[Qemu-devel] Re: [PATCH 2/3 v5] Block live migration

[Jan Kiszka <jan.kiszka@web.de>]

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Liran Schour wrote:
> jan.kiszka@web.de wrote on 25/11/2009 00:55:57:
> 
> 
>> trying to understand the code and fixing some cosmetic issues around
>> progress reporting, one potentially performance-relevant question popped
> up:
>> lirans@il.ibm.com wrote:
>>> diff --git a/block-migration.c b/block-migration.c
>>> new file mode 100644
>>> index 0000000..4b4eddf
>>> --- /dev/null
>>> +++ b/block-migration.c
>> ...
>>
>>> +static int mig_read_device_bulk(QEMUFile *f, BlkMigDevState *bms)
>>> +{
>>> +    int nr_sectors;
>>> +    int64_t total_sectors, cur_sector = 0;
>>> +    BlockDriverState *bs = bms->bs;
>>> +    BlkMigBlock *blk;
>>> +
>>> +    blk = qemu_malloc(sizeof(BlkMigBlock));
>>> +    blk->buf = qemu_malloc(BLOCK_SIZE);
>>> +
>>> +    cur_sector = bms->cur_sector;
>>> +    total_sectors = bdrv_getlength(bs) >> SECTOR_BITS;
>> Why re-calculating total_sectors here? Specifically bdrv_getlength() can
>> be a non-trivial I/O operation.
>>
>> And what is the difference to bms->total_sectors which looks a lot
> like...
>> [...]
>>> +
>>> +static void init_blk_migration(QEMUFile *f)
>>> +{
>>> +    BlkMigDevState **pbmds, *bmds;
>>> +    BlockDriverState *bs;
>>> +
>>> +    for (bs = bdrv_first; bs != NULL; bs = bs->next) {
>>> +        if(bs->type == BDRV_TYPE_HD) {
>>> +            bmds = qemu_mallocz(sizeof(BlkMigDevState));
>>> +            bmds->bs = bs;
>>> +            bmds->bulk_completed = 0;
>>> +            bmds->total_sectors = bdrv_getlength(bs) >> SECTOR_BITS;
>> ...it already contains all the information we need?
>>
> 
> You are right no need to figure it out once again. We can use the old
> total_sectors value.

Perfect.

Meanwhile I've quite a few patches here, including that change, will
hopefully manage to send them out today.

Jan


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