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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format |
Date: | Wed, 08 Sep 2010 07:55:18 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.11) Gecko/20100713 Lightning/1.0b1 Thunderbird/3.0.6 |
On 09/08/2010 03:53 AM, Avi Kivity wrote:
On 09/08/2010 11:41 AM, Alexander Graf wrote:On 08.09.2010, at 10:23, Avi Kivity wrote:Why 3 levels? Can't the L2 size be dynamic? Then big images get a big L2 map while small images get a smaller one.On 09/08/2010 01:27 AM, Anthony Liguori wrote:Maybe we should do three levels then. Some users are bound to complain about 64TB.FWIW, L2s are 256K at the moment and with a two level table, it can support 5PB of data.I clearly suck at basic math today. The image supports 64TB today. Dropping to 128K tables would reduce it to 16TB and 64k tables would be 4TB.Dunno, just seems more regular to me. Image resize doesn't need to relocate the L2 table in case it overflows.The overhead from three levels is an extra table, which is negligible.
It means an extra I/O request in the degenerate case whereas increasing the table size only impacts the size of the metadata.
A 10GB image currently has 1.2MB of metadata in QED today. A 1TB image uses 128MB of metadata. The ratio of metadata is about 0.01%.
A three level table adds an additional I/O request in order to reduce metadata. But the metadata is small enough today that I don't see the point.
Regards, Anthony Liguori
With 64K tables, the maximum image size is 32PiB, which is 14 bits away from a 2TB disk, giving us about 30 years.
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