Re: [PATCH v3 0/4] block: seriously improve savevm performance

[Denis V. Lunev]

On 6/15/20 3:17 PM, Dr. David Alan Gilbert wrote:
> * Denis V. Lunev (den@openvz.org) wrote:
>> This series do standard basic things:
>> - it creates intermediate buffer for all writes from QEMU migration code
>>   to QCOW2 image,
>> - this buffer is sent to disk asynchronously, allowing several writes to
>>   run in parallel.
>>
>> In general, migration code is fantastically inefficent (by observation),
>> buffers are not aligned and sent with arbitrary pieces, a lot of time
>> less than 100 bytes at a chunk, which results in read-modify-write
>> operations with non-cached operations. It should also be noted that all
>> operations are performed into unallocated image blocks, which also suffer
>> due to partial writes to such new clusters.
> It surprises me a little that you're not benefiting from the buffer
> internal to qemu-file.c
>
> Dave
There are a lot of problems with this buffer:

Flushes to block driver state are performed in the abstract places,
pushing
  a) small IO
  b) non-aligned IO both to
       1) page size
       2) cluster size
It should also be noted that buffer in QEMU file is quite small and
all IO operations with it are synchronous. IO, like ethernet, wants
good queues.

The difference is on the table.

Den