[darcs-users] no more checkpoints, but hashed repos for GHC in Darcs 2.4?

Ganesh Sittampalam ganesh at earth.li
Sat Sep 19 14:23:17 UTC 2009


On Sat, 19 Sep 2009, Petr Rockai wrote:

> Ganesh Sittampalam <ganesh at earth.li> writes:
>
>> On Thu, 17 Sep 2009, Petr Rockai wrote:
>>> For cache, this should be quite simple. For repos, this would be incompatible
>>> change and we probably can't afford to do it just yet.
>>
>> Isn't it a straightforward format change? Add support to hashed-storage, add an
>> extra line to the format to lock out older darcs and to drive the decision to
>> read/write the new format, perhaps add a conversion command to darcs (otherwise
>> make people re-get to switch formats), and that's it?

> Well, yes, but I'm not sure we want to start proliferating formats. It 
> comes at a maintenance cost and code bloat. And it inconveniences users 
> and I'm not sure the performance gain is enough to offset that.

Personally I think that if the GHC team really consider that they need it 
then we should do it. On the other hand I don't find the performance 
difference that Simon found so far all that compelling given that having a 
warm cache makes the difference go away - Simon, could you comment further 
on how important this is?

> Actually, the compatible change that removes the common prefix of zeroes 
> could already help with the problem.

That's a good point actually, is removing the prefix really backwards 
compatible? Old darcs binaries won't be able to read the unprefixed files, 
will they?

Ganesh


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