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Re: [Qemu-block] [Qemu-devel] [PATCH v3 17/39] qcow2: Update l2_allocate

From: Max Reitz
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v3 17/39] qcow2: Update l2_allocate() to support L2 slices
Date: Thu, 1 Feb 2018 19:22:16 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 2018-02-01 16:43, Alberto Garcia wrote:
> On Thu 01 Feb 2018 04:23:09 PM CET, Anton Nefedov wrote:
>>>> However, I'm wondering whether this is the best approach.  The old
>>>> L2 table is probably not going to be used after this function, so
>>>> we're basically polluting the cache here.  That was bad enough so
>>>> far, but now that actually means wasting multiple cache entries on
>>>> it.
>>>> Sure, the code is simpler this way.  But maybe it would still be
>>>> better to manually copy the data over from the old offset...  (As
>>>> long as it's not much more complicated.)
>>> You mean bypassing the cache altogether?
>>>      qcow2_cache_flush(bs, s->l2_table_cache);
>>>      new_table = g_malloc(s->cluster_size);
>>>      if (old_l2_offset & L1E_OFFSET_MASK) {
>>>          bdrv_pread(bs->file, old_l2_offset, new_table, s->cluster_size);
>>>      } else {
>>>          memset(new_table, 0, s->cluster_size);
>>>      }
>>>      bdrv_pwrite(bs->file, new_l2_offset, new_table, s->cluster_size);
>>>      g_free(new_table);
>>> ??
>> (I know it's a draft so you probably just skipped that but just in
>> case) It seems ok to bypass the cache read - perhaps even a flush is
>> not necessary: old_l2_offset must be read-only and flushed at this
>> point; I believe new_l2_offset might be cached too, so it needs to be
>> updated.
> One problem I see with this is that while we wouldn't pollute the cache
> we'd always be reading the table twice from disk in all cases:
>  1) Read old table
>  2) Write new table
>  3) Read new table (after l2_allocate(), using the cache this time)
> We can of course improve it by reading the old table from disk but
> directly in the cache -so we'd spare step (3)-, but we'd still have to
> read at least once from disk.
> With the old code (especially if slice_size == cluster_size) we don't
> need to read anything if the L2 table is already cached:
>  1) Get empty table from the cache
>  2) memcpy() the old data
>  3) Get new table from the cache (after l2_allocate()).

Well, then scratch the bdrv_pwrite() for the new table and keep using
the cache for that (because that actually sounds useful).

On second thought, though, it's rather probable the old L2 table is
already in the cache...  Before the guest does a write to a location, it
is reasonable to assume it has read from there before.

So I guess we could think about adding a parameter to qcow2_cache_put()
or something to reset the LRU counter because we probably won't need
that entry anymore.  But not something for this series, of course.


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