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Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded

From: Max Reitz
Subject: Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value
Date: Fri, 17 Jan 2020 12:01:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1

On 17.01.20 10:55, Kevin Wolf wrote:
> Am 17.01.2020 um 10:12 hat Max Reitz geschrieben:
>> On 17.01.20 00:26, Alberto Garcia wrote:
>>> On Tue 14 Jan 2020 03:15:48 PM CET, Max Reitz wrote:
>>>>> @@ -219,7 +219,7 @@ static int l2_load(BlockDriverState *bs, uint64_t 
>>>>> offset,
>>>>>   * Writes one sector of the L1 table to the disk (can't update single 
>>>>> entries
>>>>>   * and we really don't want bdrv_pread to perform a read-modify-write)
>>>>>   */
>>>>> -#define L1_ENTRIES_PER_SECTOR (512 / 8)
>>>>>  int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index)
>>>> Here it’s because the comment is wrong: “Can’t update single entries” –
>>>> yes, we can.  We’d just have to do a bdrv_pwrite() to a single entry.
>>> What's the point of qcow2_write_l1_entry() then?
>> I think the point was that we couldn’t, for a long time, because the
>> block layer only provided sector-granularity access.  This function
>> simply was never changed when the block layer gained the ability to do
>> byte-granularity I/O.
>> (We’d still need this function, but only for the endian swap, I think.)
> We still can't do byte-granularity writes with O_DIRECT, because that's
> a kernel requirement.

Ah, yes.  But that makes BDRV_SECTOR_SIZE the wrong choice.

> The comment explains that we don't want to do a RMW cycle to write a
> single entry because that would be slower than just writing a whole
> sector. I think this is still accurate. Maybe we should change the
> comment to say "can't necessarily update". (The part that looks really
> wrong in the comment is "bdrv_pread", that should be "bdrv_pwrite"...)

Hm.  But we wouldn’t do an RMW cycle without O_DIRECT, would we?

> Now, what's wrong about the logic to avoid the RMW is that it assumes
> a fixed required alignment of 512. What it should do is looking at
> bs->file->bl.request_alignment and rounding accordingly.


>>>>> @@ -3836,7 +3837,7 @@ qcow2_co_copy_range_from(BlockDriverState *bs,
>>>>>          case QCOW2_CLUSTER_NORMAL:
>>>>>              child = s->data_file;
>>>>>              copy_offset += offset_into_cluster(s, src_offset);
>>>>> -            if ((copy_offset & 511) != 0) {
>>>>> +            if (!QEMU_IS_ALIGNED(copy_offset, BDRV_SECTOR_SIZE)) {
>>>> Hm.  I don’t get this one.
>>> Checking the code (e.g. block_copy_do_copy()) it seems that the whole
>>> chunk must be cluster aligned so I don't get this one either.
>> Hm, how did you get to block_copy_do_copy()?  That’s part of the
>> block-copy infrastructure that’s only used for the backup job, as far as
>> I’m aware.  It’s different from copy_range.
>> I don’t see any limitation for copy_range.  I suppose maybe it doesn’t
>> work for anything that isn’t aligned to physical sectors?  But the qcow2
>> driver shouldn’t care about that.
>> On thing’s for sure, the raw driver doesn’t care about it.
> I don't understand this one either.

Good. :-)


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