Re: [Qemu-devel] [RFC V5 20/36] qcow2: Drop hash for a given cluster when dedup makes refcount > 2^16/2.

[Benoît Canet]

> You are hardcoding to a width of 16 bits; however, version 3 makes the
> refcount field variable-sized:
>
>          96 -  99:  refcount_order
>                     Describes the width of a reference count block entry
> (width
>                     in bits = 1 << refcount_order). For version 2
> images, the
>                     order is always assumed to be 4 (i.e. the width is
> 16 bits).

Currently the qcow2 code doesn't support anything but refcount_order == 4.

In qcow2.c qcow_open there is:
        be32_to_cpus(&header.refcount_order);
to get the qcow2 order followed by:
    /* Check support for various header values */
    if (header.refcount_order != 4) {
        report_unsupported(bs, "%d bit reference counts",
                           1 << header.refcount_order);
        ret = -ENOTSUP;
        goto fail;
    }

I guess the code doesn't need any special handling for now.

> Hmm, what happens if refcount_order is 0 to disable reference counting?
>  That setting is valid for creating a qcow2 file that can't be used for
> internal snapshots.  But it also interferes with dedup; so you probably
> want to add some additional requirements in the spec (patch 1/36) that
> when dedup is in use, refcount_order must be a minimum value (or require
> that it be exactly 4, for a width of 16 bits).

I'll do that.

Regards

Benoît