Re: [Qemu-devel] [PATCH 8/8] block-migration: efficiently encode zero blocks

[Peter Lieven <pl@kamp.de>]

Am 24.06.2013 16:32, schrieb Paolo Bonzini:
> Il 22/06/2013 22:58, Peter Lieven ha scritto:
>> this patch adds a efficient encoding for zero blocks by
>> adding a new flag indiciating a block is completly zero.
>>
>> additionally bdrv_write_zeros() is used at the destination
>> to efficiently write these zeroes. if the driver supports
>> it this avoids blindly allocating all sectors consumed by
>> zero blocks effectively re-thinning the device.
>>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
> This is a bit ugly because it doesn't work with drive-mirror.  Perhaps
> we can add a write-zeroes opcode to NBD, too.
That is the exact problem with NBD. If write zeroes is not supported
the routine falls back to write zeros with writev.

Peter
>
> Paolo
>
>> ---
>>  block-migration.c             |   29 +++++++++++++++++++++++------
>>  include/migration/qemu-file.h |    1 +
>>  savevm.c                      |    2 +-
>>  3 files changed, 25 insertions(+), 7 deletions(-)
>>
>> diff --git a/block-migration.c b/block-migration.c
>> index 2fd7699..99b3757 100644
>> --- a/block-migration.c
>> +++ b/block-migration.c
>> @@ -29,6 +29,7 @@
>>  #define BLK_MIG_FLAG_DEVICE_BLOCK       0x01
>>  #define BLK_MIG_FLAG_EOS                0x02
>>  #define BLK_MIG_FLAG_PROGRESS           0x04
>> +#define BLK_MIG_FLAG_ZERO_BLOCK         0x08
>>  
>>  #define MAX_IS_ALLOCATED_SEARCH 65536
>>  
>> @@ -114,16 +115,29 @@ static void blk_mig_unlock(void)
>>  static void blk_send(QEMUFile *f, BlkMigBlock * blk)
>>  {
>>      int len;
>> +    int flags = BLK_MIG_FLAG_DEVICE_BLOCK;
>> +    
>> +    if (buffer_is_zero(blk->buf, BLOCK_SIZE)) {
>> +        flags |= BLK_MIG_FLAG_ZERO_BLOCK;
>> +    }
>>  
>>      /* sector number and flags */
>>      qemu_put_be64(f, (blk->sector << BDRV_SECTOR_BITS)
>> -                     | BLK_MIG_FLAG_DEVICE_BLOCK);
>> +                     | flags);
>>  
>>      /* device name */
>>      len = strlen(blk->bmds->bs->device_name);
>>      qemu_put_byte(f, len);
>>      qemu_put_buffer(f, (uint8_t *)blk->bmds->bs->device_name, len);
>>  
>> +    /* if a block is zero we need to flush here since the network
>> +     * bandwidth is now a lot higher than the storage device bandwidth.
>> +     * thus if we queue zero blocks we slow down the migration */
>> +    if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
>> +        qemu_fflush(f);
>> +        return;
>> +    }
>> +
>>      qemu_put_buffer(f, blk->buf, BLOCK_SIZE);
>>  }
>>  
>> @@ -762,12 +776,15 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
>>                  nr_sectors = BDRV_SECTORS_PER_DIRTY_CHUNK;
>>              }
>>  
>> -            buf = g_malloc(BLOCK_SIZE);
>> -
>> -            qemu_get_buffer(f, buf, BLOCK_SIZE);
>> -            ret = bdrv_write(bs, addr, buf, nr_sectors);
>> +            if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
>> +                ret = bdrv_write_zeroes(bs, addr, nr_sectors);
>> +            } else {
>> +                buf = g_malloc(BLOCK_SIZE);
>> +                qemu_get_buffer(f, buf, BLOCK_SIZE);
>> +                ret = bdrv_write(bs, addr, buf, nr_sectors);
>> +                g_free(buf);
>> +            }
>>  
>> -            g_free(buf);
>>              if (ret < 0) {
>>                  return ret;
>>              }
>> diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
>> index 7519464..b73298d 100644
>> --- a/include/migration/qemu-file.h
>> +++ b/include/migration/qemu-file.h
>> @@ -71,6 +71,7 @@ QEMUFile *qemu_fdopen(int fd, const char *mode);
>>  QEMUFile *qemu_fopen_socket(int fd, const char *mode);
>>  QEMUFile *qemu_popen_cmd(const char *command, const char *mode);
>>  int qemu_get_fd(QEMUFile *f);
>> +void qemu_fflush(QEMUFile *f);
>>  int qemu_fclose(QEMUFile *f);
>>  int64_t qemu_ftell(QEMUFile *f);
>>  void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, int size);
>> diff --git a/savevm.c b/savevm.c
>> index ff5ece6..4d12d92 100644
>> --- a/savevm.c
>> +++ b/savevm.c
>> @@ -610,7 +610,7 @@ static inline bool qemu_file_is_writable(QEMUFile *f)
>>   * If there is writev_buffer QEMUFileOps it uses it otherwise uses
>>   * put_buffer ops.
>>   */
>> -static void qemu_fflush(QEMUFile *f)
>> +void qemu_fflush(QEMUFile *f)
>>  {
>>      ssize_t ret = 0;
>>  
>>