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Re: reftable: new ref storage format

On Thu, Jul 13, 2017 at 1:35 PM, Jeff King <peff@xxxxxxxx> wrote:
> On Thu, Jul 13, 2017 at 12:56:54PM -0700, Stefan Beller wrote:
> I agree that a full binary search of a reftable is harder because of the
> prefix compression (it may still be possible by scanning backwards, but
> I think there are ambiguities when you land in the middle of a record,
> since there's no unambiguous end-of-record character).

Its impossible to safely binary search this reftable format using a
naive divide byte count in half and find record boundary approach. I
actually did design an earlier version of reftable that was safe to
use this approach for its binary search within blocks, and wound up
discarding it. It was slower and more complex implementation than the
format I shared with the list.

> But I don't think
> it matters. If you binary-search to a constant-sized block, then a
> linear scan of the block is acceptable.

Depends on the block size. :)

> Not that I'm recommending just gzipping the whole packed-refs file. It
> ruins the fast-lookup.

As I just mentioned elsewhere in the thread:

  src file    65306185
  gzip        28338906
  reftable  28782292

The reftable format (for 64k block, 256 restart) is within spitting
distance (432 KiB) of a default level gzip of packed-refs. We can get
fast-lookup, and OK compression.

> We _could_ consider gzipping individual blocks of
> a reftable (or any structure that allows you to search to a
> constant-sized block and do a linear search from there). But given that
> they're in the same ballpark, I'm happy with whatever ends up the
> simplest to code and debug. ;)

This does help to shrink the file, e.g. it drops from 28M to 23M.

It makes it more CPU costly to access a block, as we have to inflate
that to walk through the records. It also messes with alignment. When
you touch a block, that may be straddling two virtual memory pages in
your kernel/filesystem.

I'm not sure those penalties are worth the additional 16% reduction in size.

>> When Shawn presented the proposal, a couple of colleagues here
>> were as excited as I was, but the daring question is, why Shawn
>> did not give the whole thing in BNF format from top down:
>>   initial-block
>>   content-blocks*
>>   (index-block)
>>   footer
> Yeah, I agree it took me a bit to figure out what was going on. A
> high-level overview of the format would have been nice.

Noted, I've added this to my writeup.