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Re: [Qemu-devel] write_zeroes/trim on the whole disk


From: Alex Bligh
Subject: Re: [Qemu-devel] write_zeroes/trim on the whole disk
Date: Sat, 24 Sep 2016 18:01:38 +0100

> On 24 Sep 2016, at 17:52, Alex Bligh <address@hidden> wrote:
> 
> In *your* use-case holes may be desirable. However in the general case, you 
> cannot assume a server supports holes. Optional support for holes isn't even 
> in the mainline spec yet (AFAIR).

You should also be aware that the minimum granularity for writes (currently 1 
byte in the spec, but the blocksize extension allows for larger) may be smaller 
than the minimum granularity for holes, which may depend on the underlying 
filing system.

A hole is in essence merely an optimised way of storing zeroes. And the trim 
operation says 'I'm not worried about this data any more - so zero it if you 
like'.

-- 
Alex Bligh







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