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

[DERUMIER, Alexandre]

> 

>>Upgrading libpve-storage-perl or an external storage plugin while the
>>VM
>>is running could lead to a different result for path() and thus
>>breakage, right?
mmm, yes, you are right


>>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 ?