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

From: Anton Nefedov
Subject: Re: [Qemu-block] [PATCH v6 0/9] discard blockstats
Date: Mon, 14 Jan 2019 13:16:31 +0000

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.


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