Re: [Qemu-devel] [RFC PATCHv2] block: optimize zero writes with bdrv_write_zeroes

[Markus Armbruster]

Peter Lieven <address@hidden> writes:

> On 26.03.2014 10:16, Markus Armbruster wrote:
>> Peter Lieven <address@hidden> writes:
>>
>>> this patch tries to optimize zero write requests
>>> by automatically using bdrv_write_zeroes if it is
>>> supported by the format.
>>>
>>> this should significantly speed up file system initialization and
>>> should speed zero write test used to test backend storage performance.
>>>
>>> the difference can simply be tested by e.g.
>>>
>>> dd if=/dev/zero of=/dev/vdX bs=1M
>> Got actual numbers?  Preferably for some operation that matters to
>> users.
>
> To give you a rough figure I have an iSCSI Storage for testing that
> has an 1GBit connection. The above command gives about 110MB/s
> with detect-zeroes=off and about 980MB/s with detect-zeroes=unmap.
>
> Additionally I ran a test with v1 of the patch:
>
> ---8<---
>
> I created a 60GB qcow2 container and formatted it with ext4. To
> immediately show the difference
> I disabled lazy inode table and lazy journal init (writing zero takes
> place immediately then).
>
> Timing without the patch:
> time mkfs.ext4 -E lazy_itable_init=0,lazy_journal_init=0 /dev/vda
> real 1m5.649s
> user 0m0.416s
> sys  0m3.148s
>
> Timing with the patch:
> time mkfs.ext4 -E lazy_itable_init=0,lazy_journal_init=0 /dev/vdX
> real 0m1.228s
> user 0m0.116s
> sys  0m0.732s
>
> Container Size after Format without the patch: 1150615552 Byte (1097.3MB)
> Container Size after Format with the patch: 24645632 Byte (23.5MB)
>
> --->8---
>
> Without the patch means detect-zeroes=off (default) and with the patch
> means detect-zeroes=unmap.

Recommend to document your testing in the commit message.