Re: [pve-devel] [PATCH v3 qemu-server 08/11] blockdev: convert drive_mirror to blockdev_mirror

[Fiona Ebner <f.ebner@proxmox.com>]

Am 14.01.25 um 11:03 schrieb DERUMIER, Alexandre:
>>> If we do need lookup, an idea to get around the character limit is
>>> using
>>> a hash of the information to generate the node name, e.g.
>>> hash("fmt-$volid@$snapname"), hash("file-$volid@$snapname") or
>>> whatever
> 
> yes, I think it should works
> 
>>> is actually needed as unique information. Even if we only use
>>> lowercase
>>> letters, we have 26 base chars, so 26^31 possible values.
> 
> yes, I was think about a hash too, but I was not sure how to convert it
> to the alphanum characters (valid char : alphanum , ‘-’, ‘.’ and ‘_’. 
> )
> 
> 
> 
>>> So hashes with up to
>>>
>>>> math.log2(26**31)
>>> 145.71363126237387
>>>
>>> bits can still fit, which should be more than enough. Even with an
>>> enormous number of 2^50 block nodes (realistically, the max values we
>>> expect to encounter are more like 2^10), the collision probability
>>> (using a simple approximation for the birthday problem) would only be
>>>
>>>> d=2**145
>>>> n=2**50
>>>> 1 - math.exp(-(n*n)/(2*d))
>>> 1.4210854715202004e-14
> 
> yes, should be enough
> 
> a simple md5 is 128bit, 
> sha1 is 160bit    (it's 150bits space with extra -,.,- characters)
> 
> Do you known a good hash algorithm ?

I'm not too well-read in cryptography, but AFAIK, you can shorten the
result of sha256 to get a good hash algorithm with fewer bits. We could
also have the node-name start with a "h" to make sure it doesn't start
with a number and then use base32 for the remaining 30 characters. I.e.
we could take the first 150 bits (32^30 = 2^150) from the sha256 hash
and convert that to base32.

@Shannon @Fabian please correct me if I'm wrong.


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