Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking

[Paolo Bonzini]

Il 07/03/2013 09:50, Kevin Wolf ha scritto:
> Am 06.03.2013 um 21:39 hat Paolo Bonzini geschrieben:
>> Il 06/03/2013 20:03, Peter Lieven ha scritto:
>>> Am 06.03.2013 19:48, schrieb Jeff Cody:
>>>> On Wed, Mar 06, 2013 at 07:31:51PM +0100, Paolo Bonzini wrote:
>>>>> Il 06/03/2013 19:14, Jeff Cody ha scritto:
>>>>>> QCOW breaks with it using a normal raw posix file as a device.  As a
>>>>>> test: qemu-img create -f qcow test.qcow 5G.  Now run qemu with that
>>>>>> drive mounted, and try to partition and format it.  QEMU now asserts.
>>>>>>
>>>>>> The nicety of being able to using truncate during a write call,
>>>>>> especially for VHDX (which can have relatively large block/cluster
>>>>>> sizes), so to grow the file sparsely in a dynamically allocated file.
>>>>>
>>>>> Perhaps we need two APIs, "truncate" and "revalidate".
>>>>>
>>>>> Truncate should be a no-op if (!bs->growable).
>>>>>
>>>>> Revalidate could be called by the block_resize monitor command with no
>>>>> size specified.
>>>>
>>>> I think that is a good solution.  Is it better to have "truncate" and
>>>> "revalidate", or "truncate" and "grow", with grow being a subset of
>>>> truncate, with fewer restrictions?  There may still be operations
>>>> where it is OK to grow a file, but not OK to shrink it.
> 
> What semantics would the both operations have? Is truncate the same as
> it used to be? I don't really understand what "revalidate" would do, it
> sounds like a read-only operation from its name?

Revalidate would update the current size.  Files fetch it on all calls
to bdrv_getlength, but other backends cache it.  It would visit the BDS
->file chain from the bottom (bs->file->file->file...) to the top
implicitly, with no need for an explicit recursion in the callback (like
we do for flush_to_os).  Before starting the recursion, bdrv_revalidate
does a bdrv_drain_all, so using the current iscsi_truncate for
iscsi_revalidate would be fine.

The block_resize command will always call revalidate before doing
anything.  Then block_resize will try to do a bdrv_truncate if the
callback is there.

Another change that could make sense, is to make bdrv_truncate succeed
if there is no bdrv_truncate callback but bs is larger than the
requested size.  This would be a difference from today's

    if (!drv->bdrv_truncate)
        return -ENOTSUP;

And it could even do the same if the callback is there, but returns
ENOTSUP.  It would simplify some code, like this in raw-posix.c's
raw_truncate:

    } else if (S_ISCHR(st.st_mode) || S_ISBLK(st.st_mode)) {
       if (offset > raw_getlength(bs)) {
           return -EINVAL;
       }
    } else {
        return -ENOTSUP;
    }

The "else if" branch can just go away.

The remaining question is about usage of bdrv_truncate in the formats.
One important aspect is that both QCOW and VHDX, in addition to using
bdrv_truncate to extend the file, rely on bdrv_getlength to fetch the
last _used_ byte of the file, not the available space.  I think we can
say that bdrv_getlength behaves like that iff bs->growable.  If so, QCOW
and VHDX should fail to open for write if the underlying file has
!bs->growable.  Which will never fail right now, but it will as soon as
we implement this:

>> Let's start from (c).  bdrv_file_open sets bs->growable = 1.  I think it
>> should be removed and only the file protocol should set it.
> 
> This is probably right.

Good.  More precisely, raw_open should only set it if the file is a
regular file.

Paolo