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Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format
From: |
Chunqiang Tang |
Subject: |
Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249% |
Date: |
Wed, 19 Jan 2011 10:17:47 -0500 |
> >> Moreover, using a host file system not only adds overhead, but
> >> also introduces data integrity issues. Specifically, if I/Os uses
O_DSYNC,
> >> it may be too slow. If I/Os use O_DIRECT, it cannot guarantee data
> >> integrity in the event of a host crash. See
> >> http://lwn.net/Articles/348739/ .
> >
> > You have the same issue with O_DIRECT when using a raw disk device
> > too. That is, O_DIRECT on a raw device does not guarantee integrity
> > in the event of a host crash either, for mostly the same reasons.
>
> QEMU has semantics that use O_DIRECT safely; there is no issue here.
> When a drive is added with cache=none, QEMU not only uses O_DIRECT but
> also advertises an enabled write cache to the guest.
>
> The guest *must* flush the cache when it wants to ensure data is
> stable. In the event of a host crash, all, some, or none of the I/O
> since the last flush may have made it to disk. Each of these
> possibilities is fair game since the guest may only depend on writes
> being on disk if they completed and a successful flush was issued
> afterwards.
Thank both of you for the explanation, which is very helpful to me. With
FVD's capability of eliminating the host file system and storing the image
on a logical volume, then perhaps we can always use O_DSYNC, because there
is little (or no?) LVM metadata that needs a flush on every write and
hence O_DSYNC does not add overhead? I am not certain on this, and need
help for confirmation. If this is true, the guest does not need to flush
the cache.
- [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Chunqiang Tang, 2011/01/04
- Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Christoph Hellwig, 2011/01/19
- Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Chunqiang Tang, 2011/01/19
- Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Christoph Hellwig, 2011/01/19
- Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Chunqiang Tang, 2011/01/19
- Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Christoph Hellwig, 2011/01/19
Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%, Stefan Hajnoczi, 2011/01/06