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Re: [Qemu-devel] [PATCH] Further tidy-up on block status

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] Further tidy-up on block status
Date: Fri, 20 Jan 2017 13:35:10 -0600
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On 01/20/2017 12:00 PM, Alex Bligh wrote:
>> On 20 Jan 2017, at 17:04, Vladimir Sementsov-Ogievskiy <address@hidden> 
>> wrote:
>> About extents: is 32bit length enough? We will have to send 4096 for empty 
>> 16tb disk..
> The nbd protocol uniformly uses 32 bit lengths (for better or for worse). 
> This is baked into the specification heavily.
> I'm not sure sending 4,096 items for an empty 16TB disk is any great hardship 
> to be honest.

If it truly matters, an idea that has already been floated on the list
is to add a new NBD_CMD_FLAG_SCALE (or some other spelling) that states
that a particular command is sending lengths scaled by a particular
factor (by the block size? obviously it would have to be better
specified).  Under this idea, the scaling factor would allow you to
report larger extents for fewer back-and-forth operations, but it gets
tricky if the scaling is allowed to get larger than the minimum
granularity between extent changes.

The other idea that has been floated is a way to report whether the
entire export is known to be all-zero content at the time the client
connects, which would trade 4096+ queries (you'd actually have to do
more than 4096 queries, since length is < 4G, not <= 4G) with a single
piece of information at the time the client connects.

Either way, discussion on such enhancements are probably worth a new thread.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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