Re: [Libguestfs] [PATCH nbdkit] file: Implement cache=none and fadvise=normal|random|sequential.
On 8/7/20 8:43 AM, Nir Soffer wrote:
>>
>> It's also possible to avoid polluting the page cache by using
>> O_DIRECT, but that comes with harder guarantees (aligned access
>> through aligned buffers), so we may add it as another mode later on.
>> But in the meantime, cache=none is fairly nice while still avoiding
>> O_DIRECT.
>
> I'm not sure if or even how we could ever do a robust O_DIRECT
>
We can let the plugin an filter deal with that. The simplest solution is to
drop it on the user and require aligned requests.
Maybe a filter can handle alignment?
The blocksize filter seems like a good fit. Also, I'm trying to add
support for NBD_INFO_BLOCK_SIZE (qemu already supports it), so that we
can advertise minimum granularity across to the client to avoid the need
for read-modify-write in the blocksize filter.
implementation, but my idea was that it might be an alternate
> implementation of cache=none. But if we thought we might use O_DIRECT
> as a separate mode, then maybe we should rename cache=none.
> cache=advise? cache=dontneed? I can't think of a good name!
>
Yes, don't call it none if you use the cache.
How about advise=?
I would keep cache semantics similar to qemu.
qemu-img has cache=, but it affects O_DIRECT rather than posix_fadvise.
Yeah, naming is hard, and I don't know if I have better ideas.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org