Re: [PATCH 0/1] qcow2: Skip copy-on-write when allocating a zero cluster

[Alberto Garcia]

On Mon 17 Aug 2020 05:53:07 PM CEST, Kevin Wolf wrote:
>> > Or are you saying that ZERO_RANGE + pwrite on a sparse file (=
>> > cluster allocation) is faster for you than just the pwrite alone (=
>> > writing to already allocated cluster)?
>> 
>> Yes, 20% faster in my tests (4KB random writes), but in the latter
>> case the cluster is already allocated only at the qcow2 level, not on
>> the filesystem. preallocation=falloc is faster than
>> preallocation=metadata (preallocation=off sits in the middle).
>
> Hm, this feels wrong. Doing more operations should never be faster
> than doing less operations.
>
> Maybe the difference is in allocating 64k at once instead of doing a
> separate allocation for every 4k block?

That's what I imagine, yes. I'll have a look at your patches and tell
you.

Berto