[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [RFC PATCH v2 05/26] qcow2: Document the Extended L2 Entries feature
From: |
Alberto Garcia |
Subject: |
Re: [RFC PATCH v2 05/26] qcow2: Document the Extended L2 Entries feature |
Date: |
Wed, 30 Oct 2019 23:38:01 +0100 |
User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
On Wed 30 Oct 2019 05:23:30 PM CET, Max Reitz wrote:
>> +Subcluster Allocation Bitmap (for standard clusters):
>> +
>> + Bit 0 - 31: Allocation status (one bit per subcluster)
>> +
>> + 1: the subcluster is allocated. In this case the
>> + host cluster offset field must contain a valid
>> + offset.
>> + 0: the subcluster is not allocated. In this case
>> + read requests shall go to the backing file or
>> + return zeros if there is no backing file data.
>> +
>> + Bits are assigned starting from the most significant
>> one.
>> + (i.e. bit x is used for subcluster 31 - x)
>
> I seem to remember that someone proposed this bit ordering to you, but
> I wonder why. So far everything in qcow2 starts from the least
> significant bit, for example refcounts (“If refcount_bits implies a
> sub-byte width, note that bit 0 means the least significant bit in
> this context”), feature bits, and sub-byte structure descriptions in
> general (which you reference directly with “bit x”).
>
> Soo... What’s the reason for doing it the other way around here?
The reason is that I thought that it would be better for debugging
purposes. If I do an hexdump of the L2 table to see what's going on then
starting from the most significant bit gives me a better visual image of
what subclusters are allocated.
In other words, if the first two subclusters are allocated I think this
representation
11000000 00000000 00000000 00000000 (c0 00 00 00)
is more natural than this one
00000000 00000000 00000000 00000011 (00 00 00 03)
But I don't have a very strong opinion so I'm open to changing it.
Berto
- [RFC PATCH v2 25/26] qcow2: Allow preallocation and backing files if extended_l2 is set, (continued)
- [RFC PATCH v2 25/26] qcow2: Allow preallocation and backing files if extended_l2 is set, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 13/26] qcow2: Add subcluster support to calculate_l2_meta(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 21/26] qcow2: Clear the L2 bitmap when allocating a compressed cluster, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 08/26] qcow2: Add offset_to_sc_index(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 06/26] qcow2: Add dummy has_subclusters() function, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 10/26] qcow2: Update get/set_l2_entry() and add get/set_l2_bitmap(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 22/26] qcow2: Add subcluster support to handle_alloc_space(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 05/26] qcow2: Document the Extended L2 Entries feature, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 14/26] qcow2: Add subcluster support to qcow2_get_cluster_offset(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 20/26] qcow2: Update L2 bitmap in qcow2_alloc_cluster_link_l2(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 19/26] qcow2: Fix offset calculation in handle_dependencies(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 18/26] qcow2: Add subcluster support to expand_zero_clusters_in_l1(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 07/26] qcow2: Add subcluster-related fields to BDRVQcow2State, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 12/26] qcow2: Handle QCOW2_CLUSTER_UNALLOCATED_SUBCLUSTER, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 23/26] qcow2: Restrict qcow2_co_pwrite_zeroes() to full clusters only, Alberto Garcia, 2019/10/26
- [RFC PATCH v2 09/26] qcow2: Add l2_entry_size(), Alberto Garcia, 2019/10/26
- [RFC PATCH v2 11/26] qcow2: Add qcow2_get_subcluster_type(), Alberto Garcia, 2019/10/26