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Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes |
Date: |
Mon, 24 Feb 2014 13:22:47 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 24.02.2014 um 13:10 hat Paolo Bonzini geschrieben:
> Il 24/02/2014 13:07, Kevin Wolf ha scritto:
> >>> Yeah, that's why I wrote "or should be". Those are the intended
> >>> semantics of bdrv_co_write_zeroes without BDRV_REQ_MAY_UNMAP: always
> >>> allocate a cluster that will read as zeroes (allocating even if it
> >>> does not necessarily write the zeroes).
> >Which would mean that there is no way to say "give me zeroes, and do it
> >in the cheapest way possible". Because that would be to leave the
> >allocation status as it is and just toggle the zero bit.
>
> If bdrv_co_write_zeroes is SCSI's "WRITE SAME without UNMAP", then
> it must allocate. I think "give me zeroes and do it in the cheapest
> way possible" is BDRV_REQ_MAY_UNMAP (which *may* unmap but it
> doesn't have to).
Hm, okay. So the intended behaviour for qcow2 is:
- without MAY_UNMAP: Preallocate the cluster and set the zero flag so
that its content isn't valid
- with MAY_UNMAP: If the cluster is allocated, leave it allocated and
set the zero flag; if it isn't, leave it unallocated and set the zero
flag.
> That said, the really expensive part of unmapping is probably
> re-allocating the clusters on subsequent writes. The unmap itself
> isn't that expensive (even if you have to flush the refcount blocks
> before the L2 tables), is it?
Flushes are pretty expensive. If you only have many discards in a row
and then many allocations in a row, it's probably okay. Mixing them so
that every other write is a discard might well kill performance.
Kevin
- [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/22
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Fam Zheng, 2014/02/22
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/23
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Fam Zheng, 2014/02/23
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Paolo Bonzini, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Fam Zheng, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Paolo Bonzini, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Fam Zheng, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Kevin Wolf, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Paolo Bonzini, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes,
Kevin Wolf <=
Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Kevin Wolf, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Paolo Bonzini, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/24
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Kevin Wolf, 2014/02/25
- Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes, Peter Lieven, 2014/02/25