward Incremental would cause less fragmentation since at the end of the day the VIB's are still merged into the VBK, it just happens at the other end of the chain. I'd love to know the rationale sicne I've got no problem switching to forward at this point as long as we can sustain our backup speed to tape
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Re: File Maintenance on ReFS

Veeam Logoby Gostev » Wed Mar 15, 2017 8:34 pm

Just think about how increments are created in both modes. They are sequential writes into contiguous VIB file in case of forward incremental mode, and random writes into VBK in case of reversed incremental mode (plus VRB files pointing at random existing blocks which were previously a part of VBK).
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Re: File Maintenance on ReFS

Veeam Logoby DaveWatkins » Wed Mar 15, 2017 8:38 pm

Ahh, of course, so as vib's are rolled into a VBK in FI the entire VIB file stays as a single chunk on the file system, so while you're still fragmenting the VBK but ontly by adding 2 fragments (the VIB and the remaining VBK), it's not to the same extreme as RI which could add hundreds of fragments just by writing out a single VRB because they could be spread all over the exiting VBK.

Maybe a warning on job creation for RI jobs pointing to a ReFS volume?
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Re: File Maintenance on ReFS

Veeam Logoby Gostev » Wed Mar 15, 2017 9:01 pm

There's one in the label itself, it says (slower) next to its name ;) naturally, reversed incremental is a few times slower on any file system at all - and the fragmentation issue is not specific to ReFS.
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Re: File Maintenance on ReFS

Veeam Logoby DaveWatkins » Wed Mar 15, 2017 9:04 pm

But the fragmentation is, since it's usually kept under control by File Maintenance but on ReFS File Maintenance basically does nothing other than create a new VBK pointing to exactly the same blocks as the old one
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Re: File Maintenance on ReFS

Veeam Logoby Gostev » Wed Mar 15, 2017 9:05 pm

That is correct.
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