Re: [pbs-devel] [PATCH-SERIES v4 pxar proxmox-backup proxmox-widget-toolkit 00/26] fix #3174: improve file-level backup

[Christian Ebner <c.ebner@proxmox.com>]

Thanks for your comments, some thoughts inline:

> On 13.11.2023 15:23 CET Fabian Grünbichler <f.gruenbichler@proxmox.com> wrote:
> 
>  
> 
> some (high-level) comments focused on compatibility:
> 
> the catalog v2 format is used unconditionally at the moment. IMHO it
> should be guarded/opt-in via --change-detection-method, since old
> clients cannot parse it.

While it is true that the new catalog format is not readable by an old client,
the motivation to include this unconditionally was to be able to also use
backups created with the default change detection mode as reference.
Backups with the change detection mode set to metadata would still not be
readable by older clients.

I can of course make this conditional and only ever use catalogs with format
version 2 as reference in cases of metadata based file change detection.

> 
> else, the following would happen if a client system upgrades:
> 
> - pre-upgrade backup (readable by all clients)
> - upgrade
> - post-upgrade backup *with --c-d-m data* (readable by all clients, but
>   everything catalog related only works with new clients)
> - post-upgrade backup *with --c-d-m metadata* (readable by new clients
>   only)
> 
> since the pxar format itself also changes (new entry types), it should
> also be bumped (see below). if the new formats are then only used with
> the new metadata mode, both new formats are effectively opt-in (until we
> make that the default mode). having the incompatibility between old and
> new clients encoded right in the magic value in the header also means we
> don't spend time downloading indices and chunks only to notice at some
> random point within the restore that we actually don't know how to parse
> this particular pxar archive.

I am not sure I understand your concern here, the latest patch series already
includes a bumped pxar archive format version with it's dedicated magic number.

> 
> an additional bonus point - tools like pxar and proxmox-backup-debug
> could also list the raw+parsed magic value, and in general, error
> messages like:
> 
>  Error: got unexpected magic number for catalog
> 
> are a lot easier to grasp than (pxar extract)
> 
>  Error: encountered unexpected error during extraction
> 
> or (proxmox-backup-client restore)
> 
>  Error: error extracting archive - encountered unexpected error during extraction
> 
> the magic values could also be backported to the oldstable client
> version, to make the error messages even better ("known unsupported" vs
> "unexpected").

Agreed, back-porting this should definitely improve the error messages and make
it easier to see what is going on. The obscure error context `encountered
unexpected error during extraction` was however not introduced by this patch
series, maybe this should be improved as separate patches as well?

> 
> in general, UX wise it might be nice to mark backups using the new mode,
> although I am not sure how specifically (some variants - just the
> version/mode, archives, archives+snapshots, ..?).

I have the patches to include  this in the LXC config as option for now, setting
the metadata based change detection if the option is set accordingly. Not sure
if this is the best way, any objections or suggestions? I can include these
patches in the next version of the patch series as well.

> 
> one more peculiarity I noted while testing - doing three backups in a
> row without changing the input tree at all:
> 
> - old client
> - new client, mode data
> - new client, mode metadata
> 
> the last snapshot has a bigger "logical" size, e.g., when doing this for
> my kernel clone (6.8G), the first two have a logical size of 7.736 GiB,
> while the last one is 8.064Gib. for smaller input dirs, the effect is
> even more pronounced, a 56M dir with 10 dirs with one file each is
> listed as 55M for the first wo, and 97.989MiB for the last one (almost
> double the size!). the resulting pxar archives are actually this size,
> I guess there is some optimization potential still left for this
> particular case. the actual (deduplicated) difference is just two (small
> test case) / eight (linux) very small chunks, so this issue is mostly
> cosmetic I hope unless one really goes down the "download pxar file,
> extract manually" route.

Yes, I am still looking into this somewhat unexpected behavior, as the
actual new chunk data by the backup run is rather small. I have an eye on
this especially since Dominik noticed the bloating of the index, which
should be much reduced now. But for some cases it still is rather significant.

> 
> I hope to do some more in-depth testing and code review over the course
> of the week!