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Re: [RFC PATCH 00/23] Add subcluster allocation to qcow2
From: |
Alberto Garcia |
Subject: |
Re: [RFC PATCH 00/23] Add subcluster allocation to qcow2 |
Date: |
Wed, 16 Oct 2019 09:14:33 +0200 |
User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
On Tue 15 Oct 2019 06:05:23 PM CEST, Eric Blake wrote:
>> 63 56 55 48 47 40 39 32 31 24 23 16 15 8 7 0
>> 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>> <---------------------------------> <--------------------------------->
>> subcluster reads as zeros subcluster is allocated
>> (32 bits) (32 bits)
>
> okay, in patch 5, you said you map the most significant bit to the
> first cluster. That feels backwards to me; I wonder if the math is any
> easier if you map sub-clusters starting from the least-significant,
> because then you get:
>
> bit = (address >> cluster_size) & 32
>
> rather than
>
> bit = 31 - ((address >> cluster_size) & 32)
The reason why I chose that ordering is because I think it's more
natural for debugging if you read from left to right:
63 56 55 48 47 40 39 32 31 24 23 16 15 8 7 0
00000000 00000000 00000000 00011111 11100000 00000000 00000000 00000000
<---------------------------------> <--------------------------------->
subcluster reads as zeros subcluster is allocated
Here the last five subclusters read as zeros, and the first three
subclusters are allocated.
I don't think the math is any different. What you need in the code is
1) A way to get the subcluster index. That doesn't change, it's
sc_index = (address >> cluster_bits) & 31
in both cases.
2) A way to get the "subcluster reads as zeros" and "subcluster is
allocated" masks. That's not very different either, it's a constant
shifted by the subcluster index in both cases:
LSB first:
all_zeros_mask = (1 << 32) << sc_index
allocated_mask = 1 << sc_index
MSB first:
all_zeros_mask = (1 << 63) >> sc_index
allocated_mask = (1 << 31) >> sc_index
>> Some comments about the results:
>>
>> - The smallest allowed cluster size for an image with subclusters is
>> 16 KB (in this case the subclusters size is 512 bytes), hence the
>> missing values in the 4 KB and 8 KB rows.
>
> Again reading ahead, I see that patch 5 requires a 16k minimum cluster
> for using extended L2. Could we still permit clusters smaller than
> that, but merely document that subclusters are always a minimum of 512
> bytes and therefore for an 8k cluster we only use 16 bits (leaving the
> other 16 bits zero)? But I'm also fine with the simplicity of just
> stating that subclusters require at least 16k clusters.
I can't think of any reason why you would want smaller clusters, the
numbers show that the performance starts to drop with sizes under 16KB.
> How do subclusters interact with external data files?
As far as I'm aware they work just fine (I'll add tests for that
anyway).
Berto
- [RFC PATCH 02/23] qcow2: Split cluster_needs_cow() out of count_cow_clusters(), (continued)
- [RFC PATCH 02/23] qcow2: Split cluster_needs_cow() out of count_cow_clusters(), Alberto Garcia, 2019/10/15
- [RFC PATCH 14/23] qcow2: Add subcluster support to qcow2_get_cluster_offset(), Alberto Garcia, 2019/10/15
- [RFC PATCH 18/23] qcow2: Add subcluster support to expand_zero_clusters_in_l1(), Alberto Garcia, 2019/10/15
- [RFC PATCH 19/23] qcow2: Fix offset calculation in handle_dependencies(), Alberto Garcia, 2019/10/15
- [RFC PATCH 23/23] qcow2: Add the 'extended_l2' option and the QCOW2_INCOMPAT_EXTL2 bit, Alberto Garcia, 2019/10/15
- [RFC PATCH 13/23] qcow2: Add subcluster support to calculate_l2_meta(), Alberto Garcia, 2019/10/15
- [RFC PATCH 21/23] qcow2: Add subcluster support to handle_alloc_space(), Alberto Garcia, 2019/10/15
- [RFC PATCH 17/23] qcow2: Add subcluster support to check_refcounts_l2(), Alberto Garcia, 2019/10/15
- Re: [RFC PATCH 00/23] Add subcluster allocation to qcow2, Eric Blake, 2019/10/15
- Re: [RFC PATCH 00/23] Add subcluster allocation to qcow2,
Alberto Garcia <=
- Re: [RFC PATCH 00/23] Add subcluster allocation to qcow2, Vladimir Sementsov-Ogievskiy, 2019/10/23