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

[Kevin Wolf <kwolf@redhat.com>]

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Am 17.01.2020 um 12:01 hat Max Reitz geschrieben:
> 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)
> >>>>> +#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.
> 
> 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?

file-posix with a regular file has request_alignment == 1 and wouldn't
cause an RMW cycle, I think. I won't try to make a statement about all
non-O_DIRECT cases in all protocols.

The important point is that some cases exist which would get us RMW.

> > 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.
> 
> Yes.

Who'll write the patch? :-)

Kevin

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