Can checksum from metadata be exported?

[byenilmez]

Hi,

I'm new to ZFS so I might be wrong. But as far as I understand, ZFS keeps checksum (default:flether) of each file in metadata.
Is it possible to get this metadata so that I can compare files in different systems without recalculating the checksum for each file (at least on Truenas)?

Thanks

 

[Samuel Tai]

Sorry, no.

 

[byenilmez]

Thank for the quick reply.
Just trying to understand how ZFS works. Is the checksum is block-based rather than file-based like in encryption?

 

[Patrick M. Hausen]

Is the checksum is block-based rather than file-based like in encryption?

Yes.
Checksums, snapshots, deduplication, compression, ... everything is block based - apart from the filesystem presentation itself.

 

[Samuel Tai]

Essentially, yes. If you're willing to do some reading, here are some good links on the low-level details of ZFS:

References

The TrueNAS References section includes additional information on various topics helpful for a TrueNAS user. Copyrights and Trademarks: Provides a list of copyrights and trademarks, and related logos registered as trademarks of iXsystems, dba TrueNAS. ACL Primer: Provides general information on...

www.truenas.com

Developer resources - OpenZFS

openzfs.org

 

[byenilmez]

Thank you both. I'll go over the link.
I did some reading but I couldn't figure out how deduplication would work if the checksum wasn't file based.
Anyways, thanks again for steering me in the right direction.

 

[Patrick M. Hausen]

I did some reading but I couldn't figure out how deduplication would work if the checksum wasn't file based.

It deduplicates blocks, not files.

 

[byenilmez]

I see. It makes sense now.
Thanks

 

[Arwen]

And to re-visit one of your posts about encryption, ZFS native encryption is block based as well. What this implies is that you can append, (or update part of), an encrypted file without re-writing the whole file, in ZFS. At least, I think this is what happens.

 

[AlexGG]

What this implies is that you can append, (or update part of), an encrypted file without re-writing the whole file, in ZFS

That is correct, yes.

 

[AlexGG]

Checksums and encryption are block-based. More precisely, record-based. What's more, if the compression is in use, the checksum is calculated on the compressed data. If the encryption is in use, the checksum is calculated on the encrypted data. This is done to avoid decrypting and/or decompressing each record when doing a scrub. You don't even need a decryption key to do the scrub on the encrypted volume, because checksum covers what was written to disk, not the data in the file.