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Re: [vfio-users] setting up virtual disks



On 03/28/17 15:41, Patrick O'Callaghan wrote:
> On Tue, 2017-03-28 at 15:08 +0200, Laszlo Ersek wrote:
>> On 03/28/17 14:21, Patrick O'Callaghan wrote:
>>> On Tue, 2017-03-28 at 11:47 +0200, Laszlo Ersek wrote:
>>>> I recommend the following setup:
>>>>
>>>> - hard disk(s):          virtio-blk or virtio-scsi disk, as you prefer
>>>
>>> I'm interested in why someone would prefer one over the other. Can you
>>> explain?
>>
>> I prefer virtio-scsi because it supports thin provisioning (UNMAP scsi
>> operation); it lets me conserve space in host filesystems that support
>> discard (such as ext4 or xfs, for example -- there may be more). Given
>> the right configuration, if you delete files in your Windows 8 or
>> Windows 10 VM, the space is eventually released on the host filesystem.
> 
> OK, thanks.
> 
>> With virtio-blk, the software stack is less featureful and thereby
>> thinner, which is said by some to lead to better performance. Also, as
>> far as I know, dataplane is only available for virtio-blk at the moment,
>> it is in progress for virtio-scsi. (I could be out of date on that
>> though.) YMMV.
> 
> No idea what that is. As I'm not provisioning multiple high-load
> servers, does this matter to me?

I couldn't give you more authoritative documentation than what google
turns up, so please go ahead and search for it yourself. Personally, I
have never ever set up virtio-blk dataplane, in my short or long term
guests (some of which use GPU or other device assignment as well), and I
have no complaints about IO performance. (I too don't run production
servers, like you.) The bottleneck on my laptop has always been SSD
capacity (even with two SSDs), which virtio-scsi (with unmap/discard
enabled) has remedied impeccably. I guess, if you haven't complained to
yourself about IO performance, don't bother with dataplane.

Laszlo


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