Re: [Qemu-devel] qemu-img convert cache mode for source

[Peter Lieven <pl@kamp.de>]

On 03.03.2014 11:38, Kevin Wolf wrote:
> Am 28.02.2014 um 15:35 hat Peter Lieven geschrieben:
>> On 27.02.2014 09:57, Stefan Hajnoczi wrote:
>>> On Wed, Feb 26, 2014 at 05:01:52PM +0100, Peter Lieven wrote:
>>>> On 26.02.2014 16:41, Stefan Hajnoczi wrote:
>>>>> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote:
>>>>>> I was wondering if it would be a good idea to set the O_DIRECT mode for the source
>>>>>> files of a qemu-img convert process if the source is a host_device?
>>>>>>
>>>>>> Currently the backup of a host device is polluting the page cache.
>>>>> Points to consider:
>>>>>
>>>>> 1. O_DIRECT does not work on Linux tmpfs, you get EINVAL when opening
>>>>>     the file.  A fallback is necessary.
>>>>>
>>>>> 2. O_DIRECT has no readahead so performance could actually decrease.
>>>>>     The question is, how important is reahead versus polluting page
>>>>>     cache?
>>>>>
>>>>> 3. For raw files it would make sense to tell the kernel that access is
>>>>>     sequential and data will be used only once.  Then we can get the best
>>>>>     of both worlds (avoid polluting page cache but still get readahead).
>>>>>     This is done using posix_fadvise(2).
>>>>>
>>>>>     The problem is what to do for image formats.  An image file can be
>>>>>     very fragmented so the readahead might not be a win.  Does this mean
>>>>>     that for image formats we should tell the kernel access will be
>>>>>     random?
>>>>>
>>>>>     Furthermore, maybe it's best to do readahead inside QEMU so that even
>>>>>     network protocols (nbd, iscsi, etc) can get good performance.  They
>>>>>     act like O_DIRECT is always on.
>>>> your comments are regarding qemu-img convert, right?
>>>> How would you implement this? A new open flag because
>>>> the fadvise had to goto inside the protocol driver.
>>>>
>>>> I would start with host_devices first and see how it performs there.
>>>>
>>>> For qemu-img convert I would issue a FADV_DONTNEED after
>>>> a write for the bytes that have been written
>>>> (i have tested this with Linux and it seems to work quite well).
>>>>
>>>> Question is, what is the right paramter for reads? Also FADV_DONTNEED?
>>> I think so but this should be justified with benchmark results.
>> I ran some benchmarks at found that a FADV_DONTNEED issues after
>> a read does not hurt regarding to performance. But it avoids buffers
>> increasing while I read from a host_device of raw file.
> Okay, sounds reasonable.
>
>> As for writing it does only work if I issue a fdatasync after each write, but
>> this should be equivalent to O_DIRECT. So I would keep the patch
>> to support qemu-img convert sources if they are host_device or file.
> Doing an fdatasync() is not an option (and not equivalent to O_DIRECT
> at all).
of course, fdatasync is no option. I just wanted to point out that one
should use O_DIRECT if the pagecache should be disable for the output
file.
>
>> Here is a proposal for a patch:
>>
>> diff --git a/block.c b/block.c
>> index 2fd5482..2445433 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -2626,6 +2626,14 @@ static int bdrv_prwv_co(BlockDriverState *bs, int64_t offset,
>>               qemu_aio_wait();
>>           }
>>       }
>> +
>> +#ifdef POSIX_FADV_DONTNEED
>> +    if (!rwco.ret && bs->open_flags & BDRV_O_SEQUENTIAL &&
>> +        bs->drv->bdrv_fadvise && !is_write) {
>> +        bs->drv->bdrv_fadvise(bs, offset, qiov->size, POSIX_FADV_DONTNEED);
>> +    }
>> +#endif
>> +
> This #ifdef should be in the raw-posix driver. Please try to keep the
> qemu interface backend agnostic and leave POSIX_FADV_DONTNEED and
> friends as an implementation detail of block drivers.
I had the same idee, but as far as I see the callback to the completes
request is handled in block.c. raw-posix uses the aio interface and
not coroutines.
>
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 780f48b..a4dcc3c 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -105,6 +105,9 @@ typedef enum {
>>   #define BDRV_O_PROTOCOL    0x8000  /* if no block driver is explicitly given:
>>                                         select an appropriate protocol driver,
>>                                         ignoring the format layer */
>> +#define BDRV_O_SEQUENTIAL 0x10000  /* open device for sequential read/write */
>> +
>> +
>>
>>   #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH)
> Why two additional newlines?
typo. this patch war basically an RFC :-)
>
> BDRV_O_SEQUENTIAL works for me as the external interface.
>
> Kevin

Peter