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Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded
From: |
Kevin Wolf |
Subject: |
Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value |
Date: |
Fri, 17 Jan 2020 10:55:38 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
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)
> >>> +#define L1_ENTRIES_PER_SECTOR (BDRV_SECTOR_SIZE / 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.
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"...)
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.
Kevin
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- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, (continued)
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Max Reitz, 2020/01/14
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Alberto Garcia, 2020/01/14
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Max Reitz, 2020/01/14
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Alberto Garcia, 2020/01/14
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Max Reitz, 2020/01/14
- Re: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size, Alberto Garcia, 2020/01/16
[PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Alberto Garcia, 2020/01/09
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Max Reitz, 2020/01/14
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Alberto Garcia, 2020/01/16
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Max Reitz, 2020/01/17
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value,
Kevin Wolf <=
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Max Reitz, 2020/01/17
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Kevin Wolf, 2020/01/17
- Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Alberto Garcia, 2020/01/17
Re: [PATCH v2 4/4] qcow2: Use BDRV_SECTOR_SIZE instead of the hardcoded value, Alberto Garcia, 2020/01/18