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Re: [Qemu-devel] [PATCH v6 0/9] discard blockstats

From: Anton Nefedov
Subject: Re: [Qemu-devel] [PATCH v6 0/9] discard blockstats
Date: Thu, 21 Feb 2019 08:02:53 +0000

On 14/1/2019 4:16 PM, Anton Nefedov wrote:
> On 30/11/2018 5:47 PM, Anton Nefedov wrote:
>> qmp query-blockstats provides stats info for write/read/flush ops.
>> Patches 1-7 implement the similar for discard (unmap) command for scsi
>> and ide disks.
>> Discard stat "unmap_ops / unmap_bytes" is supposed to account the ops 
>> that
>> have completed without an error.
>> However, discard operation is advisory. Specifically,
>>   - common block layer ignores ENOTSUP error code.
>>     That might be returned if the block driver does not support discard,
>>     or discard has been configured to be ignored.
>>   - format drivers such as qcow2 may ignore discard if they were 
>> configured
>>     to ignore that, or if the corresponding area is already marked unused
>>     (unallocated / zero clusters).
>> And what is actually useful is the number of bytes actually discarded
>> down on the host filesystem.
>> To achieve that, driver-specific statistics has been added to blockstats
>> (patch 9).
>> With patch 8, file-posix driver accounts discard operations on its 
>> level too.
> ping


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