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Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache |
Date: |
Tue, 14 Apr 2015 18:15:05 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 |
On 14/04/2015 08:49, Peter Lieven wrote:
> Hi,
>
> Ronnie came up with an idea to reduce latency if !bs->enable_write_cache
> for an iSCSI device.
>
> If !bs->enable_write_cache Qemu sends a flush after every single write.
> What could be done is
> the following:
>
> if (!bs->enable_write_cache)
> set FUA (force unit access) and DPO (disable page out) bits in every
> write cmd
> make iscsi_co_flush a NOOP in this case.
>
> Your thoughts?
Yes, that would work. In fact I'm not even sure you need DPO.
speed of cache=writethrough in general doesn't matter much, except if
whoever runs the guest knows that the host has battery-backed cache. In
that case this trick would improve latency. You could get the same with
-drive file.cache.no-flush=on but this would just work.
Paolo
- [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Peter Lieven, 2015/04/14
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Stefan Hajnoczi, 2015/04/14
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache,
Paolo Bonzini <=
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Peter Lieven, 2015/04/14
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Paolo Bonzini, 2015/04/15
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Peter Lieven, 2015/04/16
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Paolo Bonzini, 2015/04/16
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Peter Lieven, 2015/04/16
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Paolo Bonzini, 2015/04/16
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Peter Lieven, 2015/04/16
- Re: [Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache, Paolo Bonzini, 2015/04/16