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Re: [Qemu-devel] [PATCH v2 2/7] qcow2: Use unsigned int for both members
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v2 2/7] qcow2: Use unsigned int for both members of Qcow2COWRegion |
Date: |
Thu, 8 Jun 2017 08:38:48 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 |
On 06/08/2017 08:06 AM, Alberto Garcia wrote:
> On Wed 07 Jun 2017 06:02:06 PM CEST, Eric Blake wrote:
>
>>> - The offset of the COW region from the start of the first cluster
>>> touched by the I/O request. Since it's always going to be positive
>>> and the maximum request size is at most INT_MAX, we can use a
>>> regular unsigned int to store this offset.
>>
>> I don't know if we will ever get to the point that we allow a 64-bit
>> request at the block layer (and then the block layer guarantees it is
>> split down to the driver's limits, which works when a driver is still
>> bound by 32-bit limits). But we ALSO know that a cluster is at most
>> 2M (in our current implementation of qcow2), so the offset of where
>> the COW region starts in relation to the start of a cluster is < 2M.
>
> That's correct for the offset of the first region (m->cow_start),
> however m->cow_end is also relative to the offset of the first allocated
> cluster, so it can be > 2M if the requests spans several clusters.
>
> So I guess the maximum theoretical offset would be something like
> INT_MAX + 2*cluster_size (a bit below that actually).
At which point, your earlier claim that we bound requests at INT_MAX
takes over. But thanks for correcting me that the end cow region can
indeed be more than a cluster away from the beginning region.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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- [Qemu-devel] [PATCH v2 0/7] qcow2: Reduce the number of I/O ops when doing COW, Alberto Garcia, 2017/06/07
- [Qemu-devel] [PATCH v2 1/7] qcow2: Remove unused Error variable in do_perform_cow(), Alberto Garcia, 2017/06/07
- [Qemu-devel] [PATCH v2 6/7] qcow2: Pass a QEMUIOVector to do_perform_cow_{read, write}(), Alberto Garcia, 2017/06/07
- [Qemu-devel] [PATCH v2 7/7] qcow2: Merge the writing of the COW regions with the guest data, Alberto Garcia, 2017/06/07
- [Qemu-devel] [PATCH v2 5/7] qcow2: Allow reading both COW regions with only one request, Alberto Garcia, 2017/06/07
- [Qemu-devel] [PATCH v2 4/7] qcow2: Split do_perform_cow() into _read(), _encrypt() and _write(), Alberto Garcia, 2017/06/07