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Re: [Qemu-devel] [PATCH] qcow2: Optimize the refcount-block overlap chec

From: Alberto Garcia
Subject: Re: [Qemu-devel] [PATCH] qcow2: Optimize the refcount-block overlap check
Date: Mon, 30 Jan 2017 17:34:29 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Mon 30 Jan 2017 05:14:41 PM CET, Alberto Garcia wrote:

> This patch keeps the index of the last used (i.e. non-zero) entry in
> the refcount table and updates it every time the table changes. The
> refcount-block overlap check then uses that index instead of reading
> the whole table.

Note that while I decided to go for this approach the patch can be made
much simpler by simply stopping at the first empty entry in the refcount

    if ((chk & QCOW2_OL_REFCOUNT_BLOCK) && s->refcount_table) {
        for (i = 0; i < s->refcount_table_size; i++) {
            if (!(s->refcount_table[i] & REFT_OFFSET_MASK)) {
            if (overlaps_with(s->refcount_table[i] & REFT_OFFSET_MASK,
                s->cluster_size)) {
                return QCOW2_OL_REFCOUNT_BLOCK;

I don't think QEMU produces files where refcount_table[i] == 0 but
refcount_table[i + 1] != 0. Do they even make sense? In any case, my
patch would cover those cases too, but this simplified version wouldn't.


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