Re: [Qemu-devel] [PATCH 1/3] block/backup: deal with zero detection

[Max Reitz <mreitz@redhat.com>]

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On 30.07.19 20:40, John Snow wrote:
> 
> 
> On 7/30/19 12:32 PM, Vladimir Sementsov-Ogievskiy wrote:
>> We have detect_zeroes option, so at least for blockdev-backup user
>> should define it if zero-detection is needed. For drive-backup leave
>> detection enabled by default but do it through existing option instead
>> of open-coding.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>>  block/backup.c | 15 ++++++---------
>>  blockdev.c     |  8 ++++----
>>  2 files changed, 10 insertions(+), 13 deletions(-)
>>
>> diff --git a/block/backup.c b/block/backup.c
>> index 715e1d3be8..f4aaf08df3 100644
>> --- a/block/backup.c
>> +++ b/block/backup.c
>> @@ -110,7 +110,10 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
>>      BlockBackend *blk = job->common.blk;
>>      int nbytes;
>>      int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING : 0;
>> -    int write_flags = job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0;
>> +    int write_flags =
>> +            (job->serialize_target_writes ? BDRV_REQ_SERIALISING : 0) |
>> +            (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0);
>> +
>>  
>>      assert(QEMU_IS_ALIGNED(start, job->cluster_size));
>>      hbitmap_reset(job->copy_bitmap, start, job->cluster_size);
>> @@ -128,14 +131,8 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
>>          goto fail;
>>      }
>>  
>> -    if (buffer_is_zero(*bounce_buffer, nbytes)) {
>> -        ret = blk_co_pwrite_zeroes(job->target, start,
>> -                                   nbytes, write_flags | BDRV_REQ_MAY_UNMAP);
>> -    } else {
>> -        ret = blk_co_pwrite(job->target, start,
>> -                            nbytes, *bounce_buffer, write_flags |
>> -                            (job->compress ? BDRV_REQ_WRITE_COMPRESSED : 0));
>> -    }
>> +    ret = blk_co_pwrite(job->target, start, nbytes, *bounce_buffer,
>> +                        write_flags);
>>      if (ret < 0) {
>>          trace_backup_do_cow_write_fail(job, start, ret);
>>          if (error_is_read) {
>> diff --git a/blockdev.c b/blockdev.c
>> index 4d141e9a1f..a94d754504 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -3434,7 +3434,7 @@ static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>>      BlockJob *job = NULL;
>>      BdrvDirtyBitmap *bmap = NULL;
>>      AioContext *aio_context;
>> -    QDict *options = NULL;
>> +    QDict *options;
>>      Error *local_err = NULL;
>>      int flags, job_flags = JOB_DEFAULT;
>>      int64_t size;
>> @@ -3529,10 +3529,10 @@ static BlockJob *do_drive_backup(DriveBackup *backup, JobTxn *txn,
>>          goto out;
>>      }
>>  
>> +    options = qdict_new();
>> +    qdict_put_str(options, "discard", "unmap");
>> +    qdict_put_str(options, "detect-zeroes", "unmap");
>>      if (backup->format) {
>> -        if (!options) {
>> -            options = qdict_new();
>> -        }
>>          qdict_put_str(options, "driver", backup->format);
>>      }
>>  
>>
> 
> I'm less sure of this one personally. Is it right to always try to set
> unmap on the target?
> 
> I like the idea of removing special cases and handling things more
> centrally though, but I'll want Max (or Kevin) to take a peek.

I don’t quite know why, because this is just a block job specific
question and doesn’t have much to do with the rest of the block layer,
but OK. :-)

drive-backup always set BDRV_REQ_MAY_UNMAP, as you can see.  Maybe that
didn’t do anything because the target wasn’t opened with discard=unmap.
 But to me, it’s clear that the intention was to indeed unmap the areas
in the target (it isn’t like the user had a choice of opening the target
with discard=unmap or not).

Max


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