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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Sun, 19 Oct 2008 11:01:05 +0200
User-agent: Thunderbird (X11/20080723)

Jens Axboe wrote:
On Sun, Oct 12 2008, Avi Kivity wrote:
If you have a normal laptop, your disk has a cache. That cache does not have a battery backup. Under normal operations, the cache is acting in write-back mode and when you do a write, the disk will report the write as completed even though it is not actually on disk. If you really care about the data being on disk, you have to either use a disk with a battery backed cache (much more expensive) or enable write-through caching (will significantly reduce performance).

I think that with SATA NCQ, this is no longer true. The drive will report the write complete when it is on disk, and utilize multiple outstanding requests to get coalescing and reordering. Not sure about

It is still very true. Go buy any consumer drive on the market and check
the write cache settings - hint, it's definitely shipped with write back
caching. So while the drive may have NCQ and Linux will use it, the
write cache is still using write back unless you explicitly change it.

Sounds like a bug. Shouldn't Linux disable the write cache unless the user explicitly enables it, if NCQ is available? NCQ should provide acceptable throughput even without the write cache.

error compiling committee.c: too many arguments to function

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