Re: Proposal: Incremental Backup

On Tue, Aug 5, 2014 at 3:23 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 4 August 2014 19:30, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
>> On Mon, Aug 4, 2014 at 5:15 AM, Gabriele Bartolini
>> <gabriele(dot)bartolini(at)2ndquadrant(dot)it> wrote:
>>> I really like the proposal of working on a block level incremental
>>> backup feature and the idea of considering LSN. However, I'd suggest
>>> to see block level as a second step and a goal to keep in mind while
>>> working on the first step. I believe that file-level incremental
>>> backup will bring a lot of benefits to our community and users anyway.
>>
>> Thing is, I don't see how the LSN method is that much harder than an
>> on-disk bitmap. In-memory bitmap IMO is just a recipe for disaster.
>>
>> Keeping a last-updated-LSN for each segment (or group of blocks) is
>> just as easy as keeping a bitmap, and far more flexible and robust.
>>
>> The complexity and cost of safely keeping the map up-to-date is what's
>> in question here, but as was pointed before, there's no really safe
>> alternative. Nor modification times nor checksums (nor in-memory
>> bitmaps IMV) are really safe enough for backups, so you really want to
>> use something like the LSN. It's extra work, but opens up a world of
>> possibilities.
>
> OK, some comments on all of this.
>
> * Wikipedia thinks the style of backup envisaged should be called "Incremental"
> https://en.wikipedia.org/wiki/Differential_backup
>
> * Base backups are worthless without WAL right up to the *last* LSN
> seen during the backup, which is why pg_stop_backup() returns an LSN.
> This is the LSN that is the effective viewpoint of the whole base
> backup. So if we wish to get all changes since the last backup, we
> must re-quote this LSN. (Or put another way - file level LSNs don't
> make sense - we just need one LSN for the whole backup).

File-level LSNs are an optimization. When you want to backup all files
modified since the last base or incremental backup (yes, you need the
previous backup label at least), you check the file-level LSN range.
That tells you which "changesets" touched that file, so you know
whether to process it or not.

Block-level LSNs (or, rather, block-segment-level) are just a
refinement of that.

> * When we take an incremental backup we need the WAL from the backup
> start LSN through to the backup stop LSN. We do not need the WAL
> between the last backup stop LSN and the new incremental start LSN.
> That is a huge amount of WAL in many cases and we'd like to avoid
> that, I would imagine. (So the space savings aren't just the delta
> from the main data files, we should also look at WAL savings).

Yes, probably something along the lines of removing redundant FPW and
stuff like that.

> * For me, file based incremental is a useful and robust feature.
> Block-level incremental is possible, but requires either significant
> persistent metadata (1 MB per GB file) or access to the original
> backup. One important objective here is to make sure we do NOT have to
> re-read the last backup when taking the next backup; this helps us to
> optimize the storage costs for backups. Plus, block-level recovery
> requires us to have a program that correctly re-writes data into the
> correct locations in a file, which seems likely to be a slow and bug
> ridden process to me. Nice, safe, solid file-level incremental backup
> first please. Fancy, bug prone, block-level stuff much later.

Ok. You could do incremental first without any kind of optimization,
then file-level optimization by keeping a file-level LSN range, and
then extend that to block-segment-level LSN ranges. That sounds like a
plan to me.

But, I don't see how you'd do the one without optimization without
reading the previous backup for comparing deltas. Remember checksums
are deemed not trustworthy, not just by me, so that (which was the
original proposition) doesn't work.

> * If we don't want/have file checksums, then we don't need a profile
> file and using just the LSN seems fine. I don't think we should
> specify that manually - the correct LSN is written to the backup_label
> file in a base backup and we should read it back from there.

Agreed