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Re: [Qemu-devel] [PATCH 1/4] block: add enable_write_cache flag

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 1/4] block: add enable_write_cache flag
Date: Mon, 31 Aug 2009 17:53:23 -0500
User-agent: Thunderbird (X11/20090825)

Christoph Hellwig wrote:
If an unsafe mode is desired (I think it is, for those throwaway
testing VMs, or during OS installs), I suggest adding cache=volatile:

        O_DIRECT, fdatasync, advertise volatile write cache

        O_SYNC, do not advertise

        fdatasync, advertise volatile write cache

        nothing (perhaps fdatasync on QEMU blockdev close)

Fine withe me, let the flame war begin :)

I think we should pity our poor users and avoid adding yet another obscure option that is likely to be misunderstood.

Can someone do some benchmarking with cache=writeback and fdatasync first and quantify what the real performance impact is?

I think the two reasonable options are 1) make cache=writeback safe, avoid a massive perf decrease in the process 2) keep cache=writeback as a no-guarantees option.


Anthony Liguori

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